Key Terms in Astronomy

Astronomy is the science of things in outer space, from stars and planets to galaxies and other universes. Astronomy also deals with the large-scale properties of the Universe, including its laws and constants. Astronomy is distinct from astrology, which studies the placement of stars and constellations in the sky.

While astronomy and astrology are separate scientific fields, there are some terms that are commonly used in both. These include:

The process by which stars and other celestial objects rotate on their axes, or spin. Rotation can be caused by several factors, including the effects of gravity and other forces. The movement of the Earth around the Sun is an example of rotation.

A term for the brightness or color of a star, planet or other object in space. Albedo is a number that ranges from 0 to 1. The higher the number, the lighter an object. A white star has an albedo of 1, while a dark planet such as Mars has an albedo of 0.

The size of the primary optical surface in a telescope, usually expressed in inches, centimeters or meters. The aperture is important because larger telescopes can capture more light. A telescope with a larger aperture is more expensive to build and operate than one with a smaller aperture. Most amateur and professional telescopes use a reflector, while high-tech space observatories often use a refracting telescope.

An area of the Universe that contains galaxies and other structures such as nebulae and black holes. Astronomers can detect the presence of a galaxy by its shape, brightness and color. They can also determine its velocity by measuring the distance between it and other structures in the Universe, such as black holes.

A ring-shaped disk of dust and debris in orbit around a star. Debris disks are the next stage in planetary system formation, after the protoplanetary disk phase. They can be formed by the collision of small planetesimals, or as a result of the evaporation of a star’s core.

The point (distance and time) at which a planet or comet is at its furthest from the Sun in its elliptical orbit. Astronomers use the apoapsis to help them locate these objects in the sky.

A branch of astronomy that deals with the interaction between matter and electromagnetic radiation, such as radio waves, visible light and infrared light. This subfield of astronomy includes areas such as astrophysics, cosmology and particle physics. Astrophysics is closely related to astronomy, and many astronomers are also astrophysicists. The main difference is that astronomy uses observations to study objects in the Universe, while astrophysics uses principles of physics to understand those same objects. Astronomers who specialize in observational astronomy may still have an interest in some aspects of astrophysics, however. For instance, they may document a star that appears to be dimming and describe how this could happen using the laws of physics. This article was created by the IAU Office of Astronomy for Education and distributed under a Creative Commons CC BY 4.0 license.