| Term | Definition |
|---|
| Binary star | two stars orbiting a common center |
| Black dwarf | The remnant of a white dwarf after it has cooled down. It has very low luminosity. |
| Black hole | A singularity in space-time; the end result in the evolution of a very massive star. |
| Brown dwarf | Gas and dust that did not reach high enough temperatures to initiate fusion. These objects continue to compact and cool down. |
| Cepheid variable | A star of variable luminosity. The luminosity increases sharply and falls off gently with a well-defined period. The period is related to the absolute luminosity of the star and so can be used to estimate the distance to the star. |
| Clusters of galaxies | Galaxies close to each other and affecting each other gravitationally, behaving as one unit. |
| Comet | A small body (mainly ice and dust) orbiting the sun in an elliptical orbit. |
| Constellation | A group of stars in a recognizable pattern that appear to be near each other in space. |
| Dark matter | Generic name for matter in galaxies and clusters of galaxies that is too cold to radiate. Its existence is inferred from techniques other than direct visual observation. |
| Galaxy | A collection of a very large number of stars mutually attracting each other through the gravitational force and staying together. The number of stars in a galaxy varies from a few million in dwarf galaxies to hundreds of billions in large galaxies. It is estimated that 100 billion galaxies exist in the observable universe. |
| Interstellar medium | Gases (mainly hydrogen and helium) and dust grains (silicates, carbon and iron) filling the space in between stars. The density of interstellar mass is very low. There is about one atom of gas in every cubic centimeter of space. The density of dust is a trillion times smaller. The temperature of the gas is about 100 K. |
| Main sequence star | A normal star that is undergoing nuclear fusion of hydrogen into helium. Our sun is a typical main sequence star. |
| Neutron star | If a red giant is very large (a supergiant), the end result of the explosion throwing off mass will be a star even smaller than a white dwarf (a few tens of kilometers in diameter) and very dense. This is a star consisting almost entirely of neutrons. The neutrons form a superfluid around a core of immense pressure and density. A neutron star is an astonishing macroscopic example of microscopic quantum physics. |
| Nova | The sudden increase in luminosity of a white dwarf caused by material from a nearby star falling into the white dwarf. |
| Planetary nebula | The ejected envelope of a red giant star. |
| Pulsar | A rapidly rotating neutron star emitting electromagnetic radiation in the radio region. Pulsars have very strong magnetic fields. Periods of rotation vary from a few milliseconds to seconds. |
| Quasars | Powerful energy emitters. These are very active cores of young galaxies. The name stands for quasi-stellar radio-emitting objects, a name given since the first observations of quasars indicated a small, stellar-like size. The energy output from a quasar is greater than that of hundreds of galaxies combined. From redshift measurements, quasars are known to move away from us at very high speeds. |
| Red dwarf | A very small star with low temperature, reddish in color. |
| Red giant | A main sequence star evolves into a red giant - a very large, reddish star. There are nuclear reactions involving the fusion of helium into heavier elements. |
| Stellar cluster | A group of stars that are physically near each other in space, created by the collapse of the same gas cloud. |
| Supernova | The explosion of a red supergiant star. The amount of energy emitted in a supernova explosion can be staggering - comparable to the total energy radiated by our sun in its entire lifetime. |
| White dwarf | A red giant at the end stage of its evolution will throw off mass and leave behind a very small (the size of the earth), very dense star in which no nuclear reactions take place. It is very hot but its small size gives it a very low luminosity. |
| light year | The distance traveled by light in one year. |
| Luminosity | The amount of energy radiated by a star per second, i.e., the power radiated by the star. |
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