Stars — State

State some key facts about galaxies, the Milky Way, and how astronomical distances are measured.

(a) A galaxy is a huge collection of many billions of stars, held together by gravity. (b) The Sun is one star within the galaxy known as the Milky Way. (c) The other stars that make up the Milky Way are at much greater distances from the Earth than the Sun is from the Earth. (d) Because such distances are so vast, astronomers measure them in light-years: one light-year is the distance travelled by light through a vacuum (empty space) in one year.