Theory Stars

Stars are celestial bodies which emit energy and radiation which are originating from the core of the star, where processes of nuclear fusion occur. The study of stars in astronomy worked much upon what is called the Hertzsprung-Russell diagram, and then to nowadays astrophysics. First, astronomers worked upon stars' light to understand them and then they tried to understand how they had been born, how they evolved, and, eventually, how they were dying

A close view of star Antares, bright star to the constellation Scorpio, the Scorpion, a red supergiant. picture based upon a picture ESO

Bases of Stars' Sorting

Building of the development of spectrography in the second part of the 19th century (Robert Bunsen (1811-1899) and Gustav Kirchhoff (1824-1887), University of Heidelberg; elements of a star appear like lines in their spectrum, their scattered light), to which was added the systematic study (1884-1920s) at the Harvard College University by Annie Jump Cannon (1863-1941), stars have been sorted into 7 spectral classes: O, B, A, F, G, K, M. Strictly speaking, this sorting is not made on the basis of the chemical compounds of stars but on their surface temperature (from the stars with higher surface temperatures, class O, to stars with lower surface temperatures, class M). Spectral discrepancies which had been observed was indeed more the result of differences in temperatures at the surface of the stars than of such or such compounds being in their photosphere. However, some elements of stellar spectra are result of differences in the chemical composition, mainly differences in the so-called heavy elements (elements heavier than hydrogen and helium). Each of these spectral classes is at its turn divided into 10 spectral types; these spectral types are a range of temperatures. Class A, e.g. is divided into spectral types A0, A1, A2, A3, A4, A5, A6, A7, A8, A9. The lower the type, the warmer the surface temperature of the stars. Class O is divided only into the types O4, O5, O6, O7, O8, O9. In 1940, astronomers at the Yerkes Observatory added to sorting into spectral classes and types, an additional sorting: stars of same surface temperature are sorted into luminosity classes: class I is very bright supergiants, class II, bright giants, class III, giants, class IV, sub-giants, class V, main sequence stars, class VI (VII), dwarf stars.

On this basis, stars may so be sorted like:

• class O stars are stars of which surface temperature lies between 28,000 and 60,000 K; their spectral lines are those of ionized helium, metals and hydrogen (in feeble quantity). These are blue stars. An example of such stars is z of Orion
• class B stars are stars of which surface temperature lies between 10,000 and 28,000 K; their spectral lines are those of neutral helium, ionized metals and hydrogen (in more important quantity). These are blue stars. Examples of such stars are Rigel and Spica
• class A stars are stars of which surface temperature lies between 7,500 and 10,000 K; their spectral lines are those of Balmer hydrogen and of singly-ionized metals. These are blue-white stars. Examples of such stars are Sirius and Deneb
• class F stars are stars of which surface temperature lies between 6,000 and 7,500 K; their spectral lines are those of neutral hydrogene (in weaker quantity) and of singly-ionized metals. These are white stars. Examples of such stars are Procyon and Canopus
• class G stars are stars of which surface temperature lies between 5,000 and 6,000 K; their spectral lines are those of singly ionized calcium, of hydrogen (in weaker quantity) and of neutral metals. These are yellow-white stars. Examples of such stars are the Sun and Capella
• class K stars are stars of which surface temperature lies between 3,500 and 5,000 K; their spectral lines are those of neutral metals and molecular bands begin to appear. These are orange stars. Examples of such stars are Aldebaran and Arcturus
• class M stars are stars of which surface temperature is under 3,500 K; their spectral lines are those of titane oxyde (TiO) and of molecular lines. These are red stars. Examples of such stars are Antares and Betelgeuse

This range of sortings do that a star may be named with a code. Thus, Sirius is a A1 V star (class A star; so blue-white; spectral type 1 of the spectral class -so having a high temperature; luminosity class V -so main sequence stars); Rigel is B8 I (class B star, so blue, type 8 of class B, so low temperature; luminosity class I, so very bright supergiant). Sun is G2 V (class G star, so yellow-white; type 2 of the class, so high temperature; luminosity class V, so main sequence star)

Four Groups of the Hertzsprung-Russell Diagram

Hertzsprung (Ejnar Hertzsprung, 1873-1967, Danish astronomer) and Russel (Henry Norris Russell, 1877-1957, American astronomer), independently, at the beginning of the 20th century, placed, on a diagram, for many stars, on one side, on the horizontal axis, the spectral type of them, on the other side, on the vertical axis, their luminosity relative to the Sun (or absolute magnitude). They verified a correlation between the data. This correlation appeared under the form that the stars sorted into four groups: main sequence stars, supergiants, giants, white dwarfs. This diagram is the Hertzsprung-Russell diagram (see it below). These groups, in the same time, depict a static view of the stars (they sort stars according to such or such type) and the different stages of life of stars. Still today, astronomers are using the brightness and color of a star like the main factor to determine the age of those. The four groups of the Hertzsprung-Russell diagram, into which stars sort, allow stars to be described in a static way, as they are in the same time a statistical sorting of stars and a sorting of their physical features

As far statistical sorting of stars is concerned, main sequence contains 90 per cent of all stars. White dwarfs mean 10 per cent of stars. Red giants contain less than 0,5 per cent of stars. Blue and red supergiants are marginal. Half the stars in the Universe, generally are pairs, rather than single stars. The Milky Way's central bulge stopped making stars billions of years ago. It now is home to aging Sun-like stars and cooler red dwarfs. Giant blue stars that once lived there have long since exploded as supernovae. If there is a young star population in the bulge, it is very small or, even, in some proportions, composed of young-looking blue stragglers, those stars seemingly lagging behing in aging due a binary process. Scientists, synthesizing a snapshot of our Galaxy and all the stars it contains using models based on everything we know about how much raw material there is in our galaxy for building stars, what types of stars are made, how they evolve with time, and how long they live can eventually hold a global model of how the whole population of stars is working. Most recent database studies of stars in our Milky Way Galaxy has proved that the model is correct. A mission like NASA's Kepler further is hinting to asteroseismology a field few explored until now likely improving such a model with other data

As far as stars' physical features are concerned, the four groups of the H-R diagram are located in a reference frame which is defined by three groups of values. From the bottom to the top of the diagram (vertical axis), luminosity increases. For the left to the right of the diagram, temperatures decrease. From the left to the right of the diagram (horizontal axis), spectral classes are displayed in their order (O, B, A, F, G, K, M). These three groups of values, blended with other views on the stars' features (as above, other values), allow to describe the four groups of stars of the diagram. Stars differ in color according to their surface temperature: very hot stars are blue or white, while cooler stars are redder. They may be cooler because they are smaller, or because they are very old and have entered the red giant phase:

• main sequence is formed by the curve of stars which starts top left of the diagram and goes down to bottom right. Main sequence stars, from this fact, are stars which span over all spectral classes, from O to M (the curve goes from left to right of the diagram) and which so span too over the whole range of temperatures; from this fact also (the curve goes from the top to the bottom of the diagram), main sequence stars span over the whole range of luminosity (from the very bright stars of the class O to very weak stars of class M -this range meaning from some ten-thousandths to 100,000 times Sun's luminosity). As a whole, main sequence stars form a luminosity class, the class V. Mass of main sequence stars vary on a weak range, from 0.1 solar mass (class M stars) to 100 solar mass (class O stars). At last it has to be known that an important correlation exist between mass and luminosity of main sequence stars. Luminosity of a main sequence star is equal, in average, to the 3.8 power of its mass. This correlation allows to think that all main sequence stars have something in common as far as their internal structure is concerned. Moreover, this correlation does that, as main sequence stars have an important range of luminosity, these 3.8 power values do that the range of the masses of main sequence stars is weak (from some hundredths to some hundreds solar mass)
• red giants are luminous stars of spectral classes F, G, K and M. Their group is located above the main sequence. Giants belong to a same luminosity class, the class III. They however vary in luminosity. Generally speaking, they have a surface temperature varying from 3,000 to 7,000 K, a mass of 1 solar mass and a radius of 100 solar radii; their mean density is so 10^-6 g per cm³. A variety of stars called 'carbon stars' are similar to red giants stars, they glow brightly in longer wavelengths, and are in the late stages of their lives, with unusually high amounts of carbon in their outer atmospheres. Astronomers think this carbon comes either from convection currents deep within a star's core, or from a nearby companion star, from which it is drawned. Carbon stars are likely the generators to gamma-ray bursts. The low surface gravity of carbon stars makes that they typically may loose as much as half of their total mass by way of powerful stellar winds. The fact that their atmospheres are containing more carbon than oxygen is the opposite of the Sun as carbon monoxide forms in the outer layer of the star through a combination of these elements, until there is no more oxygen available. Carbon atoms are then free to form a variety of other carbon compounds, such as C2, CH, CN, C3 and SiC2, which scatter blue light within the star, allowing red light to pass through undisturbed
• supergiant stars sort into red supergiants if they are on the cold side of the H-R diagram (in spectral classes G, K and M), and blue supergiants if they are stars of the spectral classes O and B. Two types are stars of luminosity I and II classes . Blue supergiant stars are much larger, more massive, hotter, and possibly hundreds of thousands of times intrinsically brighter than our Sun
• white dwarfs are feeble stars, lying under the main sequence. Although few luminous, these stars have their external layers warmer than those of the Sun. They have a 1 solar mass mass, a radius of one-hundredth solar radius and a mean density of 106 g/cm³. They belong to spectral classes B, A and F. 15 percent of the stars near to our Sun are ultra-cool dwarf stars

Correlation mass-luminosity does not apply to stars others than main sequence stars. These are above or under the curve. This lack of correlation allows to think that internal structures of luminosity classes stars which do not belong to main sequence is in a substantial way different of those of main sequence stars. Blue stragglers for example are stars which seemingly lag behind in the aging process, appearing younger than the population from which they formed, being found in star clusters, nearby stars or even the core of the Milky Way Galaxy. It is not clear how blue stragglers form. A common theory is that they emerge from binary pairs. As the more massive star evolves and expands, the smaller star gain material from its companion. This stirs up hydrogen fuel and causes the growing star to undergo nuclear fusion at a faster rate. It burns hotter and bluer, like a massive young star. Blue stragglers may be considered rejuvenated stars. Delta Scuti stars are young, rapidly rotating stars which clearly pulsate. Such stars changes in brightness when internal sound waves at different frequencies cause parts of the star to expand and contract in even assymetric, hemispheric patterns. Delta Scuti stars spin so rapidly they flatten into ovals. Delta Scuti stars are between 1.5 and 2.5 times the Sun's mass. They're named after delta Scuti, a star visible to the human eye in the southern constellation Scutum that was first identified as variable in 1900. Stars with masses many times greater than the Sun generally, have lives of just a few million years as low-mass stars have expected lives of hundreds of billions of years. Our middle-sized Sun has a life expectancy of about ten billion years

->A New Stellar Count?
French astronomers since 1985 produced a theoretical model of our Milky Way Galaxy, which is termed the 'Besançon model', from the observatory of Besançon, France, which took part in that, and which is now used by most of the professional astronomers worldwide. The Milky Way Galaxy would now total 140 billion stars instead of between 200 and 400. That model further yields a statistical repartition of the stars, generally -should the new count for our Galaxy hold for other galaxies, like:
- 71 percent of stars are real thermonuclear-based ones
- 21 percent are brown dwarfs, those failed stars (the limit between gas giant planets like Jupiter and lowest mass stars like brown dwarfs is about 13 Jovian masses, a mark beyond which a celestial body is able to fuse deuterium, a heavy form of hydrogen. Brown dwarfs were theorized in the 1960s and confirmed in 1995 as albeit 70 percent larger than Jupiter, for example, they do not ignite like a star. It is still unknown weather they form like a star by contraction of gas, or like a gas gaint through accretion of material. At five Jupiter-masses star and planet formation overlaps). Brown dwarfs surface displays a face similar to that of Jupiter with their clouds organized in bands confined to different latitudes, traveling with different speeds in different bands instead of the atmospheric boiling seen at the Sun or many other stars. Brown dwarfs' clouds are hot, patchy and made of iron droplets and silicate dust as they can move and thicken or thin surprisingly rapidly, in less than a Earth day, due to giant waves causing large-scale movement of particles, changing the thickness of the silicate clouds. M-dwarf stars—hydrogen-burning stars that are smaller than 60 per cent of the size of the Sun—are the most common class of star in our Milky Way Galaxy and outnumber Sun-like stars by a ratio of 12:1
- 7 percent are white dwarfs, those remnnant of dead stars
- 0.7 percent are neutron stars (or black holes)
- supergiants are just a minute minority with 0.00001 percent. Astronomers have noticed a distinct and rapid drop-off of red giants, a type of older star, at the edge of our Milky Way Galaxy. The most massive star known in the Universe, generally, stands at more than 250 solar masses. The majority of massive stars are in binary pairs
As far as a comparison to our Sun is concerned, small stars with a feeble mass are largely the majority (of that 60 percent are red dwarfs, or there are about 10 red dwarfs for every star like our Sun -- Proxima Centauri is a red dwarf, for example), the stars with a mass between a half, and two solar masses are less than 15 percent, and the stars similar to our Sun at 1,7 percent only
That study is boding well with another one which has revised the 'stellar initial mass function' for smaller and darker galaxies. That equation was estimating that for every star 20 or more times as massive as the Sun there should be 500 stars with the sun's mass or less, as new estimations believed that the ratio should better be seen at 1 to 2,000! Many small and faint galaxies do not form a lot of massive stars but still have numerous lower-mass stars and were until now ill-studied
At last, most recent studies assess that all starry celestial objects in the whole Universe might equal to 300 sextillion, or thrice the amount estimated until now, due to red dwarfs are more common than thought, generally as elliptical galaxies accounting for one-third of the galaxies contain more of them than previously thought. Red dwarfs, about one-fifth the size of our Sun burn slowly and last much longer as they are known for stellar flareups. Despite such the complex physics of star formation, Nature cooks up stars with a consistent distribution from massive blue supergiant stars to small red dwarf stars, a study in 2015 showed. Brightest and most massive stars on a other hand, are 25 percent less abundant than predicted. Generally, that yields that mass estimates in the Universe are too low because they did not take in account that number of faint low-mass stars. Most of the Universe's stars formed about 10 billion years ago

->More Oblate Stars?
Most recent studies all go in the direction that many stars, with a speed of rotation about themselves which is very important -in the order of some 290 miles per second, for example- are displaying a strong dissymetry in diameter between their diameter at their poles and the one at the equator. Thus, such stars have an oblate shape. Vega, a Arae, or Achernar are such stars. Achernar thus has a equatorial diameter of 10.4 million of miles and of 6.7 at the poles! Those three stars all are belonging to the B or A, warm, blue and blue-white classes

Hertzsprung-Russell Diagram and Life of Stars

A gas cloud carved by the radiation of already nascent stars. picture courtesy NASA, ESA, and the Hubble Heritage Project (STScI/AURA)

The Hertzsprung-Russell diagram allows too to spot main stages of the life of stars, as these four groups picture too the four stages of this life. Life of stars further was detailed through astrophysics. Atoms transitioning into molecules are the very first step toward star formation. Interstellar space environment is not a complete void. It’s filled with clouds of dilute material remaining from stars that exploded as supernovae millions of years ago. In deep space, floating between the stars, lies an abundance of atoms -carbon, oxygen, hydrogen which may turn into a gas and dust cloud. Helium also participates into as a molecular cloud may reach hundreds of light years. Cosmic dust is mainly composed of silicon, carbon and aluminum, with grains as small as a millionth of a centimetre across as such elements mostly originate from supernovae explosions. The medium where a star forms may be, or not, rich in heavy elements. Diffuse and translucent molecular clouds are found in the interstellar medium at high galactic latitudes, which feature very low densities hence gravity, small sizes and are of a transient nature. They harbor little to no active star formation. Dense molecular clouds are the densest part of the interstellar medium, at a thousand atoms per cubic centimetre and even more in star-forming regions, which however is 10 times emptier than vacuum produced at Earth in a laboratory. Familiar molecules in molecular clouds are sulfur dioxide, nitric oxide, formaldehyde, methanol or ethanol. Different environment on a other hand, may yield diverse composition. Star-forming regions are typically not very large, stretching out for a few hundred light-years at most. Stellar winds, supernova explosions and ionization by ultraviolet photons control the lifetimes of molecular clouds as mechanical energy from the stellar wind is converted into kinetic energy causing the most disruption. Not only supernovae explosions in a forming star molecular cloud eventually are preventing further formation but stellar winds of usual appearing star also. Such winds sweep shells into the cloud. When a gas cloud collapses on itself -- a process called 'infall' -- the cloud’s own gravity causes it to contract and the contraction produces heat friction helping to create a star. In a molecular gas cloud, a multitude of dense pockets of hydrogen gas and dust form in singular, gravitationally-dense locations, and, as the area heats up under the weight of the accumulation, often becomes the seed of a star or a protostar. Magnetic fields and turbulence are involved in star formation among a cloud as both might even dominate, compared to the gravity force. Magnetic fields likely prevent dust cloud to collapse in a sense, preventing star formation -- which might hint to a lesser number of stars in the Universe. Molecular clouds literally crumple upon themselves. A cloud's collapse occurs swiftly. Magnetic fields, generally, affect the rate at which interstellar clouds condense to form new stars. Studies in 2016 have confirmed theoretical models about that. Such a collapse continues in protostars. When protostars have ignited, they begin to drift about randomly as, over time, some stars begin to fall toward a common center of gravity usually dominated by a especially large protostar. Cataclismic interactions like close encounters can occur during the process. The entire process of a star formation may be summarized like a giant cloud of interstellar gas and dust collapses under the forces of gravity. One scenario is that stars form through turbulence. In this 'hierarchical' model, a critical density of gas in a cloud causes the cloud to gravitationally collapse into a star. A different model, called 'competitive accretion,' suggests that stars begin as low-mass cores that fight over the mass of material left in the cloud. Inside the cloud, turbulent clumps of gas form and then collapse. The collapsed clumps form star clusters, and then the magnetized, swirling cores further evolve to form individual or small groups of stars. The stars formed inside a same cloud have the same age but they vary in mass because they formed at different positions within the cloud. Young stars have strong magnetic activity that heats their outer atmosphere and causes them to emit X-rays. X-ray activity in protostars with disks is, on average, a few times less intense that in ones without disks as this behavior is likely due to the interaction of the disk with the magnetic field of the host star. Much of the matter in the disks around these protostars will eventually be blown away by radiation from their host stars, and a part of it may form into planets. 'Dynamical evolution' makes that heavy stars in a cluster tend to progressively sink towards the central region of a star cluster, while low-mass stars can escape from the system. Function of how a cluster is composed, it dynamical evolution is specific. 'Hierarchical cluster assembly' consists into that several star clusters where star formation occurs, merged. It is still not clear clear whether massive stars form in a similar environment, or even in the same ways as smaller ones, as that constitutes one of the most important unsolved problems of modern astrophysics due to the influence of such massive stars in terms of spreading heavy elements in the Universe. Massive star formation is accompanied by the launching of powerful, magnetized winds that flow out from above and below a swirling disk of gas that is feeding the growing star. These winds blow cavities through the dense, dusty cloud through ionisation, whence the stars are forming. Ultraviolet radiation and violent stellar winds generally, are blowing out cavities in the gas and dust enveloping a cluster of forming stars. Stars heat up the pressurized gas surrounding them, causing it to expand into space and create bubbles as radiation may collide with large polycyclic aromatic hydrocarbons causing them to fluoresce. In some cases, bubbles may eventually burst. For a gas cloud to collapse, it has to be cold and sluggish so that it cannot resist gravity. It might that a filamentary web arises from turbulent motions of gas in the interstellar material and when gravity takes over, the densest filaments only, at the convergence of some, become unstable and fragment into compact objects –- the seeds of future stars. With dust, gas form filaments that stretch across the warmer parts. Many molecular clouds are building around filaments with dense threads snaking throughout the cloud. Such filaments potentially transport material, and, when massive enough, are known to form new stars. Filaments could feature the same characteristics in a certain radius due to a same origin in turbulence. Early on the cloud will have carbon with a missing electron, called ionized carbon. As the gas gets denser, the carbon atoms gain back their electrons, so you have neutral carbon. As you get even denser clouds, the carbon binds to oxygen creating carbon monoxide molecules, conditions that precede the collapse into a star. How long it takes before a cloud collapses to begin making a star might be anywhere between 1 to 100 million years. Dense regions of nebulosity are classified 'High-Excitation Blobs,' and likely tightly linked to the early stages of massive star formation. Molecular oxygen is so rare in the cosmos because oxygen atoms cling tightly to stardust, preventing them from joining together to form oxygen molecules. When the interstellar medium gathers together under the attraction of its own gravity, it forms a giant molecular cloud with dimensions up to hundreds of light-years across. Denser parts, just a few tenths of a light-year across, are known as molecular cloud cores. These are where stars and planets form. Inside these dense clouds, gravity pulls the gas and dust together until temperature and pressure are high enough for stars to be born. Successively embedded star forming clouds may eventually result into superbubble in a galaxy which are also due to stellar winds of supernovae. Stars are being born in huge gas and dust molecular clouds which also are termed 'HII regions' by astronomers, a environment rich in ionized hydrogen gas. As the ionised hydrogen nuclei recapture electrons they release light at different characteristic wavelengths often in the hydrogen alpha (Ha). HII regions may sometimes be the leftover from starburst regions where young stars ionized their surroundings before dying. A pinkish hue is characteristic of areas rich in hydrogen. Particularly dense molecular clouds qualify as dark nebulae because of this obscuring property. In HII regions the ionising photons come from the young hot stars within the region as such a original cloud, for example, may give birth to thousands of stars over a period of several million years. Such new stars are sculpting and dispersing the gases around them, and when the most massive of those turn supernovae, the cloud is eventually disappearing, leaving just a cluster of young stars behind. Magnetic fields play a important -and sometimes determinant- role into the evolution of such gas clouds. Intense ultraviolet light from hot young stars is causing hydrogen gas to glow. The bulk of material between the stars in the galaxy, or the cool hydrogen gas is nearly impossible to find as a study in 2013 revealed that the reservoir of raw material for making stars had been underestimated before -almost by one third- and extends farther out from our Galaxy's center than known before. Such gas is available to form new stars. In regions where the hydrogen gas is just beginning to pool into clouds, there is no carbon monoxide as it destroyed by ultraviolet light, a fact which occurs even in interstellar space. Ionized carbon however lingers there. How interactions between galaxies affect the formation, evolution, and behavior of the stars within is a subject of study. When a region inside a nebula gathers enough matter, it starts to collapse under its own gravity and the centre of the cloud grows ever denser and hotter until thermonuclear fusion begins. A web of filaments is extant, that stretches across those star-forming regions. Such brilliant hot young stars, once born, energize the hydrogen gas still present which yields a emission nebula. Solid particles in the nebula further are in turn producing some reflection nebula by scattering blue light (which is more easily scattered) from such newborns. Dark parts of the cloud are obscuring any background. The Orion Nebula was the first HII region on record, as observed in a telescope for the first time in 1610 par French astronomer Nicolas-Claude Fabri de Peires. The most recent theory in terms of how and where our own Sun had been born, is that that occurred inside a supernova remnant (SRN), the remains of a massive star a few million years older than our Sun, and named 'Coatlicue' (or 'mother of the Sun' in the Aztec language). That would be evidenced through isotope aluminum 26 found in some very primitive meteorids. Some hundred of stars were also created at the same time. As far as the formation of molecular clouds, which serve as stellar nurseries in galaxies, it is poorly understood albeit they are thought to be leftovers from the formation of galaxies, and composed mainly of hydrogen molecules. Some think that the large-scale galactic magnetic field is irrelevant at the scale of individual clouds, because the turbulence and rotation of a cloud may randomize the orientation of its magnetic field as others that such a field could be strong enough to impose their direction upon individual clouds and regulate cloud accumulation and fragmentation, hence the rate of star formation. In galaxy M33, the fields are aligned with the spiral arms, suggesting that the large-scale field in M33 anchors the clouds. Some gas clouds are called cometary globule CG, due to their comet-resembling shape. Cosmic dust, which is at the source of everything in the Universe like stars, planets and life, and made of various elements, such as carbon, oxygen, iron and other atoms heavier than hydrogen and helium, was manufactured in the early Universe through the enormous quantities released by first generations of stars going supernovae. Each such exploding stars supplied the equivalent of between 160,000 and 230,000 Earth masses of fresh dust. Process kept after that with any star turning supernova recycling again and again the dust. Cosmic dust is cold, at about minus 423 degrees Fahrenheit (about minus 217 Celsius). A molecular cloud is considered relatively small at around 2 light-years in width. Even giant clouds can further collide between them. Interstellar clouds may also contain networks of tangled gaseous filaments which could result from compression of material through sonic booms from nearby exploding stars and stretching for tens of light years in the clouds. Newborn stars are often found in the densest parts of such filaments. Regions of star formation are glowing pink regions of ionized hydrogen, hydrogen atoms that have lost their electrons. A sole, very luminous bright star can light the cloud as red in a dust cloud is probably made of dust that is more metallic and cooler than the surrounding regions. Various processes like gravity, electrostatics or magnetic fields are at play to get the materials to infall to the cloud's center, forming a "protostellar object", or protostar. As the collapsing model allows a cloud to trigger star formation by its own gravity, star formation is also possible from some other triggers. Like from the radiation emitted by one massive star located outside the molecular cloud or the 'radiation-driven implosion' (RDI) model, which drives a compression wave into the cloud while evaporating the cloud's outer layers. Or the formation of our solar system was thought to have been triggered by a nearby supernova explosion, as the 'collect-and-collapse' mechanism has shock fronts generated by massive stars sweep up material as they progress outwards. Supernovae explosions occurring inside a molecular cloud, generally, can trigger star formation from such shockwaves. Pillars like those illustrated in the famed Hubble telescope picture the 'Pillars of Creation' are structures formed when radiation and winds from massive stars in a central cluster blow gas and dust away, leaving only the densest of material, whence young stars appear. Massive stars in interstellar clouds can form nearly in isolation, with just a few lower mass stars rather than a full set of hundreds that are expected in typical star clusters. 'Champagne flow' in a nebula refers to that gas heated by young stars and expanding into the nebula eventually reaches the borders of the gas cloud and bursts outwards into the vacuum like the contents of an uncorked champagne bottle. New born stars regions may be filled by related, massive, bright blue and superhot O and B-type stars called OB, or stellar, associations. O and B type stars live fast and die young with a mass 40 times and surface temperatures eight times the ones of our Sun. Vast quantities of ultraviolet light and other radiation emitted by these stars is compressing the surrounding cloud, causing nearby regions of dust and gas to gravitationally collapse into more new stars. Such powerful radiation rapidly disperses their natal gas clouds or excavated cavities into there, which are called 'superbubbles.' Ring and bubble-like structures in cosmic dust clouds are also generated by strong winds emanating from such stars. Generally stars at a cluster's center, first create individual expanding bubbles of hot gas through their winds or supernova explosions, as such neighbouring bubbles eventually merged to form a superbubble hundreds of light-year across! More, smaller nebulae, on a other hand, usually are remaining and producing stars inside such cavities. As the stars swiflty explode as supernovae they expand the superbubble even further, to the point that it merged with other superbubbles, which forms a 'supershell,' possibly one of the largest possible structures within a galaxy. A O-type star is so luminous that the pressure of its starlight actually drives material from its surface, creating particle outflows with speeds of several million miles an hour. In a binary of such giants, which are extant, winds can collide during all or part of the orbit. Massive stars dramatically shape their environment when they explode as supernovae, but their powerful winds dominate the space around them for millions of years, altering star-formation regions throughout their energy-producing lives. Massive stars generally are relatively rare, but play a very important role in recycling materials in the Universe. They burn their nuclear fuel much more rapidly than stars like the Sun, living only for millions of years before exploding as a supernova and returning most of their matter to space. The metal content of a massive star controls the strength of its stellar wind, which determines how much of the hydrogen atmosphere it retains as it grows older. Metal content generally is also dispersed by the stellar wind of a star. As these winds push material off the star's surface, the star's rotation gradually decreases. Even during their brief lives, massive stars lose a significant fraction of their mass through fierce winds of gas driven off their surfaces by the intense light emitted from the star. Such winds might trigger the collapse of surrounding clouds of gas and dust to form new stars or, conversely, blast the clouds away before they have the chance to get started. Spiral-arm-like features superimposed on a highly fragmented wind co-rotating with the star are likely a model to such stellar winds, which, on a other hand, are just the equivalent of our solar wind. Stellar wind are not simply a uniform breeze and composed of hundreds of thousands of individual hot and cool, mostly small pieces. Supernovae shock waves also can carve out such huge cavities as hot material may also evaporate from the cavity walls. Cosmic rays are also seen in such cavities as they may have been yielded by supernovae or accelerated through repeated interaction with shockwaves produced inside the cocoon by powerful stellar winds. Such cavities holds onto its cosmic rays despite their high energies by entangling them in turbulent magnetic fields created by the combined outflows of high-mass stars. Largest star formation complex may span up to 200 light-years as the radiation blasting away from stars is pushing away the very material they are feeding from. As far as massive stars are concerned, a continuous process however by which the raw material is moved around, compressed and confined, under the influence of clusters of young, massive protostars, which keeps the density of the cloud and populations of young high-mass stars may well be able to build and maintain localised clumps of material from which they can continue to feed. Massive stars, generally, form following the same process than more usual ones. Massive stars, although rare, are important because they likely foster the formation of smaller stars and because they dissipate elements by the end of their life like supernovae. Gravity from a growing 'seed' at the center of the cloud attracts more and more matter as this process is aided by heavier elements such as carbon, which help to keep the gas falling onto the budding star cool enough to collapse. Stars' seeds are be found in classes of others of similar ages, as this is especially true when the birth of stars in a cloud of gas and dust is triggered by a external event, like the explosion of a nearby supernova which compressed the surrounding material. If the cloud gets too hot, the gas expands and escapes. Such protostellar objects are distributed into classes, with the zero class the youngest. As stars with a size approximately the one of our Sun are all thought to form from a interstellar cloud, astronomers think that stars with a mass above 10 times the one of the Sun might result from the accretion of smaller stars as the massive size of the protostar is rapidly clearing the stellar disk around. Some stellar disks however observed around massive nascent stars might also let think that the formation process is the same in both cases

Sh 2-29 or Sharpless 29, at about 5,500 light-years away in constellation Sagittarius, The Archer, is a typical view of HII star-forming region with young stars, cavities, or nebulae. picture site 'Amateur Astronomy' based upon a image courtesy ESO

The extreme conditions that reign in space have average temperatures can be as low as 100 Kelvin, densities are tens or hundreds of billionths of Earth's and interstellar molecules and ions are bathed in stellar ultraviolet and visible radiation. Nebulas are the most famed places of where on can observe the varied stages of star formation as dark clouds are seen in the infrared to be too places of star formation with stars, generally, when forming from a cloud, are beginning to be small ones and then of the massive O type. Young stars generally are still embedded deeply in a gaseous envelope, a phase which lasts perhaps 25,000 years. 'Yellow balls' are a missing link between the very young embryonic stars buried in dark filaments and newborn stars blowing the bubbles, the rims of which are made largely of organic molecules called polycyclic aromatic hydrocarbons (PAHs). The yellow color of their antecessors results from that PAHs haven't been cleared away yet. Such antecessors are also smaller because the nascent star has not produce its effects still. Many yellow balls appear to be lining the rims of the bubbles, a clue that perhaps the massive stars are triggering the birth of new stars as they blow the bubbles. Early formation steps of a star, at a few hundred thousand years old, may -rarely- bring to a transitory phase during which a close binary system appears. Lots of gas and dust are being rapidly accreted from the originating interstellar cloud and disk to form a binary system. Orbits of such protostars make that dust and gas are dragged from the inner edge of the formation disk and may trigger regular strobe-like flashes, a phenomenon, called 'pulsed accretion,' which also exists in later stages of star birth. That binary protostar also usually carves unsymetrical outflow cavities in the forming disk, or protostellar envelope. As a cluster whence binaries formed ages, binary stars often separate and are ejected from the cluster. O stars, the most massive type of star known to exist, are the most able to carve cavities out of the bubble they had been born from, through their ultraviolet radiation. O stars burn their nuclear fuel in mere tens of millions of years and thus are very rare, at only one in every three million stars in our cosmic neighbourhood. Almost three quarters of those very bright high-mass, O-type stars have a close companion star, featuring disruptive interactions, and one third are even expected to ultimately merge to form a single star. At about 10,000 to 100,000 years into the core forming process (such protostellar objects are categorized into classes; class 0 is the youngest class A sudden accumulation of gas and dust may occur at a Class 0 protostar. The Class 0 phase is short-lived, lasting roughly 150,000 years as the star then shines from the heat energy released by its contraction and by the accumulation of material from the disk of gas and dust surrounding it only), the cloud temperature is about 400 degrees below zero Fahrenheit (minus 240 Celsius). As the star keeps forming, its emission will increase in energy along the way, graduating from mainly cold sub-millimeter radiation to near-infrared through far-infrared light and finally to visible light. This earliest period of star growth lasts a mere thousands of years, a astonishingly short amount of time in astronomical term. Tiny crystals of a green mineral called olivine have been found falling down like rain on a burgeoning star, from the surrounding gas cloud presiding over a star's formation. Crystals most likely are due to jets of gas blasting away from the embryonic star as they need high temperatures to form. That also might explain why such minerals are found with comets which lie at the outskirts of a planetary system. The crystals thus might be cooked up near the surface of the forming star, then carried up into the surrounding cloud where temperatures are much colder, and ultimately fell down again. As one thought until recently that the material expelled by a forming star along magnetic field were slow moving, new studies are showing that the mechanisms at work then are similar to those forming black holes, or supermassive galactic black holes from a accreting disk. Material from the pre-protoplanetary disk is funneled to the star and forming structures expelled along the magnetic lines there into a spiral, horizontal shape as particles are moving by the speed of light. At a protostar with a rapid rotation intense magnetic fields may drive torrents of gas into the stellar surface, where they heat large areas to millions of degrees emitting X-rays. It's not until after a few million years that nuclear fusion ignites at the center of the cloud. As material falls onto the growing star, it develops a surrounding disk of swirling material and twin jets. Those jets are the leftover gas and dust that the star took from its parent cloud in order to form. As an interstellar gas cloud contracts, it spins more rapidly, just as a twirling ice skater does when she draws in her arms. The only way for the gas to continue moving inward is for some of the spin to be removed. Jets are that process but it's still not fully understood, as magnetic fields funnel some of the swirling material into twin jets. At some point in the formation process the star began to eject some of the material at supersonic speeds through space. By ejecting supersonic knots-composed jets of gas, the cloud slows down its spinning. Lumpy filaments of gas funneled from the outer to the central regions of the star, temporarily warming the object as the clumps hit its inner disk or that such material occasionally piles up at the inner edge of the disk and casts a shadow on the outer disk have a nascent star twinkle on duration of just a few weeks. Once the star ignites and shines with starlight, the jets will die off and the disk will thin out as ultimately, planets may clump together out of material left. Jets have been seen originating at one star in a area within a sphere with a radius of 3 AU which seems a average value, as one jet, for example, may be ejected a few years after a first one appeared. Such processes likely affect the formation of planets. The jets shoot off at supersonic speeds of about 100 miles per second in opposite directions through space. The jets usually are not ejected in a steady stream as they are launched sporadically in clumps instead and hinting to at what interval material still episodically fall onto the star in the last periods of its formation. Jets are an active, short-lived phase of star formation, lasting only about 100,000 years as they motion may be seen across human timescales of years. During such that epoch, the very young star is still surrounded by dusty material from its formation. Astronomers still do not know precisely what role jets play in the star-formation process or exactly how the star unleashes them as jets appear to work in concert with magnetic fields however, helping bleed excess angular momentum from infalling material that is swirling rapidly. Once the material slows down it feeds the growing protostar, allowing it to fully condense into a mature star. Jets, generally, from a newly formed star may spark a new generation of stars in the surrounding gas clouds. Jets may extend out to a distance of 118 billion miles and the jet material is moving at speeds between 200,000 and 300,000 mph as shock waves exist at the farthest ends of the jets, the main trigger to formation of new stars. The most abundant molecule in star-forming clouds, hydrogen, can be broken apart by cosmic rays, energetic particles that permeate the entire Galaxy. The hydrogen ions then combine with other elements that are present – albeit only in trace amounts – in these clouds: carbon and oxygen, or nitrogen. Normally, the nitrogen compound is also quickly destroyed, yielding more hydrogen for the carbon and oxygen compound. As a result, the latter is far more abundant in all known stellar nurseries. Mighty stellar winds of young stars might also yield the beryllium found in meteorites. Powerful, turbulent magnetic fields may also be found at stars of about 100-300 million years old when some Jupiter-like close planet accelerates the star's rotation, which keeps the fields active. Jets usually are termed Herbig-Haro (HH) objects, in honor of George Herbig and Guillermo Haro, who studied the outflows in the 1950s. Inside jets structures and bow shocks occur as material do not move at identical speeds. It is those shock fronts also forming tangled, knotted clumps of nebulosity which specifically are known like Herbig-Haro (HH) objects. Jets usually ricochet off the dense core as one suspects that such outflows of material are fueled by gas accreting onto the young star. The surrounding disk is the 'fuel tank,' the star the gravitational engine, and the jets the exhaust of the process respectively. Jets even may be diverted by a neighbouring clouds of gas, for example. End stages of the star-formation process can be violent with the young object spewing gas and shaking up and heating its stellar nursery, reaching temperatures of 10 000 degrees Celsius. When the collapsing cloud turned dense enough, that allows to atoms to fuse and ignite, like just seen. As the collapse also brings heat, generally, that heat is counteracting the process. Heavy metals in the cloud in turn are counteracting the heat. Young stars much less massive than the Sun can unleash a torrent of X-ray radiation that can significantly shorten the lifetime of planet-forming disks surrounding these stars. Stars then appear on the main sequence, at the place matching the size, the temperature and the spectral class of them at their birth. Once fully born, the theoretical limit beyond which a star lies unbound from any other is between 0.16 and 1.6 light years. The limit, in terms of mass, a star may reach is in the order of up to 300 solar masses. As stars mostly are forming in cluster, the influence of already nascent stars ultraviolet radiation and solar winds upon the rest of the dust cloud from which it had been born may be compared to the one of the Sun upon a frozen treat on a hot summer day. The radiation of the stars is carving away into cold molecular clouds, furthering the star formation process. A ring of dust and gas orbiting a young star can act like a belt and cinch the expanding nebula into an “hourglass” shape. Fomalhaut, a young star just a few hundred million years old, and twice as massive as the Sun has been observed surrounded with a dust belt in the 1980's, likely the equivalent of the Kuiper Belt, or Oort Cloud for our Sun. Grains in the dust belt are fluffy and tiny, only a few millionths of a meter across (one meter is about 3 feet) as they are similar to dust particles released from comets in our own solar system. The dust is being regenerated in the belt through continuous collisions between comets as each day, the equivalent of either two comets 6.2 miles in size (10 kilometers) or 2,000 comets .62 miles in size (1 kilometer) are completely crushed into small particles. What's more, there are a ton of comets with a estimate between 260 billion and 83 trillion in the belt. Star forming nebulae, generally, display a great array of colors and shapes, which is due to the varied regions, temperatures and components constituting the cloud. The main part of the life of a star is going to take place on the main sequence, at this place. Stars may form inside low, or high-mass stellar region. Whether our Sun, for example, formed in such or such case remains a mystery. Gas and dust clouds are at the origins of stars, containing hydrogen and varied compounds. Soccer-ball-shaped molecules, named buckyballs, are the largest molecules known to exist there as those are found too throughout our Milky Way galaxy -- in the space between stars and around three dying stars. They are named for their resemblance to the architect Buckminster Fuller's geodesic domes, an example of which is found at the entrance to Disney's Epcot theme park in Orlando. Buckyballs are made of 60 carbon atoms arranged in three-dimensional, spherical structures, alternating patterns of hexagons and pentagons as some more elongated cousin, known as C70, consist of 70 carbon atoms and are shaped more like an oval rugby ball. Both types of molecules belong to a class known officially as buckminsterfullerenes, or fullerenes. Such molecules in a planetary nebula are perhaps reflecting a short stage in a star's life, when it shared a puff of material rich in carbon. Their existence had been predicted in 1970 by Japanese professor Eiji Osawa, they were observed in lab experiments in 1985 when researchers simulated conditions in the atmospheres of aging, carbon-rich giant stars. The study of fullerenes and their relatives has grown into a busy field of research because of the molecules' unique strength and exceptional chemical and physical properties. Among the potential applications are armor, drug delivery and superconducting technologies. Fullerenes mays coexist with hydrogen, which was no thought like a possibility and fullerenes can act like cages for other molecules and atoms, might have carried substances to Earth that kick-started life. By 2012 buckyballs have also been found in a solid form in space albeit minuscule, far smaller than the width of a hair. Buckyballs are found in quantities the equivalent in mass to 15 Earth moons, in the Small Magellanic Cloud and likely are even more widespread in space than though. Buckyballs have been found on Earth in various forms. They form as a gas from burning candles and exist as solids in certain types of rock, such as the mineral shungite found in Russia, and fulgurite, a glassy rock from Colorado that forms when lightning strikes the ground. Due to their low mass, red dwarfs take longer to fully collapse and turn a star, in the order of many hundreds of millions of years as, in their early years, they can reach a important brightness. Stars have been found able to form even within extreme environment like the jets blasted out from supermassive galactic black holes at the cores of galaxies, a theory that had been agreed upon by astronomers since a while. Such stars might be hotter and brighter than stars formed in less extreme environments like in the disc as they move away from the galaxy along with the jets! Those closer to the jets' source might slow and head backwards. The other waywards stars might reach to the intergalactic medium, enriching it or participating into the cosmic infrared background radiation. During 90 per cent of their life, stars, at their place on the main sequence, are going to shine by burning hydrogen into helium. Iron plays the dominant role in controlling radiation flow through stars as heavy metals such as iron absorb different types of radiation. Young, low-mass stars are brighter than most other stars in X-rays. Forming stars, generally, are reddish and yellow in colours. Some X-rays in red giants are mostly due to the star rapidly rotating. High-mass stars are signaling their growth with flares of their masers, source of intense energy in the microwave. Length, in years, of this main leg of a star's life depends upon the spectral class to which it belongs and upon its mass: this may vary from 30 million years for a class O7 star to theoretically 200 billion years for a M0 class star; the more a star has an important mass, the less the hydrogen stage lasts; the weaker the mass, the longer the hydrogen leg. Hydrogen burning, at last, tends to move the star away from the very axis of the main sequence, and slightly increases its radius. Stars, generally, like our own Sun, possess a 'astrosphere,' that magnetic protective bubble which is a confrontation between their solar wind and the surrounding interstellar medium which (for our solar system that is called the 'heliosphere')

Most stars are very magnetically active when they are young, since the stars are rapidly rotating, exhibiting stronger magnetic fields, larger flares, and more intense X-ray emission. The energy loss then brings a slower spin as the magnetic activity level with the associated X-ray emission drops. Sun-like and less massive stars calm down surprisingly quickly after a turbulent youth, having implications for the long-term habitability of planets orbiting such stars. The material around mature stars is expanding and is in the process of escaping to the interstellar medium. Magnetic activity, flaring, and X-ray emission are linked to the star’s rotation, which generally declines with age. Stars all over the Universe have been observing slowing their rotation faster than expected. As soon as by 1967 U.S. Edmund J. Weber and Leverett Davis, Jr. had published a paper suggesting that stellar atmospheres spin along with their surfaces, and the tips of their atmospheres blew off into the stellar wind, the star loosing angular momentum. The strength of the magnetic field depends on the amount of convection in the star. Lithium, on a other hand, is usually abundant in younger stars, but over time convection carries lithium to the hot inner regions of a star, where it is destroyed by nuclear reactions. FU Orionis (or FU Ori) stars are pre-main sequence stars with changes of 100 times brighter in magnitude, and changes in spectral type. Flares are due to abrupt in-falling matter from the protostar disc on a young, low mass T Tauri star. Only 25 stars in this class exist. A T Tauri is a young star, or 'Young Stellar Object,' -- at less than 10 million years old as such stars also vary in brightness -- that is starting to contract to become a main sequence star similar to the Sun. A T Tauri star is the youngest visible stage for relatively small stars. They have not yet started to fuse hydrogen into helium in their cores, like normal main sequence stars, but are just generating heat from contraction. As these stars mature and reach adulthood they lose mass and shrink and maintain at that shape for billions of years as main sequence stars

A View of Three Astrospheres, Those Protective Bubbles Around a Star. site 'Amateur Astronomy'

On a other hand, the more the studies go, the more supposed behmoth stars of the origins of the Universe dwindle in size, with latest estimates at about tens of solar masses only, compared to earlier one thousand or hundreds. Such high numbers were due to that the early Universe consisted into hydrogen and helium as no heavy elements nor dust already existed yet, as those are allowing to cool the growing material and that astronomers supposed that early stars had to aggregate a huge mass of gas to provide for the mechanism. First stars were definitely massive, but not to the extreme we thought before, as the growth of these stars is stunted earlier than expected as matter in the vicinity of forming stars heats up to as high as 90,000 degree Fahrenheit. Gas this hot expands and escapes the gravity of the developing star into two large cones each side of the poles, instead of falling back down onto it and stopping star's growth growing earlier than predicted

->About How Long Open Clusters do Last
'Open clusters' seem a frequent consequence of star formation. Open clusters are typically found lying in the arms of spiral galaxies or in the denser regions of irregular galaxies, where star formation is still common. As the stars formed from the same initial cloud of gas and dust they are therefore very similar to one another with roughly the same age, chemical composition. However, each star in the cluster has a different mass, with the more massive stars evolving much faster than their lower mass counterparts as they use up all of their hydrogen much sooner. Individuals are very susceptible to being ejected from the main group due to the effect of gravity from neighbouring celestial objects. When a gas and dust, star forming cloud collapse, it usually gives birth not to one star only, but to several ones at the same time. Those stars, then, are seen in the sky under the form of an 'open cluster', a relatively loose cluster of stars -the best examples of which being the famed Pleiades, or Hyades, for example. Stars are hold into a cluster by their mutual gravitational attraction and also by the gas between them as those forces however are not enough to hold a cluster together against close encounters with other clusters and clouds of gas and the cluster’s own gas and dust dissipate too. Such clusters of stars are now known to 'dissipate' along about 25 million years, as their stars, on the one hand, may be short-lived, like the massive, type B, blue stars which have a live duration of some tens of millions years only, or, on the other hand, as type O, more massive stars are shorter-lived still, with them exploding supernovae after some million years only. Some clusters may also last of the order of several hundred million years. Those explosions further are pushing away the gas and dust possibly remaining in the cluster from the star births, leading to a further loss of mass in the cluster. The ultimate conclusion of such an evolution is that the cluster, eventually, do not exist anymore, in about 250 million years. That will be, for example, the case for the Pleiades. Because open clusters are only loosely bound by gravity to begin with, and because they constantly lose mass as some of their gas is pushed away by the radiation of the young hot stars, these disturbances occur often enough to cause the stars to wander off from their siblings, just as the Sun is believed to have done many years ago. Also, on their travels about the galactic center, open clusters in a galaxy are affected by the gravity of other clusters, as well as by large clouds of gas that they pass close to. During star formation, smaller star clusters may merge together to form larger ones. A single giant gas clouds out of which stars form may fragment into smaller pieces and create several cluster of stars. Runaway stars on a other hand, are fast-moving stars that have been kicked out of their stellar nurseries where they first formed as a result of dynamical interactions occurring due to a process called core collapse, in which more-massive stars sink to the center of a star cluster by dynamical interactions with lower-mass stars. When many massive stars have reached the core, the core becomes unstable and these massive stars start ejecting each other from the cluster. The smaller the cluster, the swiftier the core collapse. Runaway stars may also result from the supernova explosion of their original star or some other process. AE Aurigae, for example, formed as a binary-star system with the star Mu Columbae. Approximately 2.5 million years ago, these two stars are thought to have collided with another binary-star system in the Trapezium Cluster. This collision sent both AE Aurigae and Mu Columbae hurtling through space in opposite directions at a speed of 100 kilometers per second (over 200,000 miles per hour). Today, AE can be seen in the constellation Auriga hundreds of light-years to the North of its home, while its former companion Mu Columbae is located hundreds of light-years to the South in the constellation Columba. Stars born in a cluster, generally, eventually disperse throughout the Milky Way Galaxy. What often happens too when a multiple system in a young birth cluster falls apart is that two of the member stars move close enough to each other that they merge or form a very tight binary. In either case, the event releases enough gravitational energy to propel all of other stars in the system outward. The energetic episode also produces a massive outflow of material

A View of How a Star Life is Unfolding. site 'Amateur Astronomy'

The Eddington luminosity, or Eddington limit, is the maximum luminosity a star due to a balance between radiating working outwards and the gravitational force inwards. Mostly, stars pass from a youth when they are powered through hydrogen fusion-burning cores to a old age with helium fusion-burning cores. Intermediate stages have hydrogen fusion-burning shells expand into red giant sizes. In terms of professional observation, hydrogen shell fusion star and a helium core fusion star are indistinguishable when looking only at their surface properties as they are radically different in the inside. Red giants are stars which have almost depleted their reserves of hydrogen, which causes the atmosphere of the star to expand. Red giants, for example, may found their energy from a shell of helium outside a carbon-oxygen core, sometimes accompanied by a hydrogen shell closer to the star’s surface. Stellar seismology, or 'asteroseismology' allows to determine the age of a star as changes in brightness at a star's surface is a result of turbulent motions inside that cause continuous star-quakes, creating sound waves that travel down through the interior and back to the surface. When a helium core is present, for example, these waves interact with other waves trapped inside. By carefully measuring very subtle features of the oscillations in a star's brightness, astronomers can see that some stars have run out of hydrogen in the center and are now burning helium, and are therefore at a later stage of life. Age of stars is not always obvious from the surface as, during certain phases in a star's life, its size and brightness are remarkably constant, even while profound transformations are taking place deep inside. When the star's hydrogen is exhausted, it's made up only of helium. As burning helium requires much more pressure and temperature, the star then becomes instable, as, eventually, the central region of the star contracts until the remaining hydrogen around the helium core ignites hydrogen fusion processes. Such processes bring the star's outer region to expand. The star becomes a red giant (some stars become supergiants, as some even hypergiants -the latter with much more irregular an activity. Red giants are also called asymptotic giant branch (AGB) stars as a companion star may orbit inside the material shed and leading to a spiral strucStars of the asymptotic giant branch (AGB), that grouping of stars on the Hertzsprung-Russell diagram that is roughly asymptotic to the giant branch are carbon stars, a later stage in giant-star evolution with hydrogen and helium fusing in a shell surrounding a core where both don't anymore. With their variety of chemical compounds based on carbon and other elements in the shell, they help to recycle matter, and contribute up to 70% of the dust between stars. The star leaves the main sequence and belongs now to the group of giants. According to its mass, the star will continue the process of transforming elements, as it began to do by transforming hydrogen into helium and then burning helium: stars which mass is very much more important than one of the Sun, will be burning hydrogen, then helium, then carbon, then oxygen, neon and silicon; stars which mass is more important than one of the Sun will be burning elements down to carbon and will possibly continue down to silicon; stars of a mass equivalent to the Sun will stop at helium. Stars which mass is weaker than the mass of the Sun are burning hydrogen and possibly helium. Stars which mass is very much weaker than the one of the Sun do not become red giants (see below). A star fusing hydrogen and helium in its core into heavier elements is made through a process known as 'nucleosynthesis.' The energy made by the fusion of heavier and heavier elements balances the star against the force of gravity as such reactions continue until they formed iron and any further nucleosynthesis would consume rather than produce energy. Down to iron, stars are forming elements through nuclear reactions liberating energy as for the following ones (gold, for example), stars need energy to be added. Gold further generally originates from a merger of two neutrons stars during a 's process' occurring during the last steps of AGB stars of a mass under 10 solar masses, which can produce elements down to the polonium (a 'r process' also produces elements heavier than iron in supernovae or a merger of two neutron stars). A star burning helium as a red giant spends 10 to 25 per cent of its lifetime to do it. Such a star, which is too the fate of our own Sun, will bloat and engulf the orbit of most its planets. The innermost ones will simply be destroyed as the outermost, like Jupiter or Saturn in our own solar system, will have their orbit decaying towards the center due to frictions with the gas in the envelope and surviving into those orbits closer to the star. A dying red giant is emitting lots of gas which cools the environment, in a process similar to that of refrigerators. The process by which a star keeps on its cycle of compound burning is the following: in its red giant phase, the star cools, the core contracts again. It brings a heat of millions of degrees C, enabling helium fusion in turn. This is called "core helium burning". This may trigger an enormous amount of X-ray activity in the star's corona. The star ceases to be a read giant, it shrinks as the surface temperature increases. A dusty disk and polar jets might be featured too by stars which have come to their red giant phase and hint to catastrophic interaction with a nearby star or some giant planet. Such a engulfment further is accelerating the red giant rotation, likely triggering -through a interaction with the disk- a increased magnetic field, flares at the surface. Red giant surface gravity, due to the swell is weak. Elderly stars atmosphere generally contains only a small amount of lithium. Aging 'helium-burning' stars, which burn helium after their hydrogen fuel has been exhausted generally, are blue in color. Such a disk resulting from the expansion of a red giant may be also triggering a second and late phase of planet formation around

->A white dwarf is a dense object, as a teaspoon of such a star would weigh about 10 tons on Earth! Because white dwarfs are much denser than they were in their youth, they have stronger gravity and can produce higher-energy X-rays than normal. As a white dwarf is cooling over billions of years, reaching 107 grams per cubic centimeter in the core as carbon and oxygen ions crystallize. A white dwarf’s radius is determined by its mass, according to a theory first proposed in 1935 by Indian American astronomer Subrahmanyan Chandrasekhar. White dwarfs have their brilliant hot cores exposed, which are luminous largely in ultraviolet light. White dwarfs are expected to have chemically pure surfaces, covered only with light elements of helium and hydrogen as some white dwarf atmospheres are polluted with traces of heavier elements such as calcium, silicon, magnesium and iron hinting to a asteroid or a small planet torn apart by the star, like a remnant of a solar system existing before the star went red giant. During the process they formed from, white dwarfs stored up heat in their cores and re-radiate it slowly into space. White dwarfs get cooler and less luminous as time goes on because they have no nuclear sources of energy. The shape of the planetary nebula which is left behind has a large variety of shapes -albeit with a symmetry- which might be due to that the star had a companion, or some exoplanets and/or brown dwarfs or even a large amount of dust. Irregular shapes may also result from some occurrence disturbing symmetry. In the case of a companion -which is the case for half of the planetary nebula- the first envelopes of gaz and dust expelled by the dying star is settling like a disk about the two stars and, when the red giant collapses to a white dwarf, the envelopes then are shaped through that existing disk, expanding either side. The Boomerang Nebula is the coldest known object in the cosmos, which could have occurred because a small companion star plunged into the heart of a red giant, ejecting most of the matter of it as an ultra-cold outflow of gas and dust. In the case of some exoplanets and brown dwarfs, those bodies, in the first stage of the expellation of the envelopes of the dying star, are revolving there, thus creating shockwaves which, spiraling, are just sculpting the nebula into a variety of shapes! Some planetary nebulae shoot material in waves from their polar regions. The planetary nebulae are lightened through the ultraviolet light which is emitted by the white dwarf. A white dwarf is about the size of the Earth. Such stars may endure then a burst of activity as hydrogen from the outer envelope is brought down into the helium shell (which surrounds the carbon, oxygen, and other heavy elements core) due to heat-spurred convection. This starts a flash of fusion activity. The latter would occur as swiftly as just a few years. Such a renewed activiy has the white dwarf reheat and ionize gases in its surroundings and ejecting a large amount of carbon from the core, as gas and as dust, providing some more material to star forming regions. The Chandrasekhar limit is the greatest mass that a white dwarf star can have and support itself against gravitational collapse. It has a value of about 1.4 times the mass of the Sun. As many as 25 to 50 percent of white dwarfs are known to be polluted with infalling debris from rocky, asteroid or comet-like objects, a proof that such exo-elements of a planetary system survive a red giant phase. Planets likely also survive too. A white dwarf eventually through continual cooling, turns a 'black dwarf.' A white dwarf can also be cannibalized or evaporated by a companion star, causing the white dwarf to lose so much mass that it becomes a planetary mass object, or a helium planet or diamond planet

Several planetary nebulae seen both in the X-rays and visible light. site 'Amateur Astronomy' based upon pictures X-ray: NASA/CXC/RIT/J.Kastner et al.; Optical: NASA/STScI

At last, the star arrives at the end of its life. This end depends upon the star's mass. Intense radio sources called masers are associated with old stars. Masers occur when the molecules in certain kinds of gases get revved up and emit a lot of radiation over a very limited range of frequencies resulting in a powerful radio beacon -- the microwave equivalent of a laser. Brown dwarfs, as far as their life is concerned, simply cool off and fade away as red dwarfs keep burning until they have transformed all their hydrogen into helium and turn into a white dwarf. Stars with an initial mass of less than 8 solar masses -and stars with a weak mass generally- end their lives as they stop burning helium and eject a hydrogen envelope, a planetary nebula -which is going to last 10,000 to 50,000 years, gradually fading. As a shell of surrounding gas is formed, at the same time, the star's core shrinks and grows hotter, emitting ultraviolet light that causes the expelled gases to glow. Once enough material is ejected, the star’s luminous core is exposed, enabling its ultraviolet radiation to excite the surrounding gas to varying degrees and causing it to radiate in an attractive array of colors. After the central star cools and shrinks, turning a white dwarf, the star’s light drastically diminishes and ceases to excite the surrounding gas andthe nebula fades from view. After billions of years converting hydrogen to helium like a fuel, the star begins to run out of fuel and then ballooning in size, becoming a red giant. During burning hydrogen into helium, the nuclear outward thermal pressure is balanced by the inward pressure of gravitation. As helium is heavier than the hydrogen it tends to sink to the center of the star as the red giant phase is then generated around that inert helium by the continuing transformation of hydrogen. The color red comes from that the external layers are cooling down while expanding. During his red giant phase, the star is shedding its outer gaseous layers into space and begins to collapse as fusion reactions begin to die out. A gusher of ultraviolet light from the dying star energizes the gas, making it glow. The outer layers of the star are ejected, and get excited and ionized by the energetic ultraviolet light emitted by the bright hot core of the star, forming the nebula. Ejections of gas can be blown from the star at different speeds, which also accounts for the planetary nebula's shape. The duration of when a red star expels its layers and when it turns a planetary nebula is in the order of some thousand years. The expansion phase had the star take the form of a bloated 'red giant.' A intermediary step before the red giant phase is the subgiant oneas planets, asteroids and comet belts are expected to survive. Some scenario may bring some red giants to eject a large quantity of their outer layers and expose their blue-hot cores, shining in the ultraviolet. Most likely scenario is that the stars are rich in heavy elements, which makes radiation from the star more efficient at pushing on gas laced with those. Another possible explanation also is that the blue stars are in close binary systems and have lost mass to their partners. Such stars are found close to the core of the Andromeda Galaxy. Dust eruptions in a red giant probably occur only once every 10,000 to 50,000 years and they are thought to last less than a few hundred years each time. Such a process is known as a thermal pulse and causes the expelled layers to mix, being a determining factor into yielding heavier elements. A companion stars may sometimes cause the layers to organize into a spiral shape. This is one of the main ways dust is recycled in our Universe also providing for heavy elements star formation. Shed layers of gas cools and congeals into tiny dust particles. The ejection of mass from the star is uneven in both time and direction, which can result in fascinating structures within planetary nebulae. In case of a binary, the gas giant eventually surrounds both stars in a huge gaseous envelope and, when this cloud disperses the two -one turned a white dwarf- move closer together and form a very tight pair. Such pairs are known as post-common-envelope binaries. One brief but dramatic stage that stars pass through as they run out of nuclear fuel is called the preplanetary or protoplanetary nebula stage. A preplanetary nebula is when a dying star hot remains briefly illuminates material it has expelled before turning a planetary nebula. Before that, the star typically is ejecting material about every hundred years. Over a few thousand years, the hot remains of the aging star in the center of the nebula heat it up, excite the gas, and make it glow as a subsequent planetary nebula as stellar winds are shaping the latter. Planetary nebulae vivid colors are produced by the mix of gases present in them like for neon signs. Jets can form from the star as such a signature might hint to a binary system. Periodic burst of material form a onion-like layered structure surrounding a central cocoon, typically occurring every few hundred years. Planetary nebulae are one of the main ways in which elements heavier than hydrogen and helium are dispersed into space contributing for further star formation. A planetary nebula then typically lasts a few tens of thousands of years and, over a further few thousand years the gas slowly disperses into its surroundings. The presence of green glow from doubly ionised oxygen is used as a tool for spotting planetary nebulae. The hot, remnant stellar core, as far as it is concerned, is then cooling during billions of years, turning into a 'white dwarf'. Remains of the stars contract and the surface temperature reaches between 50,000° and 100,000° K, radiating ultraviolet photons which are absorbed by the planetary nebula and lighting it. The expansion of the shell is a slow event, supported by a stellar wind and pulsations. The material expelled by the star glows with different colors depending on its composition, its density and how close it is to the hot central star. Blue samples helium; blue-green oxygen, and red nitrogen and hydrogen. When ultraviolet light from the white dwarf reaches to the expanding shell, it creates the planetary nebula intricate structures. Such white dwarfs lately have been seen beeing often double stars, as a fast wind emanating from the hot core rams into the ejected atmosphere, pushes it outward, and creates the graceful, shell-like filamentary structures. Double stars may explain the assymetric shape of some planetary nebulae. Such a shock wave likely occurs under 5,000 years of age of the nebula. Some parts of the planetary nebula may be compressed by solar winds reaching speeds of up to 2 million mph (3.6 million km/h). The infrared-dedicated Spitzer Space Telescope found in 2007 that the swelling of the original star not only engulfes and burns some of its planets, as the remaining ones and the objects at its outskirts, like comets, asteroids, or Kuiper Belt Objects, have their orbits perturbated, which leads them to collide and to yield dust into the planetary nebula. 99 per cent of stars have the red giant period and then end as white dwarfs. A mystery might linger over white dwarfs as the younger ones (those who are hotter, thus bluer and brighter, compared to old ones), have been found, in clusters of stars, to lie at the outskirts of them, as they should have been near when their large progenitor stars had died, near the center of the clusters that is. A typical example of a planetary nebula is the Ring Nebula in constellation Lyra, the Lyre, a well-known object to amateur astronomers. The gas in there was expelled by the central star about 4,000 years ago. The outer rings were formed when faster-moving gas slammed into slower-moving material. Dark, irregular knots of dense gas embedded along the inner rim of the planetary nebula ring formed when expanding hot gas pushed into cool gas ejected previously by the doomed stars, with knots more resistant to erosion. Knots are also seen at other planetary nebulae. The nebula is expanding at more than 43,000 miles an hour, but the center is moving faster than the expansion of the main ring. The Ring Nebula will continue to expand for another 10,000 years as it will become fainter and fainter until it merges with the interstellar medium. As our Sun is smaller that the Ring Nebula progenitor, but will endure a similar fate, the resulting white dwarf however will become hot enough to illuminate the gas when the material will be farther away, hence a fainter planetary nebula because more extended. This name of "planetary" nebula was given due to such objects having the aspect of a pale planet. The hottest white star known has a temperature of 400,000° F (200,000° C), as it seems to have had an episodic activity, leading to an irregularly shaped planetary nebula. A teaspoon of a white dwarf weighs 15 tons, as, after 22,000 years of expansion, a planetary nebula is stretching across 5 light-years. The white dwarf ultraviolet radiation is lighting the nebula from the inside as the nebula most usually has a round shape. A thick ring of dust may occur around that star, or two polar jets. The conditions of how the star shed its layers however mais lead to some specific form with a good example, the Hourglass Nebula with two hemispheric flows centered upon the star. As far as some planetary nebulae are concerned, something is preventing the uniform expansion of the star’s atmosphere as, for example, a thick disc of dust surrounding the star is funnelling the outflow into two wide cones. Unstable fusion reactions in the dying star may bring to multiple shells in the planetary nebula. A planetary nebula may also be attributed to a double-star system, for example, and that one of the aging stars balloons to the point where it engulfs its companion star. The smaller star continues orbiting inside its larger companion, increasing the giant’s rotation rate. The bloated companion sar spins so fast that a large part of its gaseous envelope expands into space. Due to centrifugal force, most of the gas escaped along the star’s equator, produces a ring with embedded dense gas clumps. Both remaining stars then furiously whirl around each other, completing an orbit in a little more than a day. The process involving the binary may also have, at one point, one of the star bringing material from its companion, which forms a accretion disk and yield a precessional motion. A red giant dying star may also peridocially eject superhot blobs of gas during extended eons, which likely is due to the interaction with a binary companion. The period and ejection occurs from a disk of material drawn around the main red giant when the companion star drives into the giant's atmosphere. Such a process could explain the birth of planetary nebulae as such gaseous blobs could account from nebulae's structures. A laser emanation found in a planetary nebula might suggest the presence of a double star system inside (by coincidence, astronomer Donald Menzel who first observed in the 1920's the Ant Nebula where the phenomenon was observed was also one of the first to describe the laser process, or the light amplification by stimulated emission of radiation and said it could occur in gaseous nebulae...). This kind of laser emission needs very dense gas close to the star as that's impossible for a usual star remnant as the region close to the dead star is quite empty, because most of its material is ejected outwards and any lingering gas would soon fall back onto it. The explanation likely is that gas is settled there into a disk, hinting to a binary star. A collection of clumps, generally, may fill the central part of a nebula, and radial spokes extend well beyond. Astronomers think these features represent molecules of hydrogen gas, mixed with traces of heavier elements. Despite being broken apart by the ultraviolet light from the central white dwarf, much of this molecular material may survive intact and mix back into interstellar gas clouds, helping to fuel the next generation of stars. A study in 2012 has shown how a dying star came briefly back to life 12,500 years after casting its gassy shells out into space coughing out knots of helium and carbon-rich material in a single, violent event. The star’s outer envelope briefly expanded during this born-again episode, but then very rapidly contracted again witin 20 years. The central star generally, is bound to become a very faint white dwarf and slowly cool down over many billions of years. Like observed around white dwarf stars, the death of our Sun could too bring to the disruption of the Asteroid Belt as the asteroids' debris would rain unto the Sun turned white dwarf. How a star is dying a planetary nebula is located on the Hertzprung-Russell diagram on what is called the 'asymptotic giant branch,' or 'AGB.' A study in 2013 have shown that some stars simply did not get to this stage in their lives at all, with sodium in the stars a very strong predictor of how they ended their lives. Stars which skip the AGB phase will evolve directly into helium white dwarf stars and gradually cool down over many billions of years. When our Sun burns out, the balance of gravitational forces between the Sun and Jupiter will change, disrupting the main asteroid belt. Asteroids that veer too close to the Sun will be broken up, and the debris could be pulled into a ring around the dead Sun as a part eventually funneled unto the remaining white dwarf. Elements in a nebula may result not only from the star it originated from but also from the cloud self the star originated

Stars with an important mass (between 8 and 50 solar masses) and a surface temperature 8 times one of our Sun -as they are thus few numerous- after having burned the elements one after another -and in more and more swift sequences (the star burns helium during 500,000 years, carbon during 600 years, neon during 1 year)- have eventually their heart collapsing (in two-tenths of second) and they explode into a supernova. Of note that as many as 30 percent of most massive stars, at 25 times the Sun might not go supernova but quietly collapse into black holes. Heavy elements found in a supernova environment have been forged inside the pre-supernova star and during the supernova explosion. A supernova remnant further, is forging vast amounts of new dust from the new elements created in the progenitor star. A supernova explosion is thought to occur about every 50 years on average in the Milky Way galaxy. Many details of what goes on inside a star leading up to an explosion as well as how that explosion unfolds, remain a mystery however. A star's metal content controls the strength of its stellar wind, and this in turn determines how much of its hydrogen atmosphere it retains before collapse. The larger the star, the longer the hydrogen envelope takes to fall into the forming resulting black hole

click to a view of the explosion of a supernova

->How All is Beginning With the Explosion of a Supernova!
When a dying star comes to the point it has exhausted completely the fuel it remains to be burnt, it collapses on itself in the tremendous time of two-tenths of a second. During such this much small amount of time, a part of the material of the star is shattered and expelled to hundreds of light-years away as another part of it is collapsing down to the center, where the swift formation of the neutron star begins! A supernova explosion unleashes strong gamma rays radiation. That formation of a supernova generates in turn a 'rebound' under the form of a violent, mammoth and very short-lived shock wave. The shock wave is composed of ultraviolet and X-rays radiation! It leaves the forming neutron star back, just shattering what is remaining from the most close layers of the dying star and then, in a time which may reach, for example, up to about 160 years, eventually it reaches the layers which were first expelled. There it activates the particles through its radiation, illuminating those in the infrared! The neutron star at the core of a SRN remnant remains a tremendous dynamo yielding a powerful magnetic field, which in turn unleashes waves
As seen from the Earth, a supernova event is thus first detected through the ultraviolet and X-rays shock wave, before that the visible light of the explosion is seen. In a early phase, supernova, like seen at Betelgeuse, may turn a supergiant, 1,000 times the diameter of our Sun shedding a significant fraction of its outer layers. A series of broken, dusty arcs around the star, and ahead of the direction of its motion, are lying around like arcs of material and testify to a turbulent history of mass loss. Even a star like that is surrounding with a heliosphere with a bow shock created when star's winds are crashing against the surrounding interstellar medium

The star's layer are violently expelled towards the exterior as the star then turns into a neutron star with a very strong density. A neutron star is packing twice to thrice a solar mass into a diameter under 12 miles, with one square-half inch of matter weighing Mount Everest! A teaspoon of such a star is weighing 4 billion tons. The star remainding from a supernova explosion may also turn a pulsar, a neutron star rotating swiftly -in the order of 300 times per second!- and emitting high-intensity radio radiation bursts from its both poles, or sometimes like a 'magnetar' -a superdense neutron star, with an extremely strong magnetic field and emitting in the X-rays. Such stars were discovered by the Rossi X-ray Timing Explorer (RXTE) which worked from 1996 to 2012. Typical pulsar magnetic fields can be 100 billion to 10 trillion times stronger than Earth's as magnetar fields reach strengths a thousand times stronger still. Scientists do not know the details of how they are created. Of about 2,600 neutron stars known, to date only 29 are classified as magnetars. A outflow may be yielded by the magnetar's fast rotations and called a pulsar wind, which serves like the sources of fast-moving particles making up in a 'wind nebula.' The nebula may be corralled around the magnetar. The best-known wind nebula is the Crab Nebula, located about 6,500 light-years away in the constellation Taurus albeit existing around a mere neutron star. As a pulsar taps into its rotational energy to produce light and accelerate its pulsar wind, a magnetar outburst is powered by energy stored in the super-strong magnetic field. When the field suddenly reconfigures to a lower-energy state, this energy is suddenly released in an outburst of X-rays and gamma rays. A wind nebula generally, stores the energetic outflows over its whole active history. Astronomers find most pulsars through radio emissions as gamma rays may also be a signal. Such small stars are positioned in the midst of gas layers and elements expanding. Some neutron star have been seen moving inexplicably fast, by over 3 million miles per hour, likely because the supernova explosion quicked the remaining center while exploding. Time passing, the rotation about itself of a pulsar is braking, passing from 100 revolutions per second to only one. Unusually strong magnetic field, could slow a pulsar's rotation. Magnetars occasionally produce high-energy explosions or pulses. A magnetar's solid crust is locked to its intense magnetic field as a disruption of one immediately affects the other, like a fracture in the crust will lead to a reshuffling of the magnetic field, or a sudden reorganization of the magnetic field may instead crack the surface. A magnetar also features glitches and even anti-glitches in rotation. Theory maintains neutron star, generally, has a crust made up of electrons and ions and a interior containing oddities that include a neutron superfluid, which is a bizarre state of matter without friction and a surface that accelerates streams of high-energy particles through the star's intense magnetic field. The streaming particles drain energy from the crust. The crust spins down, but the fluid interior resists being slowed. The crust fractures under the strain. When this happens, a glitch occurs. There is an X-ray outburst and the star gets a speedup kick from the faster-spinning interior as some other process leads to a sudden rotational slowdown. The pulsar's energy then is transformed into X-rays through the star's magnetic field and then a pulsar stop working, and thus he becomes unobservable. A pulsar not associated with any other star, has a proper motion of 440,000 mph. In pulsars, the magnetic poles are especially luminous. A typical example of a remainder of supernova is the famous Crab Nebula (M1). It's 6 light-years in width, as its interior is dominated by four high-energy structures, a X-ray-emitting jet, a outflow of particles moving near the speed of light, called a "pulsar wind"; a disk of accumulating particles where the wind terminates, and a shock front where the wind abruptly slows as this environment is dominated by the pulsar's magnetic field which could be organized precariously only. A theory states that the magnetic fields become highly tangled and the motions of the particles -mostly protons- very turbulent near the expanding supernova shock wave at the front edge of the supernova remnant as high-energy charged particles can bounce back and forth across the shock wave repeatedly, gaining energy with each crossing. By the end, that leaves a messy network of holes and dense walls corresponding to weak and strong regions of magnetic fields, respectively. X-rays emissions there are due to trapped particles spiraling around the magnetic field lines. The pulsar at the center of the Crab Nebula is spinning by 30 times a second seem to sport short-lived gamma-ray superflares since 2009 at some 100 million electron volts (eV) or hundreds of times higher than the nebula's observed X-ray variations. Scientists think the flares occur as the intense magnetic field near the pulsar undergoes sudden restructuring and that such changes can accelerate electrons to velocities near the speed of light interacting with the magnetic field and emitting gamma rays. Such flares represent the highest-energy electrons known to be associated with any cosmic source as the emitting region might be comparable in size to the solar system and located within about one-third of a light-year from the pulsar. High-energy emissions from the Crab Nebula generally, are thought to be the result of physical processes that tap into the pulsar's rapid spin

->Neutron Stars and Pulsars in Details, and More!
Neutron stars are the rapidly rotating cores of massive stars left behind after a supernova event, as they are strongly magnetized. A neutron star is born when a massive star runs out of fuel and collapses under its own gravity, crushing the matter in its core and blasting away its outer layers in a supernova explosion. A neutron star gives off strong X-ray emission due to its magnetic field and rapid rotation. A neutron star's gravity is so strong it warps space-time distorting our view of the star's surface and its hot spots. Neutron stars are so dense, scientists are uncertain how matter behaves in their interiors. When neutron stars form, their atoms become crushed together and merge. As a result, the bulk of a neutron star is made up of tightly packed subatomic particles — primarily neutrons, as well as protons and electrons, in various states. Neutron stars, as they shine albeit technically dead, occasionally feed on neighboring stars if they venture too close. Although the nuclear-fusion fires that sustained their parent stars are extinguished, neutron stars still shine with heat left over from their explosive formation, and from radiation generated by their magnetic fields that became intensely concentrated as the core collapsed. A neutron star is right at the threshold of matter as it can exist; whether compressed any further, it would collapse completely in on itself and become a black hole. They host high-energy, dynamic phenomena including starquakes, thermonuclear explosions, and the most powerful magnetic fields known in the Universe as they too feature hotspots located at the stars’ two magnetic poles whence the intense magnetic field emerges from the surface. Particles trapped in the magnetic field rain down and generate X-rays when they strike the surface. A neutron star may also be engulfed by, spirals into and merges with an evolved giant star. Although its parent star could easily have been more than a million miles across, a neutron star is only about the size of a city capable of packing in an astonishing amount of matter, more than 1.4 times the content of the Sun, or at least 460,000 Earths. One teaspoon would weigh about a billion tons on Earth. When neutron stars form, they can develop a tough crust as it occasionally cracks. A neutron star is right at the threshold of matter as it can exist. If a usual neutron star gets any denser, it becomes a black hole. Isolated neutron stars with low magnetic field exist also, typically barely six miles across, yet weighing more than our Sun as they are thought to be abundant across the Universe, but they are very hard to find because they only shine at X-ray wavelengths. Two hotspots exist on a neutron star at opposite sides, one at each magnetic pole, the place where the star's intense magnetic field emerges from the surface. Here, particles trapped in the magnetic field, in the case of a binary system (a neutron star and a ordinary one) rain down and generate X-rays when they strike the surface. Neutron stars generate magnetic fields so strong they can create columns that channel material down to the surface, generating powerful X-rays in the process. But if the neutron star spins especially fast, those magnetic fields can create a barrier, making it impossible for material to reach the star's surface. Most neutron star binary systems continuously release large amounts of X-rays, punctuated by additional X-ray flashes every few hours or days. Heaviest elements in the periodic table are created in explosive neutron star mergers. With a binary linking a neutron to a average star, neutron star's magnetic field creates a gap between the neutron star and the disk around it, largely preventing it from feeding on matter from its stellar companion. Gas builds up until, under certain conditions, it hits the neutron star all at once, producing intense flashes of X-rays, or 'type-II' bursts. The Eddington limit is when the neutron star typically cannot accumulate matter any faster and give off any more X-rays. When the magnetic fields around a neutron star are extremely strong, they may in fact be helping to break the Eddington limit, reducing the pressure from the neutron star's X-rays and allowing the neutron star to consume more matter. That is called the 'cylotron line.' When the magnetic fields' strength is about 10,000 times less strong, they are not powerful enough for the flow onto this neutron star to break the Eddington limit. As the hotspots of a binary system, generally, rotate into our line of sight, they produce a pulse of light, like a lighthouse beam, giving rise to what is called a 'pulsar.' Pulsars, generally, are highly magnetized neutron stars. Pulsars send out beams of radiation ranging from radio waves to ultra-high-energy gamma rays. Many pulsars flash several times per second, because of the rapid rotation they inherit as they are born. A pulsar have been seen blinking in the gamma-rays only, or a 'gamma-ray pulsar.' As the pulsar's core shrinks, it spins faster, like a twirling ice skater pulling in her arms. Neutron stars and pulsars often come with a companion, another neutron star or a white dwarf, pulling in matter from that star when too close, as the stellar material forms a disk around the pulsar before falling on to the surface at the magnetic poles. The infalling gas can spin up a neutron star to even higher speeds; some rotate hundreds of times per second. Gas raining onto the pulsar's surface with incredible force and ultimately coating the neutron star in a layer of hydrogen and helium fuel, then builds to a certain depth and the fuel undergoes a runaway thermonuclear reaction and explodes, creating intense X-ray spike, a phenomenon called a 'bursting neutron star.' A the highest rates of accretion, the flow of fuel onto the neutron star can support continuous and stable thermonuclear reactions without building up and triggering episodic explosions. Instead of a pattern with strong spike of emission followed by a long lull a higher accretion rates has the emission spikes become smaller and occur more often. Astronomers calls the process a marginally stable nuclear fusion, where the reactions take place evenly throughout the fuel layer. That is maybe true for slow rotating pulsar only, as a faster rotation would introduce friction between the neutron star’s surface and its fuel layers, and this frictional heat may be sufficient to alter the rate of nuclear burning
'Millisecond pulsars' form when a pulsar is spun up by accreting matter from a companion star. The inflow of material from the partner star can accelerate the pulsar up to hundreds of rotations in a single second. Once the accretion ends, the rapidly rotating neutron star can be observed as a millisecond pulsar. Some millisecond pulsar are detectable solely through its pulsed gamma-ray emission, which could hint to that the theory is true that a large population of thousand of such objects are found towards the center of our Milky Way Galaxy that would account for the observed excess of high-energy gamma-radiation there. By 2011, NASA Chandra X-ray Telescope has discovered the first direct evidence for a superfluid, friction-free state of matter, at the core of a neutron star. With one teaspoon of neutron star material weighing six billion tons, the pressure in the star's core is so high that most of the charged particles, electrons and protons, merge into a mixture only composed of uncharged particles, or neutrons. In a recently formed neutron star, the formation of a neutron superfluid in the core seems to occur within about 100 years, and keep for some decades and then likely slow down. The onset of superfluidity in materials on Earth occurs at extremely low temperatures near absolute zero, but in neutron stars, it can occur at temperatures near a billion degrees Celsius. On a other hand, superfluids containing charged particles are also superconductors, meaning they act as perfect electrical conductors and never lose energy as that translates into that a young neutron star is cooling. Such a discovery is opening the way to researchers to test models of how the strong nuclear force, which binds subatomic particles, behaves in ultradense matter. These results are also important for understanding a range of behavior in neutron stars, including "glitches," neutron star precession and pulsation, magnetar outbursts and the evolution of neutron star magnetic fields. Small sudden changes in the spin rate of rotating neutron stars, called glitches, had previously given evidence for superfluid neutrons in the crust of a neutron star, where densities are much lower than seen in the core. What occurs inside these ultra-dense cores, remains a mystery. Neutrons might dominate all the way down to the centre or the incredible pressure compact the material into more exotic particles or states. Neutron stars might get more complicated the deeper one goes, in any case. Beneath a thin atmosphere made mostly of hydrogen and helium, the stellar remnants are thought to boast a outer crust just one-inch or two thick that contains atomic nuclei and free-roaming electrons. Ionized elements become packed together in the next layer, creating a lattice in the inner crust. Even further down, the pressure is so intense that almost all the protons combine with electrons to turn into neutrons, but what occurs beyond that is murky at best. A study of the Vela pulsar at about 12 miles in diameter with a complete rotation in 89 milliseconds, and with a 0.7 light-year-long, 70 percent of light speed moving, polar jet, is showing a slow wobble or precession of about 120 days. One possible cause of precession for a spinning neutron star is it has become slightly distorted and is no longer a perfect sphere. This distortion might be caused by the combined action of the fast rotation and 'glitches,' sudden increases of the pulsar's rotational speed due to the interaction of the superfluid core of the neutron star with its crust. The deviation from a perfect sphere may only be equivalent to about one part in 100 million due to the large density of a neutron star. Another possibility is the strong magnetic fields around the pulsar are influencing the shape of the jet. For example, if the jet develops a small bend caused by precession, the magnetic field's lines on the inside of the bend will become more closely spaced, pushing particles toward the outside of the bend and increasing the effect. Such source might be that of gravitational waves. X-ray binaries are composed with a massive star orbiting around either a black hole or a neutron star, with material can be pulled away from the giant star to form a disk of material around the compact object. Frictional forces heat the infalling material to millions of degrees, producing a bright X-ray source. When the massive star turns supernova, it leaves behind also a black hole or a neutron star and the end result is either a pair of black holes, a pair of neutron stars, or a black hole and neutron star. When the separation between the compact objects becomes small enough as time passes, that produces gravitational waves until a merger. A collision between two neutron stars, which released a gravitational wave, could be observed in the visible. Both stars projected dense and unstable overheated debris, some aggregating and becoming heavy elements, among the heaviest in the physical world. The merger also released a short gamma-ray burst as the cloud of heavy elements will span a entire galaxy in one millions years. Neutron stars or pulsars feature a limit of how massive and compact they can be before their force of gravity overwhelms even the ability of neutrons to resist further collapse into a black hole. That limit tends to be a mass of about 2.17 times larger than that of the Sun into a sphere only 18.64-mile across

A pulsar as seen in a comparison in size with Manhattan (left) as the insert is showing the gamma rays emission of a pulsar on a 11-day duration; a pulsar's layers (right). picture site 'Amateur Astronomy'

-> More About Pulsars!
Pulsars were discovered in 1967 by astrophysicist Jocelyn Bell Burnell, then a PhD student at the University of Cambridge, UK under under astronomer Antony Hewish. She was analysing hundreds of metres of chart paper with data collected by the array radio telescope in Cambridge to measure the random brightness of quasars, when she noticed some mysterious recurring smudges and she was able to characterize these as signs of radio pulses emanating from a spinning star. The first pulsar is now known like PSR B1919+21. Such stellar objects had been previously predicted but never observed as we eventually came to over 2,000 pulsars known nowadays. Neutron stars turning pulsars are stars between about seven and 20 times the mass of our Sun and some are found to spin hundreds of times per second, faster than the blades of a household blender, with enormously strong magnetic fields. During the next 50 years, scientists were allowed to study pulsars from space using different wavelengths of light. The core of pulsars is a environment that doesn’t exist and can’t be reproduced anywhere else and what's inside a pulsar is one of many long-standing astrophysics questions. The material of pulsars is a collection of particles familiar to scientists from over a century -- neutrons, protons, electrons, and perhaps even their own constituents, called quarks. However, under such extreme conditions of pressure and density, their behavior and interactions aren’t well understood. Pulsars generate powerful winds of high-energy particles moving near the speed of light. The power for all this comes from the pulsar's rapidly spinning magnetic field, and over time, as the pulsars wind down, these emissions fade. Theorists say pulsars could rotate as fast as 72,000 rpm before breaking apart. Astronomers, about 1980, discovered a type of pulsar revolving in 10 milliseconds or less, reaching rotational speeds up to 43,000 rpm. While young pulsars usually appear in isolation, more than half of such 'millisecond pulsars' occur in binary systems. Pulsars can be found in stellar couples, with the neutron star cannibalising its neighbour. This can lead to the neutron star spinning faster, and to pulses of high-energy X-rays from hot gas being funnelled down magnetic fields on to the neutron star. Material collected from the companion star first forms a accretion disk as it makes its way further onto the surface. Due to the strong magnetic fields of the neutron star, the material lands unevenly, traveling along the magnetic field to the magnetic poles where it creates hot spots. The layer of material builds up on the surface of the neutron star and once the pressure of this layer builds up to the point where its atoms fuse, a runaway thermonuclear reaction occurs, releasing the energy equivalent of 100 15-megaton bombs exploding over every square centimeter. 'Peculiar low-mass X-ray binary pulsars' have the companion star less massive than our Sun as intermediate-mass binary system the companion of about two solar masses. Pulsars, those possible remnants of a supernova event, slow down as they lose energy, eventually spinning too slowly to power their emissions and they become undetectable. There is a difference between 'normal' and 'millisecond' pulsars, according to their period, as both types are emitting too gamma rays as part of their process. So-called accreting millisecond pulsars are a previously unseen stage in the formation of what is called 'recycled' millisecond radio pulsars as they were discovered in the early 1980's. Pulsar's radio beams represent only a few parts per million of its total power, whereas its gamma rays account for 10 percent or more, as the beam of gamma rays is due to particles accelerated to speeds near the speed of light and arcing along curved magnetic fields lines. Yet gamma rays are few as a space telescope, for example, will just detect one gamma-ray photon from a pulsar every two minutes, that is one photon for every thousand rotations! Sometimes a slowed, terminal pulsar, can pair with a normal star, with a stream of matter from the companion spilling unto the pulsar and coming to increase the spin back, up to between 100 and 1,000 times a second. A pulsar may feature a circular structure (or ring) surrounding the pulsar and jet-like features along its polar axis. Rings may show a region where a high speed wind of particles flowing away from the pulsar, is slowing down abruptly, or the ring may represent a shock wave, similar to a sonic boom, ahead of the pulsar wind. Jets could be particles that are being fired away from the pulsar in a narrow beam at high speed. Differences in behavior shown by some pulsars might have a geometrical explanation. Some pulsars show only radio pulses and others only gamma-ray ones which might only due to our line of sight to them as steadier X-ray emission from extensive clouds of high-energy particles, called pulsar wind nebulas, are associated with both types of pulsars. A pulsar wind nebula is a type of nebula found inside a supernova remnant and powered by winds emanating from a central pulsar. Such nebulae were discovered in 1976. Pulsars allow to study the ionized material in the Milky Way galaxy through their radio pulses journeying through the interstellar medium. Gravity also revs up the star's rotation and strengthens its magnetic field. Most of the particles around a pulsar are either electrons or their antimatter counterparts, positrons. Speedy electrons emit gamma rays, the highest-energy form of light, through a process called curvature radiation. A gamma-ray photon can, in turn, interact with the pulsar’s magnetic field in a way that transforms it into a pair of particles, an electron and a positron. A simulation showed that most of the electrons tend to race outward from the magnetic poles as positrons, on the other hand, mostly flow out at lower latitudes, forming a relatively thin structure called the current sheet, where reconnection can occur. Highest-energy positrons here -- less than 0.1 percent of the total -- are capable of producing gamma rays. One population of medium-energy electrons showed truly odd behavior, scattering every which way — even back toward the pulsar. The distance where the pulsar's plasma’s rotational velocity would reach light speed is a feature astronomers call the light cylinder, and it marks a region of abrupt change. As the electrons approach it, they suddenly slow down and many scatter wildly. Others can slip past the light cylinder and out into space. A runaway pulsar may escape the blast wave of the supernova that formed it

Pulsars are now known to also emit radiation than radio, like standard thermal radiation or non-thermal radiation, via charged particles being accelerated along magnetic field lines. Some pulsars have been observed radiating in the gamma rays only as some others are ultraluminous X-ray source and called 'ultraluminous X-ray pulsars'. A pulsar might be composed of neutrons and protons and have a mass of about twice the one of the Sun as pulsars are a form of natural laboratory to study the very dense and exotic states of matter. The supernova events are well known to provide most of the dust injected into the Universe, that dust forming, then, the basis for the further formation of other stars, or the development of protoplanetary disks, and, eventually of planets and life. The dust elements are produced when the gas of the ejecta cools off, between some to several hundreds days after the explosion. Supernova events are due to that the core of the star is then composed of iron only and thus incapable of producing energy. For igniting the nuclear combustion of iron, that would need a apport of energy at the contrary. It might be frequent, on the other hand, that some supernova eventually turn like a dust-shrouded whimpler instead of bang. The region of the Carina Nebula is well known like a place of past, and soon to come, ramping supernova activity. Stars between 20 and 40 solar masses alreday exploded in the last few million years as a population of young massive stars are bound to explode. The most famous, examplifying constituent of the Carina Nebula is Eta Carinae, a massive, unstable star that may be on the verge of exploding as a supernova. When it does explode, it will likely be a spectacular –yet safe- light in the Earth's sky. Stars which mass is very important (mass greater than 50 solar masses) have the same story (they burn elements, the core collapses, they explode into supernovae) but, instead that the remainder of the star holds as a neutron star or as a pulsar, the collapse keeps on and the star becomes a black hole, a star which density is so high that spacetime is there warped closed and that, if matter may still come in, nothing more is able to go out, not even light! Supernovae events occur due to the fact that stars, core of which have turned into iron are no able anymore to yield energy. They would need, at the contrary, an input of energy instead to nuclear-burn the iron! It's likely that an intermediate range of stellar mass is yielding the smallest black holes possible in the Universe, with a 3-solar masses mass. Black holes may also result from another type of star death, when a progenitor star collapses without a explosion as it looses its mass in a vigorous stellar wind

->A "Bang" or "Hiccups"?
Most recent data using observations by the gamma-ray bursts observing satellite Swift are showing that the last stages of a star going supernova are maybe not as simple as thought when the birth of a black hole is involved. The star, in this case, might not go "bang" once for all but it might endure a "bang" and a series of "hiccups". After the stars falls on itself as it runs out of energy and is no more able to sustain its mass, an initial blast is obliterating the outer shells of the star, as a chaotic black hole activity re-energizes the explosion again and again, creating multiple bursts all within a few minutes. First comes a blast of gamma rays followed by intense pulses of X-rays. The newly formed black hole gets immediately to work. Some matter falls as some other is expelled. Another explanation might involve no black hole at all and simply rest on the jets shooting away from the dead star falling back unto itself, creating shockwaves in the jet core

On a more longer timescale, it may be useful to know another categorization of stars, as 'Population I stars' are young stars (some billion years old, like the Pleiades), 'Population II stars' are stars which appeared 10 billion years ago during the bloom of most galaxies. 'Population III stars' are the oldest stars of the Universe and thought to be a hundred times more massive than our Sun, as short-lived -some million years only. The name 'Population III' arose because astronomers had already classed the stars of the Milky Way as Population I (stars like the Sun, rich in heavier elements and forming the disc) and Population II (older stars, with a low heavy-element content, and found in the Milky Way bulge and halo, and globular star clusters). Population III stars however did not yield any of the dust which the stars provided into the Universe. They formed in a environment devoided of dust or complex molecules but of hydrogen, helium and lithium instead, those primordial components from the Big Bang. One thought they had a size of some hundreds of solar masses only as one discovered that they might form from the dissociation of a flat disk created by forming protostars accreting gas through their rotation. Population III stars thus would weigh some solar masses only and they could not be, thence, be at the origin of the black holes in the beginnings of the Universe, which gave birth to quasars, those galaxies possessing a black hole of billion of solar masses! That needs that primitive stars have existed with masse up to 1 million solar masses, which would have concentrated most of the mass of a embryonic galaxy where they stood. They would have had a life of some hundreds of millions years only before exploding supernova and turning black holes. Astronomers think also that Population III stars might end their lives as blue supergiants and likely at the origin of a rare, very-long-duration gamma-ray bursts, a category of those most energetic events in the Universe. Blue supergiants are hot stars with about 20 solar masses that retains its deep hydrogen atmosphere and contain only a very small fraction of elements heavier than helium, making it larger still

A few stars in the Universe are escaping their galaxy, seeming to have velocities that will eventually take them out of there. Such unbound stars usually come from the galactic center when a pair of binary stars gets too close to the supermassive black hole at the heart of the Milky Way. They also can be yielded out of a close binary of which a white dwarf. The white dwarf’s gravity sucks material from its companion until the dwarf grows big enough to ignite fusion inside it and it is destroyed in a violent explosion known as a type Ia supernova, propelling the companion on a escape route

Most stars are powered by nuclear fusion. Other types of star activity are: rotation (neutrons stars), accretion (type Ia supernovae; a white dwarf pulls matter from a red giant companion and eventually goes supernova), maybe magnetic field. Magnetars are the bizarre super-dense remnants of supernova explosions and a unusual form of neutron star following a supernova explosion. They are the strongest magnets known in the Universe and millions of times more powerful than the strongest magnets on Earth. Magnetars further have their surfaces releasing vast quantities of gamma rays when they undergo a sudden adjustment known as a starquake as a result of the huge stresses in their crusts. The reason why a dying star turns into a magnetar and not a black hole refered to that it results from a binary system which is exchanging matter between its both components, building up strong magnetic fields until one of the stars reaches a sufficient low mass to turn magnetar and not a black hole. The stronger brand of magnetars are called Soft Gamma-ray Repeaters (SRGs) or Anomalous X-ray Pulsars (AXPs); they can flare 100 times per 20 minute, each flare releasing more energy than our Sun does along 20 years; that is followed by periods of calm during some months, as the flares are expelling dusts of material around the star; they might be much numerous in our Milky Way than the 6 of them currently found)

In terms of rotation, massive stars tend to rotate slowly, while less massive stars rapidly. Larger stars have a huge core enveloped in a thin layer of stellar material undergoing a process called convection as small stars consist almost entirely of convective, roiling regions. As stars mature, on a other hand, the braking mechanism from magnetic fields more easily slows the spin rate of the thin, outermost layer of big stars than the comparatively thick, turbulent bulk of small stars. The age is also a factor as when a star keeps moving into adulthood, it looses momentum because its stellar wind makes charged particules carried away along the star’s magnetic fields, which overall exerts a braking effect on the rotation rate of the star

A star, generally, is no smaller than 75 Jupiter masses. Two categories of most faint stars exist about that limit. Brown dwarfs, which until now, are few in numbers, as that might be due to an observational limit only, with those stars needing deep infrared searches to be seen. Brown dwarfs have masses somewhere between those of a star and a planet, typically 10 times the mass of Jupiter as they start out like stars as collapsing balls of methane, hydrogen sulfide and ammonia, but they lack the mass to fuse atoms together at their core and shine with starlight. Brown dwarfs are thought to have masses up to 80 times that of Jupiter. 'Brown dwarf desert' is the name given to that less than 1 percent stars the mass of our Sun have a brown dwarf orbiting within 3 AU (which is very close). As time goes on, these lightweights cool off, until they can only be seen in infrared light, with temperatures of a chilly 600 Kelvin. There is just one brown dwarf for every six stars. Astronomers think that brown dwarfs can form by several different mechanisms, including having their growth stunted by a variety of factors that prevent them from becoming full-blown stars but they still don't know exactly how this process works. With time, they cool and fade. Brown dwarfs glow because of the heat leftover from their formation. The first brown dwarf wasn't discovered until 1995, though these objects had been predicted to exist as far back as the 1960s. Wind-driven, planet-sized clouds have been observed by 2013 enshrouding such strange worlds which are similar somehow to gas giant planets. Clouds on brown dwarfs, amazingly, are composed of hot grains of sand, liquid drops of iron, and other exotic compounds as water vapor and methane compose highest dry layers of the atmosphere and cloudy and silicate-rich layers are found below. Weather is extreme at brown dwarfs as they are actually hot by earthly standards, at about 1,100 to 1,300 degrees Fahrenheit (600 to 700 degrees Celsius). Brown dwarfs have massive, organized cloud systems, perhaps akin to giant versions of the Great Red Spot on Jupiter. Brown dwarfs are classified according to their temperature. T-dwarfs are about 1,400 to 500 Kelvin as the elusive Y-dwarfs could be as cold as 200 Kelvin. A pair of brown dwarfs named CFBDSIR 1458+10 is the coolest pair of brown dwarfs found with one of both the cooler brown dwarf ever, at 212° F (100° C) only. That might mark the limit between giant exoplanets and stars as they also could hold water in their external layers. By 2011 NASA's Wide-field Infrared Survey Explorer (WISE) have discovered the coldest class of star-like bodies, with temperatures as cool as the human body, or Y-dwarfs. Astronomers had chased those objects for more than a decade without success. When viewed with a visible-light telescope, they are nearly impossible to see. The Ys are the coldest members of the brown dwarf family as these objects cool and fade with time since their inception, until what little light they do emit is at infrared wavelengths only and considered failed stars. Such dark stars are part also of our neighbourdhood, at 40 light-years, for example. L dwarfs might look red, Ts dark purple and Ys dark blue. L dwarf stars feature a low mass and barely meet the threshold for nuclear fusion. Red dwarfs, on the other hand, are largely 20 percent the mass of our Sun and are roughly half its diameter and temperature. In a red dwarf, convection inside the sphere should occur all the way into the star's core, resulting that it shouldn't experience a regular cycle of activity like our Sun. Such a cycle however is observed at such stars. Red dwarf stars have been studied able to unleash powerful eruptions, or flares much stronger than our Sun's as albeit smaller than our own star, they have a deeper convection zone hence a stronger magnetic field. Star spots can cover half of the dwarf's surface instead of 1 percent only for the Sun! Dwarf stars however are thought to flare 15 times less frequently than other, normal stars. Red dwarfs, or M dwarfs, are very luminous and active in their youth blasting high-energy X-rays and extreme ultraviolet during hundreds of millions of years after they form. M dwarfs are common in the Milky Way and are expected to host many Earth-sized planets. On the other hand of the spectrum, Wolf-Rayet stars, named after astronomers Charles Wolf and Georges Rayet, are massive stars, like V385 Carinae, this star is 35 times as massive as our Sun, with a diameter nearly 18 times as large, as they super-hot stars characterized by a fierce ejection of mass. Some of the most massive stars evolve into Wolf-Rayet stars towards the end of their lives. This stage is short-lived, and Wolf-Rayets survive in this state for only a few hundred thousand years. In that time, they throw out huge amounts of material in the form of a powerful stellar wind, hurling matter outwards at millions of miles per hour. Some Wolf-Rayet star system may also emit gamma-ray burst (GRB). Vast amounts of mass are ejected by Wolf-Rayet stars through their thick winds, creating nebulae of ejecta, bubbles and knots of gas around them. V385 is hotter, too, and shines with more than one million times the amount of light. They burn quickly however during only a few million years. As they age, they blow out more and more of the heavier atoms cooking inside them, such as oxygen that are needed for life as we know it. The material is puffed out into clouds. Wolf-Rayet stars are burning so hot they drive out their outermost hydrogen layer like a stellar wind and they are large enough to turn a black hole albeit too small to generate jets. Proto-typical Wolf-Rayet stars are extremely bright, massive, and short-lived star that have lost most of their outer hydrogen and fusing helium into heavier elements. A disk of gas may circle a Wolf-Rayet star due to powerful stellar winds. Wolf-Rayet-type stars, on the other hand, are stars which represents a late stage in the evolution of Sunlike stars as they share many observational characteristics with Wolf–Rayet stars, which are much larger. Both Wolf–Rayet and Wolf–Rayet type stars are hot and bright because their helium cores are exposed: the former because of the strong stellar winds, the latter because the outer layers of the stars have been puffed away as the star runs low on fuel. The exposed helium core, rich with heavier elements, means that the surfaces of these stars are far hotter than the Sun, typically 45,030 to 90,030 Fahrenheit (25,000 to 50,000 degrees Celsius). Only a few hundred such stars are known. The intense heat of Wolf-Rayet stars forces their matter apart, making them extraordinarily windy stars, blowing winds at up to 5.6 million mph. Wolf-Rayet stars usually die like a supernova the remnant of which subsume any previous material ejected

->Heliosismology and Asterosismology to Better Understand our Sun and the Nearest Stars!
Astronomers are well equipped now to understand, from Earth, how the inside of our Sun is working, due to the study of how the sound waves (of a very deep frequency), which are generated at the surface through the gas running into each other, are seen until deep into the solar sphere! That technique, called 'heliosismology' is now applied too to the nearest stars, with networks of telescopes or a satellite -like the European Space Agency (ESA), 2009, Corot, or 2013, Siamois)- allowing to finely study those sound waves. This is done by studying, with a great accuracy, the Doppler effect (which is complicated as those variations of the surface barely reach some tens of years!)