Astronomy is a natural science that deals with the study of celestial objects such as stars, planets, comets, nebula, star clusters, and galaxies. Here are some definitions of things that involve astronomy.

Nebula- a interstellar cloud of dust, hydrogen gas, helium gas, and other ionized gasses.

Star- a celestial body of hot gases that radiates energy derived from thermonuclear reactions in the interior.

Planet- any of the nine large celestial bodies in the solar system that revolve around the sun and shine by reflected light; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto in order of their proximity to the sun.

Comet-a relatively small extraterrestrial body consisting of a frozen mass that travels around the sun in a highly elliptical orbit

Constellation-a configuration of stars as seen from the earth