What holds together the stars, planets, gases, and dust in a galaxy?
Gravity is the "glue" that holds a galaxy together. The Milky Way galaxy consists of hundreds of billions of stars plus enough gas and dust to make many billions more. Although this material is spread across a disk that measures 100,000 light-years in diameter, there’s just so much of it that its gravitational pull is considerable. And the visible stars and nebulae account for only one-tenth of the galaxy’s total mass; the rest consists of mysterious "dark matter" that surrounds the disk, boosting the galaxy's total gravity.
A supermassive black hole inhabits the core of the Milky Way, but it’s not sucking in the solar system and the rest of the galaxy. It is about four million times as massive as the Sun, which means it accounts for only about .0004 percent of the galaxy’s total mass (when you include the dark matter). Only the stars and gas clouds very close to the black hole feel any appreciable effects from its gravity.
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