Can there be more than one black hole in a galaxy?

This artist's conception illustrates one of the most primitive supermassive black holes known (central black dot) at the core of a young, star-rich galaxy. Astronomers using NASA's Spitzer Space Telescope have uncovered two of these early objects, dating back to about 13 billion years ago.  This illustration also shows how supermassive black holes can distort space and light around them (see warped stars behind black hole). Stars from the galaxy can be seen sprinkled throughout, and distant mergers between other galaxies are illustrated in the background. Image credit: NASA/JPL-Caltech

This artist's conception illustrates one of the most primitive supermassive black holes known (central black dot) at the core of a young, star-rich galaxy. Astronomers using NASA's Spitzer Space Telescope have uncovered two of these early objects, dating back to about 13 billion years ago.  This illustration also shows how supermassive black holes can distort space and light around them (see warped stars behind black hole). Stars from the galaxy can be seen sprinkled throughout, and distant mergers between other galaxies are illustrated in the background. Image credit: NASA/JPL-Caltech

Sure!  But it depends on what type of black hole.  There are two main flavors of black hole: supermassive black holes, and stellar mass black holes.  As their names indicate, stellar mass black holes are about the mass of a single star, and supermassive black holes are super massive.

By supermassive, we mean that they’re at least a million suns worth of mass crammed into a very tiny space. (For this kind of thing, astronomers have defined the handy unit of the solar mass: this is the mass of our sun.) They can go up to solar masses of several billion (several billion times the mass of the sun). These are usually the types of black holes people think about when they’re talking about galaxies, and the center of each galaxy should have exactly one of these. If we find a galaxy with more than one supermassive black hole in its center, we have found ourself a very unusual galaxy, which has very likely just devoured another galaxy about as large as itself. The two black holes will eventually fall together and combine to make one, larger, black hole, as the rest of the galaxy recovers from the consumption of another galaxy. We think that supermassive black holes got to be supermassive by gradually going through this process of munching on smaller galaxies and absorbing their black holes over the course of the universe’s lifetime.

Stellar mass black holes, on the other hand, are about as common as dirt. You get a stellar mass black hole any time a large star (usually more than about 8 solar masses) reaches the end of its life and dies in a spectacular supernova fashion. The mass of the star plus the energy of the supernova compresses the remaining star matter down past the density you would need to make a white dwarf or neutron star, and you’ve got yourself a new stellar mass black hole. There are tons of these black holes in the galaxy, but since they’re only as massive as the star that they formed from, they don’t have nearly as big an effect on their surroundings as the supermassive ones do. This makes them much harder to find, so we’re still getting a handle on exactly how many there should be in the galaxy, but the number can be safely rounded to “a lot.”

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