Astronomy Glossary

Astronomy Concepts

Absolute magnitude measures intrinsic luminosity—how bright an object would appear if placed at standard distance of 10 parsecs (32.6 light-years).

Astronomy Concepts

Absorption lines are dark lines in a spectrum where atoms in cooler foreground gas absorbed specific wavelengths from background continuous source.

Galaxies

An Active Galactic Nucleus is a compact, extraordinarily luminous central region of a galaxy powered by accretion onto a supermassive black hole.

Astronomy Concepts

Albedo is the fraction of incoming light reflected by a surface, ranging from 0 (perfect absorber) to 1 (perfect reflector).

Telescopes Equipment

An altazimuth mount (alt-az) moves in altitude (up-down) and azimuth (side-to-side), like a camera tripod.

Units Distance

Angular size is the apparent angle an object spans in the sky as viewed from Earth, regardless of its physical size or distance.

Telescopes Equipment

Aperture is the diameter of a telescope's primary light-collecting element (lens or mirror), the single most important specification determining teles...

Solar System

Aphelion is the point in an object's orbit farthest from the Sun, where orbital velocity is slowest.

Solar System

Apogee is the point in an object's orbit around Earth that is farthest from Earth (analogous to aphelion for Sun orbits).

Astronomy Concepts

Apparent magnitude measures how bright an object appears from Earth, with lower/negative values being brighter.

Units Distance

An arcminute (symbol: ' or arcmin) is 1/60th of a degree, used for medium-scale angular measurements.

Units Distance

An arcsecond (symbol: " or arcsec) is 1/3600th of a degree, the standard small-angle measurement in astronomy.

Solar System

Asteroids are rocky remnants from the Solar System's formation, too small to be rounded by gravity, predominantly found in the asteroid belt between M...

Solar System

The asteroid belt (main belt) is a torus-shaped region between Mars and Jupiter containing hundreds of thousands of rocky bodies, remnants from Solar ...

Units Distance

The average distance from Earth to the Sun.

Units Distance

An Astronomical Unit is the average distance between Earth and the Sun, serving as a fundamental measuring stick for the Solar System.