Supermassive black holes are greedy entities that weigh-in at millions to billions of times more than the mass of our Sun. Our own Milky Way Galaxy plays host to just such a gravitational beast, that resides in its secretive heart. Our resident supermassive black hole is named Sagittarius A*, and as supermassive beasts go, it is of relatively low mass. Sagittarius A* (pronounced saj-a-star) weighs a “mere” millions–in contrast to billions–of solar-masses. Our Milky Way’s dark heart is quiet now. It is an elderly beast, and it awakens only occasionally to feast on an unfortunate celestial object that has wandered too close to where it waits. Even though it is mostly dormant, when both Sagittarius A* and the Universe were young, it dined greedily, and glared brightly, as a quasar. Quasars are the brilliantly glaring accretion disks encircling active supermassive black holes haunting the centers of galaxies.
Despite their misleading name, black holes are not just empty space. Indeed, they come in more than one size. Besides the supermassive variety, there are black holes of stellar mass that form when an extremely massive star runs out of its necessary supply of nuclear-fusing fuel and violently explodes as a core-collapse (Type II) supernova. The gravitational collapse of an especially massive star heralds its natural “death”. When a doomed heavy star has no more nuclear-fusing fuel to burn, it has reached the end of the stellar road. Nuclear-fusion within a still-“living” roiling, broiling, brilliant star, creates radiation pressure that tries to push all of the stellar material outward. In the meantime, the star’s own gravity tries to pull everything inward. This creates a delicate balance that keeps a star bouncy. Alas, when a giant, massive star runs out of fuel, and contains a heavy iron-nickel core, it can no longer churn out pressure. Gravity wins in the end. The star’s core collapses and it goes supernova. Where once a star existed, there is a star no more.
Astronomers have also found convincing evidence of the existence of intermediate mass black holes that weigh less than their supermassive kin, but more than their stellar-mass “relatives”. Crush enough mass into a small enough space and a black hole will form every time. Some scientists have proposed that these intermediate mass objects met up with one another and merged in the early Cosmos. For this reason, it has been suggested that they served as the “seeds” that created the supermassive black holes that haunt the mysterious hearts of most, if not all, large galaxies, including our own.
The Milky Way’s resident supermassive black hole is not a lonely gravitational beast. Sagittarius A* has plenty of company. Indeed, theoretical studies indicate that a large population of black holes of stellar mass–possibly a many as 20,000–could be tripping the light fantastic around our own Galaxy’s resident central black hole. A study published in 2018, that was based on data acquired from Chandra, suggests the existence of a treasure trove of stellar mass black holes haunting the core of our Milky Way.
Some current theories propose that supermassive black holes already existed in the ancient Universe. During that very early era, clouds of gas and doomed stars whirled around and then down into the hungry beast’s waiting, greedy, gravitational snatching claws, nevermore to to return from the violently swirling maelstrom encircling this bizarre entity. As the captured, doomed material swirled down to its inevitable demise, it formed a brilliant, violent storm of glaring material around the black hole–its accretion disk (quasar). As this bright and fiery material became hotter and hotter, it hurled out a raging storm of radiation–especially as it traveled ever closer to the event horizon , which is the point of no return.
In the 18th-century, John Michell and Pierre-Simon Laplace proposed the possibility that there could really exist in nature such insults to our Earth-evolved common sense as black holes. In 1915, Albert Einstein, in his General Theory of Relativity, predicted the existence of objects bearing such powerful gravitational fields that anything unfortunate enough to wander too close to their pull would be consumed. Nevertheless, this concept seemed so outrageous at the time that Einstein rejected his own idea–even though his calculations proclaimed otherwise.