Universe
Variable Star
Variable Star -- Most stars are of nearly constant luminosity. Our own Sun is a good example which goes through practically no measurable variation in brightness. There are, however, stars which do vary in brightness, called variable stars. They fa...
White Dwarf
White Dwarf -- A white dwarf is a a star supported by electron degeneracy. A star like our Sun will become a white dwarf when it has exhausted its nuclear fuel. Near the end of its nuclear burning stage, such a star goes through a red giant phase a...
X-ray Binaries
X-ray Binaries -- X-ray binaries are a class of binary stars that are very luminous in X-rays. The X-rays are produced by matter falling from one component (usually a relatively normal star) to the other component, which is a neutron star or a blac...
X-ray Burster
X-ray Burster -- X-ray bursters are a class of binary stars which are luminous in X-rays. They contain a neutron star and a low-mass companion star. The companion fills its Roche lobe and therefore the neutron star is accreting matter from it. The ...
X-ray Pulsar
X-ray Pulsar -- This dramatic artist's vision shows a city-sized neutron star centered in a disk of hot plasma drawn from its enfeebled red companion star. Ravenously accreting material from the disk, the neutron star spins faster and faster emitti...
Nebula
Nebula -- in astronomy, observed manifestation of a collection of highly rarefied gas and dust in interstellar space. Prior to the 1960s this term was also applied to bodies later discovered to be galaxies, e.g., the so-called Great Nebula in the c...
Nova
Nova -- A nova is an enormous nuclear explosion caused by the accretion of hydrogen onto the surface of a white dwarf star. When a white dwarf has a close companion star, the companion will often begin to have its outer atmosphere drawn away from i...
Open Cluster
Open Cluster -- An open cluster is a group of star formed from a molecular cloud, the illuminated parts of which we see as one or more nebulae. They are also called galactic clusters since they exist within the galaxy. All the stars in an open clus...
Orbit
Orbit -- An orbit is the path that an object makes around another object under the influence of some force. The classical example is that of the solar system, where the Earth, other planets, asteroids, comets, and smaller pieces of rubble are in or...
Photosphere
Photosphere -- The photosphere of an astronomical object is the region at which the optical depth becomes one. In other words, the photosphere is the place where an object stops being transparent. It is typically used to describe the Sun or another...
Planet
Planet -- A planet is a body of considerable mass that orbits a star and that doesn't produce energy through nuclear fusion. Until recently, only nine were known (all of them in our own Solar system). As of the end of 2002 over 100 are known, with ...
Planetary Nebula
Planetary Nebula -- A planetary nebula is an astronomical object that usually appears nebulous and disk-like in low-resolution observations. Because of this appearance, similar to the appearance of planets in early observations, the "planetary" adj...
Planetary Ring
Planetary Ring -- A planetary ring is a ring of dust and other small particles orbiting around a planet in a flat disc-shaped region. The most spectacular and famous planetary rings are those around Saturn, but all four of the solar system's gas gi...
Precession
Precession -- Precession is the phenomenon by which the axis of a spinning object "wobbles" when a torque is applied to it. The phenomenon is commonly seen in a spinning toy top, but all rotating objects can undergo precession. As a spinning ob...
Pulsar
Pulsar -- A pulsar, which originally stood for pulsating radio source, is a rapidly rotating neutron star, whose electromagnetic radiation is observed in regularly spaced interval, or pulses. Pulsars are closely related to magnetars, the main diffe...
Quasar
Quasar -- A quasar (from quasi-stellar radio source) is an astronomical object that looks like a star in optical telescopes (i.e. it is a point source), but has a very high redshift. The general consensus is that this high redshift is cosmological,...
Red Dwarf
Red Dwarf -- A red dwarf is a small star on the main sequence, either late K or M spectral type. They have a diameter and mass of less than one-third that of the Sun (down to 0.08 solar masses, which are Brown dwarves) and a surface temperature of ...
Red Giant
Red Giant -- A red giant is a large star of spectral class K or M which has evolved off the main sequence. They are believed to be stars of solar mass or higher which have exhausted the supply of hydrogen in their cores and started burning hydrogen...
Redshift
Redshift -- Redshift is the phenomenon that the frequency of light when observed, under certain circumstances, can be lower than the frequency of light when it was emitted at the source. This usually occurs when the source moves away from the obser...
Reflection Nebula
Reflection Nebula -- In astronomy, reflection nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars. The nearby star or stars are not hot enough to cause ionization in the gas of the nebula like in emission nebu...
Ring Galaxy
Ring Galaxy -- A subclass of interacting galaxies, ring galaxies, provides a unique laboratory for studying unusually large bursts of non-nuclear star formation. The rings in these systems are often large (10s of kiloparsecs) and contain what appea...
Seyfert Galaxy
Seyfert Galaxy -- Seyfert galaxies are spiral or irregular galaxies containing an extremely bright nucleus, most likely caused by a supermassive black hole, that can sometimes outshine the surrounding galaxy. The light from the central nucleus vari...
Solar Nebula
Solar Nebula -- In astronomy, the solar nebula is the gaseous cloud from which, in the so-called nebular hypothesis of the origin of the solar system, the Sun and planets formed by condensation. In 1755 the German philosopher Immanuel Kant suggeste...
Space
Space -- The definition of space in physics is contentious. Various concepts used to try to define space have included: -- the structure defined by the set of "spatial relationships" between objects -- a manifold defined by a coordinate system ...
Spacetime
Spacetime -- In special relativity and general relativity, time and three-dimensional space are treated together as a single four-dimensional manifold called spacetime (alternatively, space-time; see below). A point in spacetime may be referred to ...
Spiral Galaxy
Spiral Galaxy -- Among the galaxies, there are apparently three main categories, according to their appearance: the disk galaxies (`cosmic frisbies' according to P. Murdin, D. Allen, and D. Malin), consisting of a huge disk of stars and interstellar ma...
Star
Star -- A star is a self-gravitating sphere of plasma in hydrostatic equilibrium that generates energy in its interior through the process of nuclear fusion. Energy from this process radiates into space as electromagnetic radiation and neutrinos. ...
Star Cluster
Star Cluster -- Star clusters are physically bound systems of stars. In order of low compactness to high compactness (and in some sense also age) they range from stellar associations to open clusters to globular clusters. Star clusters are held tog...
Star Designation
Star Designation -- The International Astronomical Union (IAU) is the internationally recognised authority for assigning designations to stars (and other celestial bodies). Many of the star names in use today were inherited from the time before the...
Star Catalogue
Star Catalogue -- In astronomy, many stars are referred to simply by catalogue numbers. There are a great many different star catalogues which have been produced for different purposes over the years, and this article covers only some of the more frequ...
Star Formation
Star Formation -- Star formation is the process by which gas in molecular clouds gets transformed into stars. In the current paradigm of star formation, cores of molecular clouds (regions of specially high density) became gravitationally unstable, ...
Stellar Evolution
Stellar Evolution -- Stellar evolution is the process of formation, life, and death of stars. It is one of the major topics of cosmogony. Star Birth and Life A star starts out as an enormous cloud of gas and dust many light-years across. Star...
Strange Matter
Strange Matter -- Strange matter (also known as quark matter) is an ultra-dense phase of matter that is theorized to form inside particularly massive neutron stars (which are then known as "strange stars" or "quark stars"). It's theorized that when...
Supergiant Star
Supergiant Star -- A star of 15 solar masses exhausts its hydrogen in about one-thousandth the lifetime of our sun. It proceeds through the red giant phase, but when it reaches the triple-alpha process of nuclear fusion, it continues to burn for a ...
Supermassive Black Hole
Supermassive Black Hole -- A Supermassive black hole is a black hole with a mass in the range of millions or billions solar masses. A supermassive black hole has some interesting properties differing from his low-mass cousins: -- The average ...
Supernova
Supernova -- A supernova is a star that increases its brightness drastically within a matter of days, making it appear as if a "new" star was born (hence "nova"). The "super" prefix distinguishes it from a mere nova, which also involves a star incr...
Supernova Remnant
Supernova Remnant -- A supernova remnant (SNR) is made up of the materials left behind by the gigantic explosion of a star in a supernova. There are two possible routes to this end: either a massive star may cease to generate fusion energy in its c...
T Tauri
T Tauri -- T Tauri stars are a class of stars thought to represent extremely young pre-main sequence stars, in an early stage of life. They are seen near many molecular clouds in our galaxy. The first ones were found in 1945, identified by their op...
Terrestrial Planet
Terrestrial Planet -- A terrestrial planet is a planet that is mostly composed of silicate rocks and may or may not have a relatively thin atmosphere. The term is derived from the Greek word for Earth, so an alternate definition would be those plan...
Triple Star
Triple Star -- A triple star system consists of three gravitationally bound stars. The stars are in orbits around a common center of mass, usually so that two of the stars form a close binary star and the third is further away. This configurati...
Accelerating Universe
Accelerating universe -- In the late 1990s, observations of type I supernova produced the unexpected result that the expansion of the universe appears to be accelerating. These observations appear more firm as new data has appeared. This means that...
Accretion Disk
Accretion Disk -- An accretion disk is a structure formed by material falling into a gravitational source. Conservation of angular momentum requires that, as a large cloud of material collapses inward, any small rotation it may have will increase. ...
Active Galaxy
Active Galaxy -- An active galaxy is a galaxy where a significant fraction of the energy output is not emitted from normal stellar populations or interstellar gas. This energy, depending on the active galaxy type, can be emitted across most of the ...
Asteroid
Asteroid -- An asteroid, also called a minor planet or planetoid, is a member of a group of small, planet-like bodies that are part of our solar system. They are believed to be remnants of the interstellar clouds, nebula, that were not incorporated int...
Aurora
Aurora -- The Polar Aurora are natural displays of light in the sky that can be seen with the unaided eye only at night. An auroral display in the Northern Hemisphere is called the aurora borealis, or the northern lights; in the Southern Hemisphere it ...
Big Bang
Big Bang -- The Big Bang theory is the dominant theory in cosmology about the early development and current shape of the universe. According to this theory, the universe expanded rapidly starting 13.7 � 0.2 billion years ago. Extrapolating the...
Binary Star
Binary Star -- A binary star system consists of two stars both orbiting around a gravitational point between them. The term "binary star" was apparently first coined by Sir William Herschel in 1802 to designate "a real double star - the union of tw...
Black Hole
Black Hole -- Black holes are objects so dense that not even light can escape their gravity. They are believed to form from the gravitational collapse of astronomical objects containing two or more solar masses. Astronomical observations suggest th...
Cataclysmic Variable Star
Cataclysmic Variable Star -- Cataclysmic variables are a class of binary stars containing a white dwarf and a companion star. The companion star is usually a red dwarf, although in some cases it is another white dwarf or a slightly evolved star (subgia...
Chromosphere
Chromosphere -- The chromosphere (literally, "color sphere") is a thin layer of the Sun's atmosphere just above the photosphere, roughly 10,000 kilometers deep. The chromosphere is more visually transparent than the photosphere. The most common sol...
Circumpolar Star
Circumpolar Star -- Circumpolar stars are those stars which are located near the poles of the celestial sphere. As the Earth rotates, the sky appears to rotate; and most stars will be hidden below the horizon at some point in their circular paths. ...
Comet
Comet -- A comet is a small body from the outer reaches of the solar system similar to an asteroid but composed of ice. Often described as "dirty snowballs," they are composed largely of carbon dioxide ice, methane ice, and water ice with a mixture of ...
Constellation
Constellation -- A constellation is a group of stars visibly related to each other in a particular configuration. In three-dimensional space, most of the stars we see have little relation to one another, but can appear to be grouped on the imaginar...
Corona
Corona -- The corona is the luminous "atmosphere" of the Sun extending millions of kilometers into space, most easily seen during a total solar eclipse. An interesting feature of the corona is the fact that it is much hotter than the visible "surfa...
Cosmic Background Radiation
Cosmic Background Radiation -- The Big Bang theory predicts that the early universe was a very hot place and that as it expands, the gas within it cools. Thus the universe should be filled with radiation that is literally the remnant heat left over fro...
Inflation
Inflation -- Inflation is the idea - first proposed by Alan Guth (1981) - that the nascent universe passed through a phase of exponential expansion that was driven by a negative vacuum energy density (positive vacuum pressure). This expansion can b...
Cosmic Rays
Cosmic Rays -- Cosmic Rays. Cosmic rays are energetic particles that are found in space and filter through our atmosphere. Cosmic rays have interested scientists for many different reasons. They come from all directions in space, and the originatio...
Cosmological Constant
Cosmological Constant -- The cosmological constant (usually denoted by the Greek capital letter lambda: Λ) is a value occurring in Einstein's theory of general relativity. The units of Λ are 1/second2; its value is unknown but believed to b...
Crater
Crater -- A crater is a circular depression on the surface of a planet, moon, asteroid, or other celestial body. Craters are typically caused by meteorite impacts, although some are caused by volcanic activity. In the center of craters on Earth a c...
Quintessence (Dark Energy)
Quintessence (Dark Energy) -- Quintessence or dark energy is a hypothetical form of energy postulated to exist in order to explain observations of an accelerating universe. This energy would act like a vacuum pressure, pushing things apart. Other a...
Doppler Effect
Doppler Effect -- The Doppler effect is the apparent change in frequency or wavelength of a wave that is perceived by an observer moving relative to the source of the waves. For waves, such as sound waves, that propagate in a wave medium, the veloc...
Double Star
Double Star -- When two stars are so nearly in the same direction as seen from Earth that they appear to be a single star to the naked eye but may be separated by the use of telescopes, they are referred to as a double star. There are two different...
Drake Equation
Drake Equation -- The Drake equation (also known as the Green Bank equation) is a famous result in the speculative fields of xenobiology and the search for extraterrestrial intelligence. This equation was devised by Dr. Frank Drake in the 1960s in ...
Eclipse
Eclipse -- An eclipse occurs when an astronomical body such as a planet, or satellite gets between a source of light (e.g. the Sun) and another body. For instance, Jupiter eclipses its moons when it gets between them and the Sun. -- Lunar eclipses ...
Ecliptic
Ecliptic -- The ecliptic is the geometric plane that contains the orbit of the Earth. The orbits of most planets in the Solar System lie very close to it. Seen from the Earth, this is the great circle on the celestial sphere that contains the diffe...
Electromagnetic Radiation
Electromagnetic Radiation -- Electromagnetic radiation is a combination of oscillating electric and magnetic fields propagating through space and carrying energy from one place to another. Light is a form of electromagnetic radiation. The theoretic...
Electromagnetic Spectrum
Electromagnetic Spectrum -- The electromagnetic spectrum describes the various types of electromagnetic radiation based on their wavelengths. Radio, representing wavelengths from a few feet to well over a mile, is at one end of the spectrum. Gamma ...
Elliptical Galaxy
Elliptical Galaxy -- In astronomy, one of the main classes of galaxy in the Hubble classification and characterized by a featureless elliptical profile. Unlike spiral galaxies, elliptical galaxies have very little gas or dust and no stars have rece...
Emission Nebula
Emission Nebula -- In astronomy, emission nebulae are clouds of ionized gas, emitting light of various colors. The most common source for ionization are high-energy photons emitted from a nearby young, hot star. Usually, a young star will ionize pa...
Escape Velocity
Escape Velocity -- An escape velocity is the minimum speed at which an object without propulsion can move away from a source of a gravitational field indefinitely if there is no friction. This definition may need modification for the practical prob...
Event Horizon
Event Horizon -- The event horizon is a boundary beyond which information will never reach an observer. An event horizon is a mathematical construct and not a physical object and a person passing through an event horizon will not notice any odd beh...
Extraterrestrial Life
Extraterrestrial Life -- Extraterrestrial life is life beyond planet Earth (apart from humans travelling in space, and living organisms they bring along or send). There are many questions about extraterrestrial life, including: -- Does it exist...
Extrasolar Planet
Extrasolar Planet -- An extrasolar planet is a planet orbiting around a star other than the Sun. Extrasolar planets were first discovered in the 1990s as a result of improved telescope technology, CCD and computer-based image processing which allowed f...
Galaxy
Galaxy -- Stars are almost always found in collections called galaxies, together with gas, dust, and large amounts of dark matter detected by its gravitational effects. These are all held together by gravitational attraction and orbit a common centre. ...
Gamma-Ray Burst
Gamma-Ray Burst -- In astronomy, Gamma-ray bursters (GRBs) are flashes of gamma rays that last from seconds to hours, the longer ones being followed by several days of X-ray afterglow. They occur at random positions in the sky several times each da...
Gas Giant
Gas Giant -- A gas giant is a generic astronomical term invented by the science fiction writer James Blish to describe any large planet that is not composed mostly of rock or other solid matter. Gas giants may still have a solid core - in fact, it ...
General Relativity
General Relativity -- General Relativity is the common name for the theory of gravitation published by Albert Einstein in 1915. According to general relativity the force of gravity is a manifestation of the local geometry of spacetime. Although the...
Geocentric Model
Geocentric Model -- The geocentric model of the cosmos is a paradigm which places the Earth at the center of the universe. Common in ancient Greece, it was believed by both Aristotle and Ptolemy. Most Greeks assumed that the Sun, Moon, stars, and plane...
Geodesic
Geodesic -- In mathematics and specifically in differential geometry, a geodesic is a path that furnishes shortest paths between any points on it that are close enough together. The most familiar examples are the straight lines in Euclidean geometr...
Globular Cluster
Globular Cluster -- A globular cluster is a cluster of stars that is spherical in shape and extremely dense towards its core. Globular clusters are usually composed of hundreds of thousands of old stars, similar to the bulge of a spiral galaxy but ...
Gravitational Lens
Gravitational Lens -- A gravitational lens is formed when the light from a very distant, bright object (such as a quasar) is "bent" around a massive object (such as a massive galaxy) between the bright object and the viewer. The process is known as gra...
H II Region
H II Region -- An H II region is an emission nebula associated with hot, young, blue stars, and star forming regions. H II, or singly-ionized hydrogen, is nebular gas ionized by ultraviolet light emitted by these hot, young stars of spectral type O...
Heliopause
Heliopause -- The heliopause is the boundary where our Sun's solar wind is stopped by the interstellar medium. The solar wind blows a "bubble" in the interstellar medium (the rareified hydrogen and helium gas that permeates the galaxy). The point w...
Hypernova
Hypernova -- A hypernova is a theoretical type of supernova produced when exceptionally large stars collapse at the end of their lifespan. In a hypernova, the core of the star collapses directly into a black hole and two extremely energetic jets of...
Interstellar Cloud
Interstellar Cloud -- Interstellar cloud is the generic name given to accumulations of gas and dust in our galaxy. Depending on the density, size and temperature of a given cloud, the hydrogen in it can be neutral (HI clouds) or molecular (molecular cl...
Irregular Galaxy
Irregular Galaxy -- In astronomy, a class of galaxy with little structure, which does not conform to any of the standard shapes in the Hubble classification. The two satellite galaxies of the Milky Way, the Magellanic Clouds, are both irregulars. ...
Lenticular Galaxy
Lenticular Galaxy -- The lenticular galaxies are disk galaxies without any conspicuous structure in their disks. This is probably because they have either used up most of their interstellar matter, so that they consist of old stars only, which have...
Magnetar
Magnetar -- A magnetar is a neutron star with a strong magnetic field. The theory around these objects was formulated by Robert Duncan and Christopher Thompson. When in a supernova a star collapses to a neutron star, its magnetic field increases dr...
Massive Compact Halo Object (MACHO)
Massive Compact Halo Object (MACHO) -- Massive compact halo objects, or MACHOs, are a type of astronomical body proposed as one possible explanation for the presence of dark matter in galactic halos. A MACHO is a small chunk of normal baryonic matt...
Meteorite
Meteorite -- A meteorite is a relatively small extraterrestrial material body that reaches the Earth's surface. While in space these bodies are called meteoroids. Upon entering the atmosphere air drag and friction will cause the body to heat up, em...
Microquasar
Microquasar -- Microquasars are smaller cousins of quasars. They are named after quasars, as they have some common characteristics: strong and variable radio emission often seen as radio jets, and an accretion disk surrounding a black hole. In quas...
Molecular Cloud
Molecular Cloud -- Molecular clouds are interstellar nebulae that have a density and size sufficient to permit the formation of H2, molecular hydrogen. However, this molecule is difficult to detect, and the molecule most used to trace the H2 is CO (car...
Multiverse
Multiverse -- The term Multiverse was invented in December 1960, by Andy Nimmo, then vice chairman of the British Interplanetary Society, Scottish Branch, for a talk on the Everett many-worlds interpretation of quantum physics which had been published ...