(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")
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"
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
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 clouds :vast zones of interstellar gas and dust, found inside galaxies. Dust clouds are the zones were stars are forming
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
Exoplanets : term designating a planet orbiting a star other than Sun
Galaxy : ensemble of 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
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
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
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-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)
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
Milky Way : the Milky Way is our Galaxy. As the Sun is located inside it, we can not see it from the outside. 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
Moon : accessory object to a planet. A moon, or satellite, is an object smaller than the planet and orbiting around it. On the other hand, absolutely, Moon is the name for Earth's only moon
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
Oort Cloud :vast zone of leftovers of the solar system formation which stretches to about 1 or 2 light-years from the Sun
Orbit :path which a celestial body is following around another
Planetology :science studying planets of the solar system, as exoplanets
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. 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"
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
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
Star : celestial body working on the base of nuclear fusion
Sun : central star to the solar system. Sun is our star
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 : ensemble of all what composes our space surroundings
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