Theory Supernovae

Definition, Classification

Kepler's supernova (that type of supernova is described further in the text). picture courtesy X-ray: NASA/CXC/NCSU/M.Burkey et al.; optical: DSS

Supernovae are explosion events occurring at a moment in the life of certain stars. At the moment of the explosion a supernova may illuminate an entire galaxy as those seen in our own Galaxy can be many times brighter than the planet Venus and may even have rival the brightness of the Moon, being so bright at maximum that it casts shadows and visible during the day. Once the nuclear-fusion fuel in the star's core exhausted, the star then begins to collapse, which compresses protons and electrons together and converts them into neutrons. Astronomers tend to hypothesize that interacting neutrino heating -- neutrinos are produced through particle interactions in the newly formed neutron star -- convection and standing accretion shock instability (SASI) oscillations are behind the explosions. The spectacular initial increase in luminosity of a supernova matches the moment when the explosion shockwave gets out ot the star's surface once it crossed the interior of it at supersonic speeds. Within months the supernova is fading back to invisibility as the expelled shell, called a "supernova remnant" (SNR), is remaining visible during for millenia. Hypernovae are supermassive supernovae. Turbulent, fast-moving filaments of gas and dust are left behind after a supernova explosion. One supernova occurs once or twice a century in our home galaxy, the Milky Way as one once every second somewhere in the Universe. The most ancient documented record of a supernova dates back to 185 AD, when Chinese astronomers saw a ‘guest star’ that remained visible for several months, in the vicinity of the two stars Alpha and Beta Centauri. Supernovae fall into two broad classes. Stars born with more than about 8 times the Sun's mass collapse under their own weight and explode as core-collapse supernovae. White dwarf stars may become unstable due to interactions with a nearby star and explode as so-called Type Ia supernovae. Core-collapse supernovae mostly scatter elements ranging from oxygen to silicon, while white dwarf explosions release predominantly heavier elements, such as iron and nickel. The overall composition of a large volume of space depends on the mix of supernova types contributing to it. For example, accounting for the overall chemical makeup of the Sun and solar system requires a mix of roughly one Type Ia supernova for every five core-collapse explosions. Supernova explosions may feature a asymmetric wind of gas blowing away from the star or take in account a stellar companion to the deceased star. Two neighbouring separate stars can undergo supernova explosions as resulting neutron stars may be so close that gravity can pull them together until they merged and collapsed into a black hole. Astronomers once thought that supernovae were built uniformly, like fireworks in a cosmic assembly line. That changed in the 1990s, when astronomers noticed that some of the supernovae were dimmer than the others. The brightest supernovae generally seem to fade more slowly than their dimmer kin. Supernovae indeed are the progenitors of large amounts of primordial chemical elements useful to ground exo-life. Cassiopeia A (Cas A), for example, one of the most intensely studied of supernova remnants has had its supernova churn out prodigious amounts of key cosmic ingredients, like 10,000 Earth masses worth of sulfur alone, and about 20,000 Earth masses of silicon, 70,000 of iron and a whopping three solar masses worth of oxygen. Carbon, nitrogen, phosphorus and hydrogen have also been detected. All of the oxygen in the Solar System comes from supernovae, about half of the calcium and 40 percent of the iron, with the balance of these elements being supplied by explosions of smaller, white dwarf stars. During a supernova event, the stellar envelope is ejected and sometimes the star's core launches a jet through the envelope, generating a hot bubble known as a cocoon. If the jet successfully breaks out of the stellar envelope, it produces a bright burst of ?-rays as the cocoon also breaks out, and spreads to engulf the star. According to theoretical models, the cocoon moves faster than the envelope, but more slowly than the jet as the jet is expected to drag material from the core of the exploding star and to deposit it in the cocoon

Indian-born physicist Subrahmanyan Chandrasekhar, at the University of Cambridge, calculated in 1935, building on the equations of general relativity and quantum mechanics, that very massive stars were unstable and would collapse into nothingness at the end of their lives, producing black holes -a name coined decades later- as astronomers until then thought that by the end of the life, stars were just fading away into inert, white dwarfs. His ideas were rejected by the old guard of the time, which prompted him to leave Britain and settle into the USA. In some cases, the star's core survives like a rapidly rotating neutron star or a pulsar. The frequency of supernovae in our own Galaxy is about one each 40 years. Some are remaining unseen however as obscured or hidden by gas clouds. As the explosion ejecta of a supernova expand, they are cooling and slowing down, with their emission progressively moving to longer wavelengths, from X-rays to radio waves! A supernova blast wave generally runs into a cocoon of dust and gas which had been expelled by the star during the early phase of the phenomenon. When the star explodes, it is transformed into a supernova remnant (SRN), a rapidly expanding shell of hot gas bounded by the blast's shockwave. Such a shock front is similar to the sonic boom produced by an aircraft going supersonic. A supernova associated shock waves rumble through interstellar space at speeds of millions of miles per hour. After a supernova event, stellar dust is much more abundant than theories estimated as dust particles can re-form or grow rapidly, even after the catastrophic damage caused during the passage of the blast wave. There is evidence that such shock waves may be responsible for some of the cosmic rays that pervade the Galaxy. Oxygen-rich supernovas are of great interest to astronomers because they are one of the primary sources of the heavy elements, expanding in space elements that were forged in the star before it exploded. A supernova remnant also may take the shape of a thick ring, or torus, of X-ray emission where jets of a central pulsar are yielding a elaborate web of loops and swirls. Both swarms of high-energy particles and powerful magnetic fields are extant. SRNs may display a assymetric shape when stellar debris field expand into a nearby interstellar cloud. As the outward-moving shock wave sweeps up interstellar gas, a reverse shock wave, in a more general way, is driven inward, heating the material ejected by the star. Meanwhile, the rapid rotation and intense magnetic field of a possible pulsar, combine to generate a powerful wind of high-energy particles. The motion of a pulsar inside a SRN maybe resulting from a asymmetric supernova explosion. Supernovae explosions usually kick the central remnant neutron star out, which may move at a speed of at least 3 million miles per hour, for example. A supernova explosion, generally, fuses smaller elements into larger ones. The concentrations of massive stars exist in certain areas, or 'galactic superbubbles,' which are regions where many supernovae explode within a few million years. Some rare explosions are extant, in which matter is ejected at higher speeds along the poles than the material emanating from its equator. Iron then is found in only half of the remnant as other elements such as sulfur and silicon were spread throughout, which matches predictions for an asymmetric explosion. Remnants of such supernovae are much more elongated and elliptical than most other remnants as they are also jet-driven like GRBs are. The famed V838 Mon, or a light echo upon dust layers of a star closing supernova, results from a bright flash in 2002 which rapidly faded as its moving light echo is reflected by such layers. A study in 2014 about supernova remnant Cassiopeia A (Cas A), revealed how shock waves likely rip massive dying stars apart, with the original shockwave starting from a star's center first slowing close to that center and then sloshed and re-energized before blowing out the celestial body. The study also revoke the idea that a star going supernova is rapidly rotating just before it dies and launching narrow streams of gas that drive the stellar blast. The blast wave after the supernova explosion in Cas A, travels outwards at speeds of about 11 million miles per hour, it encounters surrounding material and slows down, generating a second shock wave called a "reverse shock" that travels backward. Reverse shocks is usually observed to be faint and much slower moving than the blast wave, as those in Cas A appear bright and fast moving. That is likely caused by the blast wave encountering clumps of material surrounding the remnant, causing the blast wave to slow down more quickly, which re-energizes the reverse shock, making it brighter and faster. Particles are also accelerated to colossal energies by these inward moving shocks, reaching about 30 times the energies of the LHC. Some supernova remnants hint to partially failed supernova, in which the explosion failed to decimate the whole star. A supernova remnant may take a few tens of thousands of years before dissolving as it may reach 57 light-years in diameter

Supernovae fall into two broad classes based on the triggering event. Those called core-collapse supernovae occur in stars born with more than eight times the Sun's mass when they undergo a terminal energy crisis, collapse under their own weight and explode. The other variety, known as type Ia supernovae, involves the total destruction of a white dwarf, a compact remnant produced by stars like the Sun when paired with other objects in binary systems. This occurs either by merging with a companion white dwarf or, when partnered with a normal star, by stealing gas from its consort

->Old White Dwarfs and Young, Massive Stars
A Type Ia supernova -which is triggered by the infall of matter from a companion star onto an old (some billions years old) white dwarf- is leaving a gas shell which contains a lot of iron. In contrast, a Type II supernova -which is yielded by the explosion of a massive, young (some million years old) star contains a much lower quantity of iron. Such white dwarfs began, like usual, as the relatively peaceful death of a star like our Sun, shrinking to a dense, small star after a phase of expansding layers. Recent observations show that a white dwarf can create a cavity around it before blowing up in a Type Ia event as cavities were believed to be the telltale of explosive supernovae only. Supernova called Type Iax has been discovered in 2012, essentially a mini-supernova working like binary systems Type Ia supernova. In Type Iax, the companion star lost its outer hydrogen and is left dominated by helium wich is the element accumulated by the main star. Either the helium in the companion star's outer shell might undergo nuclear fusion, blasting a shock wave at the white dwarf that makes it detonate or all the helium the white dwarf accumulated from its companion star could alter the density and temperature of the white dwarf's interior, forcing carbon, oxygen and maybe helium within the star to fuse, triggering an explosion. Type Iax supernovase are the faintest at only one-hundredth as bright as a Type Ia. In any case, it appears that in many Type Iax supernovas, the white dwarf actually survives the explosion, unlike in Type Ia supernovas, in which the white dwarfs are completely destroyed

A classification of supernovae was devised by the Caltech astronomer Fritz Zwicky during the 1960s as 3 sub-types were added to Type I in the 1980s. All supernovae except type Ia are caused by a star's core collapse. The star needs to be above 8 solar masses as it continues to burn heavier and heavier elements beyond hydrogen. Once the core transformed into iron it increases in size. At a moment, the pressure coming from the star's center becomes so high that electrons merge with protons into neutrons leading to the pressure to vanish. Neutrinos are produced too. The star just collapses on itself. Type II are just caused by a core collapse. Type Ib are caused in the same way at the exception that the star has already shed their hydrogen envelope. Type Ic are stars which have further shed their helium enveloppe. Type Ic supernovae are usually associated the Gamma Ray Bursts (GRBs). Most of the GRBs' energy is released by gamma-ray jets traveling almost at the speed of light. Most or all type Ic supernovae produce bipolar jets, but the energy content of these outflows varies dramatically, whereas the total energy of the explosions is much more standard. It might that a majority features relatively dim and mildly relativistic jets that only can be detected nearby. The explosion dynamics in typical supernovae limit the speed of the expanding matter to about three percent of the speed of light. Type Ia supernovae occurs in binary systems only where the material of a companion star is infalling unto the primary's surface leading to an eventual thermonuclear explosion. The event may last less -and less into the star's life- than usual when the involved primary star is a high-mass one. Core collapse-type events may lead to the formation of neutron stars, that is a star which is formed from the concentrated neutrons yielded in the last phase of the collapse. Such neutrons stars then tend to cool as neutrons further collide and keeps producing neutrinos which, as weakly interacting, are taking the star's heath away. Generally, only six progenitor stars have been found out of 3,200 supernovae observed since 1885. Usual supernovae have massive stars blow material away from them before they blow up, carving out holes around them. The first supernova observed from the Earth was by Chinese astronomers about 2,000 years ago, in 185 A.D., staying in the sky during 8 months. Chinese named those stars 'guest stars.' Recent studies show that the stellar explosion, a type Ia, white dwarf/companion supernova, located at 8,000 light-years, took place in a hollowed-out cavity, allowing material expelled by the star to travel much faster and farther than it would have otherwise. Such a characteristic was until now reserved to other type supernovae

->The Magnetars
Magnetars, those superdense neutron stars, with an incredibly strong magnetic field and emitting in the X-rays, may sometimes be the other type of dense stars like left by a supernova explosion!

->The Most Usual Way Supernovae Work Pinpointed?
The discovery of the increase in X-rays emission of two supernovae events, which were well studied, might point to the most usual way supernovae work! It might that, before the dying star explodes, it somehow carves a large cavity around it either through a fast solar wind, or a blast. The supernova explosion occurs then, with the blast wave expanding unimpeded into the cavity. The explosion wave then only would catch the star's expelled layers, having it glow. This might explained that increase of the X-rays glow along a period of time

->A New Class of Very Luminous Supernovae
A recent class of supernovae have been observed ten times more luminous than a typical one resulting from the collapse of a massive star, a class termed 'very luminous supernovas.' Different explanations have been proposed to explain these energetic supernovas including the interaction of the supernova's blast wave with a dense shell of matter around the pre-supernova star, radioactivity resulting from a pair-instability supernova (triggered by the conversion of gamma rays into particle and anti-particle pairs), or emission powered by a neutron star with an unusually powerful magnetic field. The first, shock wave scenario means that matter around the supernova is heated and ionized by X-rays generated when the blast wave plows through

to a table of supernovae types

Rarer, unusually weak supernova explosions of the Type Iax may let the white dwarf extant. Such supernova are thought to feature a white dwarf and a helium star binary

Miscellaneous

Type II, Ib and Ic supernovae constitute about 80 percent of the supernovae in our Milky Way Galaxy. Pulsar in M1 (the Crab Nebula, the nebula of which is expanding at around 932 miles/s) or in 3C 58 are of type II, type Ib or type Ic. Supernovae are the purveyors of heavy elements in the Universe. At the exception of type IIs which stops at iron, all supernovae are making heavier elements like gold, or uranium. Without the supernovae such elements would not exist in the Universe, hence no planets, and no humans would. The Spitzer Space Telescope, in December 2007, definitively brought the evidence that it's really the supernovae in the early Universe which provided for the dust in the Universe. A nova is another stellar event. Novae are lesser events where the star involved is not destroyed. 'Calcium-rich supernovae' are fast-and-faint ones because they are less luminous than other types of supernovae and also evolve more rapidly, and they reveal spectra dominated by strong calcium lines. Calcium-rich supernovae are often observed at large distances from the nearest galaxy, raising curious questions about their progenitors

->About Tycho's and Kepler's Supernovae
In early November 1572, observers on Earth witnessed the appearance of a new star in the constellation Cassiopeia, an event now recognized as the brightest naked-eye supernova in more than 400 years. It's often called 'Tycho's supernova' after the great Danish astronomer Tycho Brahe, who gained renown for his extensive study of the object. The supernova of 1572 was one of the great watersheds in the history of astronomy. The star blazed forth at a time of astronomy changing and when the starry sky was regarded as a fixed and unchanging part of the universe. The supernova first appeared around Nov. 6, but poor weather kept it from Tycho until Nov. 11, when he noticed it during a walk before dinner. 'When I had satisfied myself that no star of that kind had ever shone forth before, I was led into such perplexity by the unbelievability of the thing that I began to doubt the faith of my own eyes, and so, turning to the servants who were accompanying me, I asked them whether they too could see a certain extremely bright star…. They immediately replied with one voice that they saw it completely and that it was extremely bright,' he recalled. The supernova remained visible for 15 months and exhibited no movement in the heavens, indicating that it was located in the domain of stars, far beyond the Sun, moon and planets. Modern astronomers estimate that the remnant lies between 9,000 and 11,000 light-years away. The Tycho SRN was optically seen expanding between 2000 and 2015. Clumps seen in the Tycho SRN came from the explosion itself. While scientists are not sure how, one possibility is that star's explosion had multiple ignition points
In 1604, a new star appeared in the night sky that was much brighter than Jupiter and dimmed over several weeks. This event was witnessed by sky watchers including the famous astronomer Johannes Kepler. Centuries later, the debris from this exploded star is known as the Kepler supernova remnant. That Type Ia supernova explosion was most powerful. Most Type Ia supernova remnants are very symmetrical, but the Kepler remnant is asymmetrical with a bright arc of X-ray emission in its northern region. The pre-supernova star and its companion might have been moving through the interstellar gas and losing mass at a significant rate via a wind, creating a bow shock or the X-ray arc is caused by debris from the supernova expanding into an interstellar cloud of gradually increasing density. Kepler's supernova lies by about 23,000 light-years from the Earth as its progenitor star held a larger fraction of heavy elements. A large amount of iron indicates a explosion more energetic than the average Type Ia supernova. A small cavity could also have been cleared out around the star before it exploded and produced by a fast, dense outflow from the surface of the white dwarf before it exploded, as predicted by some models of Type Ias. Recent studies in 2013 has shown that the 1604 explosion was triggered by a interaction between a white dwarf and a red giant star, such a couple likely explaining some of the assymetrical features of the supernova. The material during the supernova explosion was expelled in a disk-like structure, with a gas density that is ten times higher at the equator

As much of the energy of the blast of a supernova is dedicated to heat up the bubble of expelled star layers, about 30 percent of the energy goes into accelerating particles there into cosmic rays; the outer layers in a supernova might be spherical, and the inner layers flattened into a disk-shaped form. Those inner layers are responsible for the high-velocity plumes (also said 'jets') of silicon or iron or other metal that are shooting out from the explosion. High-velocity jets in the SRN occurs in multiple directions. The energy which is dedicated to accelerating particles makes that the gas in a SRN is much colder than theory might predict. SRN thus are the main generators of cosmic rays, those charged particles, mostly protons, that come from the interstellar space, beyond the solar system and carrying a large energetic punch. The blast of a supernova wave travels fast – at about six percent the speed of light. CMEs' wave-particle interaction known like Whistler waves might also happen in solar flares

It has been observed in stars of some 200 solar masses that the pressure reigning at the heart of the star is such important that the light emitted by the nuclear reactions is able to generate couples of electrons-antielectrons (antimatter that is!). That is accelerating the star's collapse. Such a process had been theorized since 40 years as one recently only came to get evidences for it. As far as such supernovae are concerned, the light signature is that the cooler, red light is seen first, and then the high-energy blue one only, as the order is inverted for the usual supernovae. Such supernovae too are emitting radioactive nickel as it might that that process be at work in the very first supernovae going off in the early Universe, and maybe too as far as 7 billion years old supernovae are concerned!

Supernovae remnants endure for 100,000 years and affect regions of space thousands of light-years across. The blue and green glow at the edges of the bubble are often X-ray emission from hot gas, heated to millions of degrees by shock waves generated after the explosion. After thousands of years, gas within these stellar may retain the imprint of temperatures of the original explosion, with atoms then stripped away from their electrons are recapturing some in the cloud! A typical evolution of a SRN is that aging stars are sheding material in the form of an outflow called a stellar wind and creating a cocoon of gas and dust. Then the star goes supernova and the blast wave traverses the dense cocoon and heats it to temperatures as high as 100 million degrees F (55 million C), or 10,000 times hotter than the sun's surface. Eventually, the shock wave breaks out into true interstellar space, where it rapidly expands, cooling the electrons and thinning the remnant's gas. Collisions between particles become rare events thus as over thousand of years, the remnant gradually heats up again. Ultraviolet observations in 2012 of SRN Cassiopeia A (or Cas A) which mapped the distribution of elements in the remnant has shown that some elements which were found at the center of the pre-exploding star are now found at the edges of the SRN likely hinting to a strong instability in the explosion process that somehow turned the star inside out! A supernova remnant mass might well be a average of three solar masses as last nuclear reactions in the core of a pre-exploding star might be able to produce clumps of iron

-> About Eta Carinae
At the turn of the 19th century, the binary star system Eta Carinae, at 8,000 light-years from us, was faint and undistinguished as in the first decades of the century, it became brighter and brighter, until, by April 1843, it was the second brightest star in the sky, outshone only by Sirius, a event named the 'Great Eruption.' In the years that followed, it gradually dimmed again and by the 20th century was totally invisible to the naked eye. The star has continued to vary in brightness ever since, and while it is once again visible to the naked eye on a dark night, it has never again come close to its peak of 1843. What had been observed was a stellar near-death experience as the larger of the two stars in the Eta Carinae system is a huge and unstable star that is nearing the end of its life and bound to a supernova explosion. Astronomers call such events supernova impostor events, because they appear similar to supernovae but stop just short of destroying their star. The huge clouds of matter thrown out a century and a half ago, known as the Homunculus Nebula are seen on the picture below. The material from the star was not thrown out in a uniform manner, but forms a huge dumbbell shape. Eta Carinae thus is one of the closest stars to Earth that is likely to explode in a supernova in the relatively near future (though in astronomical timescales that could still be a million years away). When it does, expect an impressive view from Earth, far brighter still than its last outburst. The binary system played a significant role in sculpting the nebula, which now is now about a light-year long and continues to expand. The system contains a pair of massive stars whose eccentric orbits bring them unusually close every 5.5 years as the stars contain 90 and 30 times the mass of our Sun respectively and pass 140 million miles (225 million kilometers) apart at their closest approach. At that moment, their solar winds dramatically interact and accelerate electrons in violent shock waves along the boundary of colliding stellar winds -- a acceleration akin to what occurs during a supernova explosion -- The process is yielding both hard X-rays and gamma-rays, which produce cosmic rays which can reach down to the solar system. The faster wind from the smaller star carves a tunnel through the denser wind of its companion. The winds from each star have markedly different properties: thick and slow for the primary, lean and fast for the hotter companion. Twins to Eta Carinae, containing a high mass star buried in five to 10 solar masses of gas and dust exist in other galaxies. By 2019, a completely new luminous magnesium structure was found in the space between the dusty bipolar bubbles and the outer shock-heated nitrogen-rich filaments. It was ejected in the Great Eruption but hasn't yet collided with the other material surrounding Eta Carinae. It represents the fast and energetic ejection of material that may have been expelled by the star shortly before the expulsion of the bipolar lobes

The Eta Carinae system as seen by the Hubble Space Telescope. picture ESA/NASA

Type Ia Supernovae in Details

As type Ia supernovae are so consistently bright that astronomers refer to them as standard candles used to measure vast cosmological distances, the knowledge of how they are working is of importance further as such supernovae were instrumental for the discovery of the dark energy. White dwarfs are much stable stars and the idea of a large companion star siphoned by the white dwarf gained the support as a merger between two dwarfs might also be the trigger. Astronomers consider that both processes likely exist. Type Ia supernovae result from the interaction inside a binary system -- the stability limit of a white dwarf is about 1.4 solar masses -- or type Ia also may result from the merger of two low-mass white dwarfs. A single white dwarf could also turn a Type Ia supernova. Some supernovas originate from a binary pair of massive stars, as one companion drawing matter from the other, generates instability in the primary star, causing it to episodically blow off a cocoon and shells of hydrogen gas before the supernova explosion. Interaction occurrence is thought to be couples either white dwarf/red giant-headed normal star or two white dwarfs. Recent search are showing how that second type might be prevalent, with two carbon-oxygen white dwarves implied, one of those reaching its so-called 'Chandrasekhar mass limit' which triggers the thermonuclear explosion and a merger between both stars. A flash of ultraviolet may be emitted when supernova’s blast wave slams into and engulfs the companion star. In the first case one of the stars enters its red giant phase and spills gas onto the companion, eventually engulfing it as the system becomes included into a common envelope. This brakes the orbits and when the common envelope is ejected the two stars are closer than at the beginning. The remaining core of the giant becomes a mere white dwarf. And it's when the other star in turn begins to become a red giant spilling material onto the white dwarf that the latter eventually explodes into a supernova. After the event a supernova remnant is seen that is a bubble of gas and material running away from the explosion. The companion star surviving the explosion is ejected away retaining its orbital speed hence moving swiftly. Such Type Ia supernova immerse the area in a cloud of gas that produces a significant source of X-rays after the explosion as a merger of two white dwarf stars result in little or no X-rays after the explosion. Several smaller eruptions on the surface of the white dwarf might clear the region of the couple prior to the supernova, preventing the white dwarf to be immersed in a cloud of gas resulting from the infalling gas. Type Ia supernovae are more symmetric than remnants of supernovas involving the collapse of massive stars. A double central stars may explain the odd shapes of some supernovae remnants as a very lopsided explosion may have produced this Type Ia supernova or the remnant has been expanding into a uneven environment
When the system is two white dwarfs, the stars are braking each other through their gravitational waves. They eventually merge via their carbon nuclei when the resulting fusion tips the scales at around 1.4 times the Sun's mass. It's this merger which triggers the supernova event which is due to the formation of heavier elements and the release of a vast amount of energy. This wave of nuclear fusion rapidly propagates throughout the star, ultimately triggering a supernova explosion. The metallicity of a Type Ia progenitor is likely function of its distance to the crowded center of our Milky Way Galaxy. A type Ia supernova may also result from a white dwarf star growing in mass and exceeded its weight limit, like the case for the Tycho remnant. The extra mass triggers a thermonuclear explosion in any case that blows the dwarf. Astronomers in 2011 were able to determine another type of both the main and companion star in a Ia system, a white dwarf and a star still burning hydrogen at its core rather than a larger red giant. Observations of type Ia are of importance because such supernovae are commonly used as 'standard candles' to probe the expansion of the Universe. As no ultraviolet have been observed about Type Ia supernovae explosions as studied few after the explosion, that leads to that supergiant and even Sun-like stars in a later red giant phase are not present into the Type Ia binary systems. Most recent mounting evidence, by early 2012, suggests that such supernovae are either of the white dwarf -normal star model or two white dwarfs in a binary system eventually spiraling inward and colliding. The rate of Type Ia supernovae due to two white dwarfs generally sharply declines between roughly 7.5 billion years ago and more than 10 billion because most stars in the early Universe are too young to become Type Ia supernovae. Such exploding stars produced about half of the iron in the Universe, the raw material for building planets, and a substantial amount of the material from which planets form, generally, and life also as their study in the early Universe allows to know how quickly the Universe enriched itself with heavier elements. Type Ia supernovae, generally are hydrogen poor for one third of them

Evidence has been mounting recently that the Type Ia explosions are triggered when two orbiting white dwarfs collide, with one notable exception. Kepler's supernova, named after the astronomer Johannes Kepler, is thought to have been preceded by just one white dwarf and an elderly, companion star called a red giant. A second such supernova was found in the roughly 1,000 year-old supernova remnant N103B. Scientists know this because the remnant sits in a pool of gas and dust shed by the aging star. Both the Kepler and N103B explosions are thought to have unfolded as follows: a aging star orbits its companion -a white dwarf. As the aging star molts, which is typical for older stars, some of the shed material falls onto the white dwarf. This causes the white dwarf to build up in mass, become unstable and explode. This scenario may be rare

->Fewer Supernovae in the Milky Way Galaxy!
As it's still ill-known, it looks like there are fewer supernovae exploding in our Milky Way Galaxy than in the other galaxies

->More About a Couple Neutron-Companion Stars
The case of a neutron star/companion star is specific in that they don't explode into a supernova, as the atmosphere of the neutron star only is exploding once filled with the material coming from its companion star, without destroying the star nor the system. A regular X-ray signal ticking every 120 seconds from the gas atoms mixing unto the neutron star surface, is allowing to know, when it slows to 125 seconds, that a binary (neutron/other star) system is to explode. Such a system endures about 7 to 10 bursts per day