A galaxy is a huge system of of stars and solar system containing millions or billions of them. The name of our galaxy is "Milky Way".


Our Milky Way is a barred spiral galaxy containing over 100 billion stars.  The spiral disk of stars, gas, and dust is about 100,000 light-years across and 2,000 light-years thick — flatter than a pancake. The central bulge of stars is elongated in the shape of a bar.
Galaxies show a range of shapes that astronomers group into three basic classes: spiral, elliptical, and irregular.

Astronomers aren't certain exactly how galaxies formed. After the Big Bang, space was made up almost entirely of hydrogen and helium. Some astronomers think that gravity pulled dust and gas together to form individual stars, and those stars drew closer together into collections that ultimately became galaxies. Others think that the mass of what would become galaxies drew together before the stars within them were created. Astronomers are also refining their techniques of measuring the mass of individual galaxies, such as this 2018 study that used the three-dimensional movements of several galaxies to better narrow down the Milky Way's mass.

Most galaxies have black holes at their centers that can produce a tremendous amount of energy, which astronomers can see over great distances. In some cases, a galaxy’s central black hole is extremely large or active, even in relatively small galaxies. Material circling the black hole may be accelerated outward by its jets. Other galaxies may contain quasars — the most energetic bodies in the universe — at their core.


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