Beginners Astronomy Glossary

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Altazimuth

A type of telescope mount that only moves in Altitude and Azimuth.

Astronomical Unit - AU

The mean distance from the Sun to the Earth. Very close to 93,000,000 miles. This measurement is used when discussing distances within the Solar System. It is also used when measuring the distances between close double stars.

Binary Stars

Stars that are gravitationally bound to each other. They rotate around a common center of gravity. There are 3 kinds of binaries: Visual, Spectroscopic and Eclipsing. (see These terms)

Chromatic Aberration

When light is bent by a lens, each wavelength (or color) is bent at a different angle. This will cause an image to be bordered by red on one side, and blue on the other. This is common in refractor type telescopes, and is corrected with various other lenses.

Galaxy

A large aggregations of stars bound together gravitationally. There are three major classifications of galaxies-- spiral, elliptical, and irregular -- and several subclassifications. The sun belongs to a spiral galaxy, the Milky Way galaxy.

Light Year

The distance light travels in one year, which is 5.8 x 10¹² (approx. 6 trillion) miles

Magnitude

The brightness of a star or planet, expressed on a scale in which lower numbers mean greater brightness. Apparent magnitude indicates the brightness of objects as we see them from Earth, regardless of their distance. Absolute magnitude is defined magnitude a star would have if viewed from a standard distance of 10 parsecs . (1 Parsec is 3.26 light years. See below). Each step in magnitude equals a difference of 2.5 times in brightness: the brightes stars in the sky are apparent magnitude 1; the dimmest, 6. The magnitudes of extremely bright objects are expressed in negative numbers. e.g. the apparent magnitude of the Sun is around -26.

Parsec

Abbreviated pc. Another unit of stellar distance, frequently used in place of the light-year, is derived from the Parralax method. This unit is the distance at which a star has a parallax of one second of arc. One Parsec = 3.26 light years, or 206,265 Astronomical Units. The distance to Proxima Centauri, for example, is 1.3 pc.

Precession

The slow (once per 26,000 years) gyration of the earth's axis.

Quasars

Pointlike sources of light whse redshifts indicate that they lie at distances of billions of light-years. Thought to be the nuclei of youg galaxies.

Redshift

Displacement of the spectral lines in light coming from the stars of distant galaxies, thought to be produced by the velocity of the galaxies outward in the expanding universe.

Refractor

A telescope type that has the lens in front and the eyepiece in the back. The light path is straight to the eyepiece. Probably what you think of when you visualize a telescope. See Telescope Types.

Reflector

This type of telescope uses a mirror to focus the image instead of a lens. See Telescope Types.

Spectrum

A record of the distribution of matter or energy (e.g. light) by wavelength. Spectra can be studied to learn the chemical composition and motion of stars and galaxies.

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