Stellar-mass black holes — which weigh between a few and 100 times the mass of the Sun — speckle the universe. In our Milky Way alone, there are an estimated ten million to one billion stellar-mass black holes. That sounds like a lot, until you consider there are an estimated 100 to 400 billion stars in our galaxy.
But what exactly are stellar-mass black holes? And how do these mysterious voids in space differ from their supersized cousins?
Created by destruction
Not every star has the potential to become a black hole; only the most massive reach this coveted status. The smallest stellar-mass black holes come from stars packed with at least 2 to 3 times the mass of our Sun. (If you’re wondering, our petite Sun is too small to collapse into a black hole and instead will one day become a white dwarf).
Stars in the primes of their lives, like the Sun, burn hydrogen in their cores through a process known as nuclear fusion. This converts the hydrogen to helium and creates an outward pressure that counteracts the inward force of gravity. Following this hydrogen-burning phase, the most massive stars are hot enough to burn through their helium (just like less massive stars), then carbon, neon, oxygen, and, finally, silicon. After silicon, however, the star’s core is basically a hunk of iron, at which point no further energy can be unlocked through nuclear fusion. At this point, the inward crush of gravity has the upper hand.
In the most basic sense, the outer shell of the star, with no internal pressure to support it, implodes. For stars slightly more massive than the Sun, those collapsing outer layers rebound off the star's core, detonating it as a supernova. But in the case of the most massive stars, nothing can stop the crushing collapse. Such stars are destined to become stellar-mass black holes upon their deaths.
But stellar old age isn’t the only way to form a black hole.
A white dwarf or neutron star remnant from a smaller star can also become a stellar-mass black hole, but it needs some help. It must syphon enough material from a nearby binary companion that it eventually climbs about the mass threshold needed to collapse into a black hole. Alternatively, the merger of a binary neutron star system could also create an object too massive to sustain itself as anything except a black hole.
There are also supermassive black holes, which weigh in at millions to billions of times the mass of the Sun. These gravitational Goliaths reside in the centers of most, if not all, galaxies. But although they are well documented, exactly how they first formed is still up for debate.