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Last updated on April 16, 2014 at 17:34 EDT

Universe

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 fal...

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 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 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 -- 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 -- 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 -- 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 -- 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 -- 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 -- 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 -- 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 -- 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 -- 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 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 -- 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 -- 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 -- 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 -- 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 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 -- 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...