Where Do They Come From?

No, today’s entry is not about automobile drivers in Massachusetts. It’s about black holes — those ubiquitous conglomerations of huge amounts of mass — that have such strong gravitational influences that nothing (not even light) can escape their grasp. These are truly mass holes — or think of them as “mass sinks”, where mass (stars, gas, dust, etc.) is deposited and can never be retrieved.

Black holes have been around as theoretical constructs (i.e. an idea in somebody’s head) since at least the 18th century. As actual objects, however, they’ve been around probably since the beginning of the universe and they come in various flavors (or types, and if you want a more rigorous discussion of the physics behind a black hole, go here or here).

So, where do these mass holes come from?  How do they form?

The ones we’re most familiar with are those that form when a supermassive star collapses in on itself (in a supernova explosion) or when a pair of massive stars (a massive binary) somehow manage to merge together.  In either case, the matter in the stars is so dense and there’s so much of it that not even the individual atoms and neutrons in the star’s core can withstand the pressure to keep collapsing.  When it does, a new black hole is born — and becomes what astronomers call a “stellar mass” black hole.

The other astronomically interesting types of black holes are the supermassive ones that lie at the hearts of galaxies and gobble up stars and clouds of gas and dust. In very active galaxies — that is, the ones with jets shooting out from their hearts — the black holes are incredibly massive, often containing the equivalent of the mass of millions or billions of stars.

Our own galaxy has at least one black hole at its heart, and although it doesn’t shoot out a jet, it does eat up material and it does influence the motions of stars in its nearby neighborhood.

The big mass holes at the hearts of galaxies probably formed when the galaxies they live in were built through the collisions of two or more older galaxies that already had central black holes. As time went by, those black holes just continued to eat up more and more stars, growing ever larger.

There is a class of black holes that probably exist called intermediate-mass. They most likely form when smaller black holes (those that are several times the mass of a stellar black hole) collide with each other.  And, at the other end of the black hole spectrum, we have the micro black holes (sometimes called “mini black holes” if you want to be cute about it).

We haven’t seen any of these forming,yet. But they could have blipping in and out of existence during the very earliest epochs of the formation of the universe, and if they have formed, the gamma-ray radiation from their evaporation could be detectable. It’s also possible that (if it gets working again) the Large Hadron Collider could create some short-lived mini black holes during its experiments. There’s nothing to worry about, though. They wouldn’t last long enough to do damage.

No matter how they form, black holes are among the most interesting creatures in the cosmic zoo of objects that astronomers study.  And, as long as galaxies keep making massive stars and/or colliding with each other, astronomers will have plenty of them to study “in the wild.”