So what are black holes?
Black holes are astronomical objects that have such intense gravity that they absorb all particles and light in their vicinity. This extreme gravity is a result of their extreme density (a large amount of mass in a tiny amount of space) and it bends all light and particle paths inward, making it impossible for anything to escape from black holes.
But there are a couple of other ways to answer the question of “what is a black hole?”. From the point of view of Einstein’s theory of General Relativity, a black hole is a vacuum spacetime with all the mass concentrated in an infinitesimally small region at the center, a so-called singularity. In fact, this was the original, mathematical definition of a black hole, which was accepted by the physics community as early as 1915 as a mathematical construct but rejected as an actual physical object in the Universe. The theory predicts that dense objects can deform spacetime to form a black hole, but even Einstein himself was highly bothered by the singularity and rejected their existence.
Luckily, nature does make black holes, even if coming to that realization took many decades and many different kinds of observations. And perhaps how they appear in the universe is the most interesting definition of what black holes are. They form as the collapsed remnants of what were once massive stars. Upon running out of the fuel that supports them against their own inward pull of gravity, very massive stars implode and contract indefinitely, forming an infinitely dense center. Black holes are surrounded by a virtual surface called the event horizon, which forms the barrier between the black hole’s sphere of influence and the universe outside of it. Since no information can leave from within the event horizon, it is not possible to observe the interior of this region and the singularity at the center is hidden from view.
This artist’s impression depicts a rapidly spinning supermassive black hole surrounded by an accretion disc. This thin disc of rotating material consists of the leftovers of a Sun-like star which was ripped apart by the tidal forces of the black hole. Shocks in the colliding debris as well as heat generated in accretion led to a burst of light, resembling a supernova explosion.
Credit: ESO, ESA/Hubble, M. Kornmesser/N. Bartmann
Why doesn’t every star form a black hole?
During their lifetime, stars use fusion fuel to generate the energy that prevents them from collapsing onto themselves and the light that makes them shine. When they exhaust this fuel, they start to contract inwards under the pull of their own gravity. For small stars like our Sun, this inward collapse can be stopped by the outward pressure of electrons pushing against one another (more precisely called the electron degeneracy pressure) and the collapsed remnant becomes a white dwarf.
For more massive stars, the collapsed remnant becomes even more compact but this time, the pressure generated by degenerate neutrons and interactions of nucleons prevents an endless collapse. But there is a limit to the mass that can be held up by that pressure. For even more massive stars, the contracting mass exceeds this limit and gives rise to a black hole.
How about big black holes?
Black holes come in many sizes. Those formed by collapsing stars are called stellar mass black holes and range from 5 to perhaps 100 times the mass of our Sun. But there are much more massive black holes that we find at the centers of galaxies and they can get to millions or even billions times the mass of the sun. We don’t yet know exactly how they form and how big they are when they’re first born but we do know that their birth must have happened very early in the life of the Universe.
Whether these were individual massive stars that grew quickly or entire clusters of stars that collapsed into black holes all at once, the “supermassive” black holes are ubiquitous in galaxies. Our Galaxy, the Milky Way, has its own supermassive black hole called Sagittarius A* (pronounced Sag A star) at its center. All nearby galaxies we can observe in sufficient detail also have black holes. Collective observations of their properties show that these supermassive black holes not only exist in galaxies but help shape what the host galaxies look like.