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Active galaxies: Galaxies that have unusually bright centers, possibly because they harbor nurseries of hatching stars, or because they may contain central engines, possibly black holes, that are producing enormous energy.

Angstrom: A unit of measure equal to one ten-billionth of a meter. An angstrom is about the size of two atoms. As many as one million angstroms would fit within the diameter of a human hair. It is used to measure the wavelengths of light and sometimes also to express sizes of tiny objects like viruses and molecules.

Arc minute: 1/60 of a degree. The full moon is about 30 arc minutes, or half a degree, across.

Arc second: A unit of measurement for very small angular sizes; equal to 1/3,600 of a degree, or 1/60 of an arc minute. An arc second is about the smallest angular size typically measurable from a good ground-based observing site.

Cataclysmic variables: These are two stars locked together by powerful gravitational attraction in very close orbits, with typical orbital periods of only three to six hours. One of the stars is a white dwarf, and the other is an average star less massive but otherwise not unlike our own sun. As the normal star loses mass, the white dwarf's gravitational attraction captures this material, which then spirals onto the surface of the white dwarf. These binary star systems are ideal laboratories for observing and understanding the process of accretion because the geometry of the stellar orbits can be determined accurately.

Diffraction grating: A device etched with many fine lines that is used to break light into its component wavelengths, or spectrum.

Electromagnetic spectrum: A term used to describe the entire range of wavelengths of electromagnetic radiation, including gamma rays, X-rays, ultraviolet, visible (optical), infrared and radio radiation. Any part of the electromagnetic spectrum can be categorized according to the size of its wavelengths. The shorter its wavelength, the higher its energy. Visible light is just a small piece of the electromagnetic spectrum. Its wavelengths are from about 4,000 to 7,000 angstroms.

Ionization: the removal of electrons from atoms, creating ions, which have an electrical charge. Ionization can occur when atoms are exposed to powerful electromagnetic radiation (called photoionization), or when atoms are bombarded by particles moving in shock waves or stellar winds (called collisional ionization).

Light year: The distance light travels in one year. Equivalent to about 5.9 trillion miles.

Main sequence stars: the term used to describe stars of all temperatures that shine by fusing hydrogen into helium. This is the most stable period of a star's evolutionary history. Our sun is a main sequence star.

Nebula: A general term describing a diffuse cloud of dust and gas in interstellar space.

Nucleosynthesis: A process whereby lighter elements are fused together, creating the heavier elements in the centers of stars. The only way for astronomers to directly test this phenomenon is by observing the remnants of a supernova's explosion.

Parsec: A unit of distance used by astronomers. It is equivalent to 3.26 light years, or 19.2 trillion miles. A kiloparsec is 1,000 parsecs and a megaparsec is one million parsecs.

Quasars: The most luminous objects in the universe, quasars are extremely distant objects that look like stars when viewed through a conventional telescope, but actually emit much more energy than an entire galaxy. They were common long ago, when the universe was young, and some scientists believe they played a role in the formation of galaxies.

Recombination: The process whereby ionized atoms recapture free electrons, thus changing their state of ionization.

Seyfert galaxies: A form of active galaxy. These are spiral galaxies with very bright, compact cores that produce as much light as an entire normal galaxy of stars, suggesting the presence of a massive black hole in the nucleus.

Spectrograph: An instrument that separates radiation into a spectrum, allowing quantitative measurements. Just as visible light can be separated into its component colors, or wavelengths, ultraviolet light can be separated into a spectrum that can be studied in detail.

Supernova: Although many stars like our sun can remain stable for billions of years, more massive stars can race through their entire life cycles in a relatively short 10 million years or so, ending in a titanic explosion called a supernova that literally tears the aging star apart.

Ultraviolet spectrum: Ultraviolet light has shorter wavelengths than visible light. It is designated by three ranges, the near, far, and extreme. Near ultraviolet light extends from about 2,000 to 4,000 angstroms. Far UV is typically from 912 to about 2,000 angstroms, and extreme UV is the region below 912 and extending down to X-rays.

White dwarfs: Stars that have burned all of their nuclear fuel and shrunk to the size of a planet. These small objects have retained much of their mass, however, resulting in matter so dense that a piece the size of a golf ball would weigh 35 metric tons. Our sun will eventually collapse to a star of this type.

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