aurora australis

Sporadic radiant emission from the upper atmosphere that usually occurs about the North and South magnetic poles of the Earth. Note 1: Auroras are most intense at times of intense ...

astronomical unit (AU)

Astronomy; General astronomy

Approximately equal to the mean earth-sun distance, which is about 150,000,000 km or 93,000,000 miles. Formally, the AU is actually slightly less than the earth's mean distance ...

celestial coordinates

Astronomy; General astronomy

Any system for locating the positions of objects on the celestial sphere. Various systems are defined, according to the chosen point of observation (the origin) and the plane of ...

distance modulus

Astronomy; General astronomy

The difference between the apparent magnitude of an object, m, corrected for *interstellar absorption, and its absolute magnitude, M. This difference is directly linked to the ...

luminosity class

Astronomy; General astronomy

A classification of stars according to their luminosity, which can vary widely for a given *spectral type. Luminosity class indicates, for example, whether a star is a supergiant, ...

asteroid

Astronomy; General astronomy

Any of the many small rocky or metallic objects in the Solar System, mostly lying in a zone (the asteroid belt) between the orbits of Mars and Jupiter; also known as a minor ...

elliptical galaxy

Astronomy; General astronomy

A type of galaxy with a smooth, featureless circular or elliptical appearance, no spiral arms, and little or no interstellar gas or dust; symbol E. Elliptical galaxies range from ...