The Evolving Universe

Smithsonian Institution

What is a Galaxy?

cute illustration of stars in the form of a galaxy

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A galaxy is a vast island of dust, gas, and millions of stars moving through mostly empty space. Our own Milky Way Galaxy is 100,000 light-years across and contains 200-400 billion stars.

Galaxies began to take shape soon after the birth of the universe, and gravity holds them together. Astronomers estimate that our galaxy settled into its disk shape about 9 billion years ago.