sentence: Stars are a source of a gravity field.
sentences_after: This is what keeps planets close to them. It is also not unusual for two stars to  orbit each other. This happens when they are close together. This is also because of gravity, in the same way as the Earth orbits the Sun. There are even groups of three or more stars orbiting each other.
concept_name: star
headings: Star Numbers, distances
categories: Astrophysics