Black holes are among the largest and most massive objects in the universe. A black hole is defined as any object whose gravitational pull is so strong that even light cannot escape, which is why they are called black holes. Most black holes form during the death throes of the most massive stars. Black holes, which form from dying stars, are usually less than a hundred times the mass of the sun. However, there are some black holes that are too massive to have formed directly from dying stars. These black holes are called supermassive black holes and can be anywhere from a million times the mass of the sun to over 100 billion times the mass of the sun. What are supermassive black holes and how do they form?
Galactic core
The centers of most large galaxies in the universe contain a supermassive black hole. Image credit: NASA/ESA
Supermassive black holes do not exist everywhere. Rather, they tend to exist only in the centers of large galaxies of galaxies. Even the Milky Way has its own supermassive black hole located at the center. Called Sagittarius A*, it is estimated to be about one million times the mass of the sun and about the same size as our solar system. The majority of large galaxies discovered have a supermassive black hole at their center, and so it seems likely that the presence of supermassive black holes plays an important role in the formation and evolution of galaxies.
Where did supermassive black holes come from?
The first image of a black hole was of the supermassive black hole at the center of a large elliptical galaxy called Messier 87.
How supermassive black holes formed in the early universe and where they came from remains one of the greatest mysteries in astronomy. Their mass suggests that they could not have formed from collapsing stars, but instead must have formed through another process. There are two possible explanations. First, supermassive black holes formed before the first stars. During the first few million years after the Big Bang, the universe was saturated with an enormous amount of hydrogen and helium. Some regions gradually become denser, leading to a process called accretion, where huge amounts of material stick together and increase in mass. With accretion, more and more material is pulled inward as mass and in turn gravity increases. With so much material in the early universe, the process would have been exponential. There may have been enough material to form supermassive black holes directly from the primordial gas of the Big Bang. Over time, these supermassive black holes would attract stars, leading to the formation of the first galaxies.
The second possibility is that supermassive black holes formed from the merger of countless smaller black holes. The first stars in the universe would have been extremely massive, and most, if not all, would have eventually formed black holes. Over time, millions of black holes would merge together, increasing their total mass. If enough black holes merge together, they would eventually form a supermassive black hole. It remains unknown how exactly the first supermassive black holes formed and what their role was in the formation of the first galaxies. As technology advances and telescopes become more powerful, one day we may have an answer.
Aidan Remple August 1, 2022 in Science
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