galaxy

A galaxy is a large astronomical structure containing several billions of stars and other astronomical bodies, looking like a more or less regular spiral with a thick core and thinner edges.

Example: The Milky Way is the galaxy where our Earth is located, it has about 200 billion stars.

The galaxy: the galaxy we live in, the Milky Way (very often employed with the definite article).

Example: Some science fiction writers imagine that mankind will one day settle an empire throughout the galaxy.