Basic Definitions in Astronomy

Planet A planet is a celestial body that revolves around a star. A planet gives off no light and is illuminated by its star.

Galaxy Stars are always found in large groups called galaxies. Galaxies have between 1 billion and 1 trillion stars. Our Sun is part of the Milky Way Galaxy. Estimates for the number of stars in the Milky Way Galaxy range from 100 billion to 200 billion stars. Astronomers are constantly increasing their estimates of the number of galaxies; however, present estimates are around 200 billion.

Nebula A nebula is a general term for clouds of gas and dust in space. Dark clouds indicate that the nebula is blocking light from more distant, brighter objects. Some nebulas also emit light by being excited by nearby stars.

Open star clusters Within a galaxy, there are areas where stars concentrate in groups. Open star clusters can have a few dozen to several thousand stars.

Globular cluster Within a galaxy, there are many locations that have a concentration of gas from which many stars develop. Globular clusters can have from a few hundred thousand to several million stars.

Binary stars Binary stars are two or more stars that orbit around a common center of gravity. More than half of all stars are binary stars.

Variable stars Variable stars are stars that change in their brightness over periods of time. Variable stars are classified into various types, including the following: nova, supernova, Cepheid variable, and eclipsing variable.

Red giant Red giant stars are stars that greatly expand during their declining years. When our Sun begins to die, it will expand to become a red giant and its diameter will encompass Earth’s orbit.

White dwarf White dwarf stars occur in the last stage of stellar evolution. After the star has exhausted its nuclear fuel, the star collapses under its own gravity.

Neutron star A neutron star is a remnant of a star that collapses because of gravitational force. It is thought that supernovas result in the formation of neutron stars.

Black holes Black holes are regions in space where the gravitational force is so great that even photons of light cannot escape. It is thought that black holes occur when massive stars collapse. There is no hard evidence that black holes exist.