How Do Black Holes Form?

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There is a black hole at the center of this spiral galaxy: Image courtesy of NASA, ESA and the Hubble SM4 ERO Team

Supermassive Black Holes in Galactic Nuclei

In the nuclei, or centers, of galaxies such as our own Milky Way galaxy, stars are much more densely packed than they are near the Sun and other regions outside galactic nuclei. Early in the history of galaxies, the most massive stars in the galactic nuclei exhausted their fuel, exploded as type II supernovas, and formed black holes.

After this, the possibility of stellar collisions continued; if two black holes collide, they merge into a single, more massive, black hole. If a black hole and a star collide, the black hole absorbs the star’s material, and increases in mass.

After many generations of such collisions, supermassive black holes formed in the nuclei of many, perhaps most, galaxies. Astronomers think that there is a supermassive black hole in the core of our own Milky Way galaxy. These supermassive black holes can easily be millions to billions of times as massive as the Sun.

Primordial Black Holes

The original big bang that formed the universe was a violent chaotic event, which created many fluctuations in the density of the matter in the universe. These fluctuations eventually led to the formation of structure, such as galaxies, in the universe.

In the 1970s, Stephen Hawking suggested that some of these density fluctuations might have compressed matter enough to form primordial black holes, with masses less than the masses of typical stellar black holes. Astronomers have found strong evidence for stellar black holes and supermassive black holes – they have not yet, however, found evidence for primordial black holes.

Understanding Black Holes

Black holes form from the death throes of the most massive stars, but the eventual evaporation of a black hole isn’t related to star death. In galactic nuclei, many stellar black holes can merge into a supermassive black hole, possibly even at the center of our own Milky Way Galaxy! In addition to black holes and supermassive black holes, the theoretic primordial black holes suggested by Hawking may have formed during the Big Bang, but astronomers haven’t found them yet. As science develops, however, our abilities to explore the universe grow – it may be only a matter of time before we find evidence to support or refute black hole theories.

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