Observation Theory Glossary

(note that this glossary is pertaining to both sections "Observation" and "Theory". For a list of definitions pertinent to celestial mechanics, see too the tutorial "Celestial Mechanics Definitions List")

Albedo

the reflecting nature of a celestial body, or the percentage of light its surface is reflecting. A body with a albedo of 1 is reflecting light at 100 percent as, with a albedo of 0, it is reflecting no light and completely dark

Asteroid

minor planet mostly found in the asteroid belt, located between Mars and Jupiter. Asteroids originated as planetesimals, these building blocks of the solar system. Such planetesimals collided, yielding the asteroids. Asteroids are found too in orbits outside the asteroid belt. "Minor planet" is an alternative term for "asteroid"

Asteroseismology

science of studying stellar sound waves with telescopes to understand stars better. Just as Earth’s seismic waves tell us about the inside of Earth, stellar waves -- resulting in vibrations or 'star quakes' -- reveal the secret inner workings of stars

Astronomy

science studying stars and the Universe

Astrophysics

modern aspect of astronomy. Astrophysics is studying deep-sky object on the point of their composition and dynamics

Big Bang

now well-accepted conception of the beginnings of the Universe. Our Universe originated as a minute point seen as a quantum singularity. Expanding swiftly in all directions, such an event created all the particles of matter, then the atoms, then the large-scale structures of the Universe. Universe is still expanding today

Blazar

supersized black hole-powered galaxies called blazars have matter falls toward the supermassive black hole at the galaxy's center, some of it is accelerated outward at nearly the speed of light along jets pointed in opposite directions. When one of the jets happens to be aimed in the direction of Earth, as illustrated here, the galaxy appears especially bright and is classified as a blazar. Gamma rays produced in blazar jets travel across billions of light-years. Blazars appear bright in all forms of light. NASA's Fermi mission by the 2010's brought to increase the number of blazars observed. Blazars are formed from quasars, which wind magnetic fields as they spin at speeds approaching half the speed of light, resulting in high-energy jets that contain the material surrounding the black hole

Bose–Einstein Condensate

a Bose–Einstein condensate (BEC) is a state of matter in a form of a clouds of hundreds of thousands of atoms that, when chilled to just above absolute zero, behave as waves that synchronize into a single quantum object. Gravitaty on Earth usually disperses a BEC within a few seconds. BECs are characterized as a fifth state of matter. The wave nature of atoms is typically only observable at microscopic scales, but BECs make this phenomenon macroscopic, and thus much easier to study. The ultracold atoms all assume their lowest energy state, and take on the same wave identity, becoming indistinguishable from one another. Together, the atom clouds are like a single super atom, instead of individual atoms. The first laboratory BECs were produced in 1995, but the phenomenon was first predicted 71 years earlier by physicists Satyendra Nath Bose and Albert Einstein. On Earth, BECs can only be studied for fractions of a second as microgravity allows scientists to observe individual BECs for five to 10 seconds

Brahe (Tycho Brahe)

Danish astronomer of the 16th century, the observations of the planetary motions of whom allowed his pupil, Kepler, to device his theory of the planetary motion

Comet

icy body orbit of which brings its more or less close to the Sun. In the process, the comet develops a tail. The term "comet" comes from the Latin "comata", f. "long-haired"

Constellation

stars which are looking close to each other in the night sky. Since the oldest Antiquity such groups of stars have been given names and significances

Copernicus

Polish scientist of the 16th century who displaced the center of the solar system from the Earth to the Sun, bringing the first important revolution in the view of our Universe

Cosmology

reflexion about the Universe at large and its origins

Dark Energy

recently discovered part of the energy-mass budget of the Universe. An anti-gravity of sort, dark energy began to overwhelm matter about 7 billion years ago. Since that time, Universe expansion pace is progressively accelerating. Dark energy represents 75 percent of the Universe's energy-mass budget

Dark Matter

recently discovered concept which designates an unseen part of the Universe. Although still unknown in its composition, dark matter accounts for 25 percent of the mass budget of the Universe

Dust (in the solar system)

dust consisting of crushed-up remains from the formation of the solar system like rubble from asteroid collisions or crumbs from blazing comets is dispersed throughout the entire solar system. It mostly collects in areas overlying the orbits of Earth and Venus as another dust ring exists around the Sun at Mercury. The source of the dust ring at Venus’ orbit might be a group of asteroids co-orbiting with the planet as the one at the Earth’s orbit comes largely from the Asteroid belt. A dust-free area is seen just close to the Sun, starting a little over 7 million miles from the Sun

Dust clouds

vast zones of interstellar gas and dust, found inside galaxies. Dust clouds are the zones were stars are forming

Earth (the)

the Earth is our planet, the third in the solar system, from the Sun. The Earth is a balanced system allowing for life, and eventually manking

Ecliptic

the plane of the Earth's orbit around the Sun, as it considered further, like the plane of reference to determine, for example, the inclication of the orbit's plane for the other planets of the solar system

Eddington Limit

the Eddington Limit is defined by a balance between the outward pressure of radiation from the hot gas in the torus of a galactic black hole and the inward pull of the gravity of it

Einstein

German scientist whose theory (the Relativity) is describing the Universe. Relativity surpassed the old Newtonian vision of the Universe. The main base to Einstein theory is that space is bent by matter

Equinox

one the two types of the remarkable positions of a planet on its orbit around the Sun. The equinox is when the poles' axis of the planet are neutral, neither away nor towards, relative to the Sun. There are two equinoxes along the year, determining the spring and fall on the planet. The vernal equinox determines spring (it is also called the 'vernal equinox' as the terms has some further, more technique sense) and the autumnal equinox determines fall

Event horizon

the barrier where a black hole's escape velocity -- the minimum speed at which a object must travel to escape the gravitational pull -- is greater than the speed of light. The faster a black hole spins, the more egg-shaped the event horizon becomes as it warps space-time around it

Exoplanets

term designating a planet orbiting a star other than Sun

Galaxy

ensemble of billion stars. Galaxies are large-scale structures in the Universe. They comprise stars and gas clouds. Most of them have a supermassive black hole at their center

Galaxy Cluster

most of galaxies are found grouped into clusters. Such clusters in turn are grouped into superclusters. Both these large-scale structures are now understood like embedded in a filamentary network of gas and matter encompassing the whole Universe

Galileo Galilei

Italian physicist and astronomer of the 16th and 17th centuries who is considered one of the founders of modern science. In astronomy, he was the first to use an astronomical instrument. Due to that he supported the Copernician view of the solar system, he had troubles with the Roman Catholic church

Gravitational Lensing

Gravitational lensing may be considered a cosmic magnifying lens. That occurs when a massive celestial object like a galaxy or cluster of galaxies is interposing itself between the objet whence the light is coming from -most usually a distant galaxy- and a observer. As the light rays are bent through the mass of the intervening object, the observer is seeing -or photographying- two, or more, distorted images of the distant galaxy. Light is curved through mass according to Einstein's Theory of Relativity. Gravitational lensing allows astronomers to reach further into the early Universe

Gravitational Microlensing Events

Gravitational microlensing events occur when something, such as a star or planet, passes in front of another, more distant star. The passing body's gravity warps the light of the background star, causing it to magnify and brighten. Heftier passing bodies, like massive stars, will warp the light of the background star to a greater extent, resulting in brightening events that can last weeks. Small planet-size bodies will cause less of a distortion, and brighten a star for only a few days or less

Heliosphere

the heliosphere is the domain of the Sun among the interstellar medium. It is, alike the Earth's magnetosphere, a comet-shaped region, with a bow shock in front of it, and a comet tail-shaped form in the trailing direction. The heliosphere is shielding the solar system from what is found in the interstellar medium

HII Region

A HII region is created when the radiation from hot, young stars strips away the electrons from neutral hydrogen atoms (HI) to form clouds of ionized hydrogen (HII)

Hubble Constant

The speed at which the Universe is expanding (measured in km/s per million of light years between each galaxy)

Hypervelocity Star

hypervelocity star usually originate from a binary system; the closer the stars in the system, the faster they orbit about one another and able to reach speeds fast enough to become hypervelocity stars. Hypervelocity stars move, isolated like runaway stars in a galaxy

Inflation

refinement to the Big Bang theory of the Universe. This theory describes how the Universe endured a rapid and enormous increase in size at its very beginnings. The inflation theory is useful as it clarifies numerous characteristics of the Universe

Infrared

part of the electromagnetic spectrum found beyond the visible red color. It is this part of the spectrum that coming science will focus on as dusty places where important objects of the early Universe are forming are best studied in this wavelength

Innermost Stable Orbit

The closest point to a black hole that material can orbit without being irresistibly drawn inwards by the latter's immense mass

Interstellar Medium

all what is inter-stellar, or 'located between stars.' The term mostly is used like a term for the space between star (the 'interstellar space') and for the clouds of gas and dust where the stars are forming, which are called the 'interstellar clouds.' In the interstellar medium there drifts vast clouds of neutral atoms and molecules, as well as charged plasma particles. The interstellar medium pervades any galaxy and the denser the clouds turn, the more they turn the locus to star formation

Kepler

German astronomer of the 16th and 17th centuries. Building upon databases by others of the planetary motions, he deviced an elaborated theory of the planetary motion, with the famed "three Kepler's laws'. The planetary orbits are ellipses with the Sun at one of focuses; the area of the ellipse journeyed by a straight line between the Sun and a planet is proportional to the time used for that; the squared time of the yearly revolution of a planet is proportional to the thirded time of the major axis of the orbit

kilonova

'kilonova' is the other name for the massive explosion caused by a collision of two neutron stars

Kuiper Belt

recently discovered region of the solar system, beyond the orbit of Neptune, where icy planetesimals, leftovers of the formation of the solar system, are found

Light

scientists progressively catched how electric currents generate magnetic forces, and how moving magnets in turn create electric currents and showing a connection between electricity and magnetism. A changing electric field generates a changing magnetic field that generates a changing electric field, and the whole system is light. Light in the Universe is also a way to transfer energy

Light-Year

unit of measure conveniently used in astronomy to qualify the important distances existing in the Universe. 1 light-year is 5.9 trillion miles (9,463 billion km)

Magnetar

magnetars are dense, collapsed stars, or 'neutron stars,' that possess enormously powerful magnetic fields. When magnetars form, they can develop a tough crust as it occasionally cracks. The magnetars are rare, with only some 24 discovered in our Milky Way Galaxy

Magnitude

unit of measure of how bright a star is appearing. Each class of magnitude represents a star brightness 2.512 fainter than the previous. Classes starts at 0 with negative classes for brighter stars

Maser

a maser is a naturally occurring source of intense energy, typically in the microwaves as the emission may arise in the right stimulated physical condition in molecular clouds, comets, planetary or stellar atmospheres, or various other conditions of interstellar space. Megamasers are 100 million times brighter than a maser

Milky Way

the Milky Way is our Galaxy, there were our Sun is located, hence the Earth. As the Sun is located inside the Galaxy, we can not see the latter from the outside. Our Milky Way Galaxy is similar to those well-know, popularized pictures of the Andromeda Galaxy, or the Whirlpool Galaxy The Milky Way, in the sense of the faintly glowing strip seen in the night sky, is a glimpse of our Galaxy. It's our Galaxy seen edge on

Milky Way Galaxy

(see at Milky Way)

Minor Planet

equivalent of Asteroid

Moon

a moon is a celestial object of small size, generally, and orbiting around a planet. Some moons of the gas giants, in the solar system are large enough to be comparable in size with some terrestrial planets

Moon (the)

natural satellite of the Earth, Earth's moon. One thinks now that Moon was created from a gigantic impact which hit Earth 30 to 50 million years after that the Earth was created

Nebula

nowadays usually a interstellar cloud of dust and gas (resulting from the remaining dust and gas from star formation which occurred there) as, until the early 20th century a 'nebula' represented any astronomical object that was not clearly localized and isolated. Hydrogen, the main component to nebula, glows brightly in the characteristic red colour. It may be, or not, associated to one, or more stars at varied stage of their development. The stars' radiation strips electrons of the gas cloud from atoms, a process known as ionisation, and when they recombine they release energy in the form of light. Each chemical element emits light in characteristic colours. X-ray emission, ionised hydrogen and oxygen are part of what a nebula is shining with as a Wolf-Rayet star, old and massive may lie at the center of one. Astronomers make the difference between 'reflection nebulae', which are clouds of interstellar dust, which scatter light from a massive, nearby or embedded star, appearing blue as dust grains more efficiently scatter blue, than red light, and 'emission nebulae', which are a variety of cloud hot enough to emit light by themselves, tending to appear red. Starlight that is more energetic, such as the ultraviolet radiation pouring forth from very hot new stars, can ionise nearby gas, making it emit light of its own. Emission nebulae usually are places where new stars are forming. The reddish glow of hydrogen gas surrounding a cluster is typical of emission nebulas found in massive star-forming regions La couleur rouge de l'hydrogène qui entoure un amas est typique des nébuleuses d'émission des régions massives de formation d'étoiles. Reflection nebulae also, as radiation from new born stars may strip electrons from atoms within the gas, which eventually recombine with other atoms and release light. Mixed in with the gas are dark clumps of dust that absorb rather than emit light, creating weaving lanes and dark shapes across the nebula. 'Dark nebulae' are obscuring clouds of inert material which are only seen by contrast relative to the background of a distant light source as a significant number of the dust particles in it are coated with frozen water, nitrogen, carbon monoxide and other simple organic molecules. Reflection nebulae are termed HII regions when ultraviolet radiation from hot stars ionizes the surrounding gas and dust

Nebula (Wolf–Rayet Nebula)

A Wolf–Rayet nebula is a interstellar cloud of dust, hydrogen, helium and other gases, created when speedy stellar winds originating from a massive, fast living, windy Wolf-Rayet star, interact with the outer layers of hydrogen ejected by the latter. These nebulae are frequently ring-shaped or spherical

NEO

asteroid orbit of which may represent a hazard to Earth. As NEO stands for "Near-Earth Object", the category may encompass comets too. Strictly a grazing asteroid is a NEA (Near-Earth Asteroid)

Newton

English scientist of the 17th century who built a comprehensive view of the solar system working on previous studies. He deviced, in 1672, the reflector, like an astronomical instrument

Oort Cloud

vast zone of leftovers of the solar system formation which stretches to about 1 or 2 light-years from the Sun

Open cluster

a open cluster or stars cluster is group of stars born at almost the same time and place, from a large cosmic cloud of gas and dust, due to shock waves scouring the arms of a galaxy. Typical open clusters can contain anything from a few tens to a few. There are over 1,000 known open star clusters within the Milky Way, with a wide range of properties, such as size and age thousands of stars

Orbit

path which a celestial body is following around another

Parallax

The shift of any object's position relative to the background when observed from two different locations. Stars' parallax is measured each six months when the Earth is 180-degree apart on its orbit as trigonometry allows to determine their distance

Photoionization

photoionization is that oxygen atoms in a gas cloud absorb light from a source light and slowly re-emit it over many thousands of years

Planet

a celestial body, which is usually small and inactive (in terms of energy) and which orbits around a star

Planetology

science studying planets of the solar system, as exoplanets

Proper Motion

The fact that a star's position in the sky is moving over eons, which is due to the star's motion inside the Milky Way Galaxy

Protoplanetary Disk

disk of gas and material which forms in the equatorial plane of a star during the formation process of the latter. Such a protoplanetary disk may further evolves into planets

Quantum Theory

quantum theory is to the particles world what Einstein's Relativity is to the Universe. The main base of quantum theory is that particles behavior may be described in terms of likelihood only

Quasars

very active galaxies found in the early Universe where giant black holes are feeding on nearby gas. A quasar is the visible, energetic signature of a black hole. Black holes cannot be observed directly as they are the energy source at the heart of quasars. Such galaxies are thought to be harbouring supermassive black holes which are outshining them. Such objects might be the proof that galaxies and black holes are developing interacting. 'Quasar' stands for "quasi-stellar object". Gas trapped in the black hole's powerful gravity is compressed and heated to millions of degrees, giving off intense light and/or radio energy. Quasars are typically brighter than a billion suns as most of them quasars lurk in the outer reaches of the cosmos, over a billion light years away. 3C 273, in a giant elliptical galaxy in the constellation of Virgo at 2.5 billion light-years from us and the closest to us was the first quasar ever to be identified in the early 1960s by astronomer Allan Sandage. Super-fast jets are shot into the surrounding space which may reach some 200 000 light-years in length

Redshift

unit of measure mostly used for the farthest objects of the Universe. As the American scientist Hubble demonstrated, all galaxies in the Universe are seen receding from each other, and the further they are from an observer, the faster they seem to recede from him. As when a faraway celestial body is receding from an observer its wavelengths become longer, that is is shifted to the red part of the spectrum, the redshift allows to know of how the wavelength is shifting, hence how quickly the galaxy is receding, hence how far it is

Reflector

astronomical instrument working on the principle of a primary optical device reflecting the incoming light

Refractor

astronomical instrument working on the principle of a primary optical device refracting the incoming light

Resonance

the fact that, between two or more orbiting bodies around a larger one, like moons around a planet, for example, there is a precise ratio for their orbital periods

Rogue Star

a rogue star, generally, or a intergalactic star, is a star moving independently in the intergalactic void

RR Lyrae

RR Lyrae are variable stars which are used for measuring distances in the Universe. With their well-defined luminosities, astronomers compare that to how bright the stars appear in the sky, allowing to their distance easily calculated

Satellite

'satellite' is the term equivalent for moon, as the latter is most often used in the Anglo-Saxon world. see at moon

Satellite (manmade)

manmade satellite is another name for manmade artifical satellite of the space age

Solar system

ensemble composed of the Sun and its nine planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto in this order). The asteroid belt composed of asteroids is part of the solar system too

Solstice

one the two types of the remarkable positions of a planet on its orbit around the Sun. The solstice is when the poles' axis of the planet is the most away, or the most towards the Sun, determining either the winter (by the 'winter solstice'), or summer (by the 'summer solstice') on the planet

Space Conquest

the space conquest is the set of the human techniques and aims which are taking place in the extra-atmospherical space and which began in 1957 when the USRR was the first nation to launch a manmade satellite in orbit around the Earth. The space conquest may limit itself to the activites in the Earth's orbit or engage into the interplanetary exploration, like the one of the Moon or the planets of the solar system. The current technology doesn't allow to extend the space conquest to even the closest stars, as they are featuring most extreme distances

Space Shuttle

re-usable U.S. manned spacecraft which first flew by the early 1980's. It provided the inhabited access to the Earth orbit to the U.S.A, with the ability further to glide back to surface, and to be re-used for other launches

Space Telescope

a astronomical telescope which is embedded into a space satellite structure, and mostly orbiting the Earth. A space telescope may be of the permanent type, like the Hubble Space Telescope, working for an extended period of time, or of the transient type, mostly assimilated to a space mission, having a specific target, and bound to be shut off once its specific observation mission completed. The advantage of a telescope placed in orbit around the Earth is that it avoids the light pollution question, or may gain access to wavelengths which are not making their way down to the Earth's surface

Spacecraft

any manmade satellite orbiting about the Earth, or an interplanetery mission craft, or a manned capsule. 'Probe' may also be used in the case of a interplanetary mission, or 'mission'

Star

a star is a celestial body which usually is of a large size, spherical as it's radiating an energy proper to it, and strong. That energy is yielded by the nuclear fusion processes which are work in the core of the star. A star, generally, finds its origin from the gravitational collapse of the whole -or the part- of a interstellar cloud of gas and dust

Sun (the)

the Sun is our star, the one about which the 9 planets of the solar system -the Earth included- are orbiting

Supernova

form attained by some stars at the end of their life. Various processes have the dying star collapsing and expelling its outside material in a gigantic explosion. Nebulae like the Crab Nebula are the remaining form of a supernovae event. Plural form of "supernova" is "supernovae"

Telescope

in English, term used to designate any optical instrument used for the purpose of astronomical observation

Universe (the)

the set of all the visible or unvisible objects which are forming the global frame of the galaxies and the other groups of celestial objets, and celestial objects. The Universe is to be considered, at the same time, in those terms but in terms of a lapse of time too. The Universe is stretching from its origine, the Big Bang, 13.7 billion years ago, to nowadays. Advanced theories are thinking that the event which gave birth to the Big Bang might have given birth further to other universes, or mushrooming universes, each of those having their own set of laws of Nature