At the center of our seemingly tranquil galaxy, there's a black hole massive enough that it significantly warps spacetime, swallows any matter that gets close enough, and in the process emits truly colossal amounts of radiation. Named Sagittarius A*, it was discovered in 1954 because of its enormous output in the radio region of the spectrum. [N. B. Throughout this post, when I refer to the black hole's radiation output, I am not of course talking about anything coming from inside its event horizon; that's physically impossible. But the infalling matter that gets eaten by it does emit electromagnetic radiation before it takes its final plunge and disappears forever. Lots of it.]
This thing is a real behemoth, at an estimated four million times the mass of the Sun. There is a lot of interstellar dust between it and us -- after all, when you're looking at the constellation of Sagittarius, you're looking down a line going directly along the plane of the galaxy toward its center -- but even without the dust, it wouldn't be all that bright. Most of its output isn't in the visible light region of the spectrum. This doesn't mean it's dim in the larger sense; not only are there the radio waves that were the first part of its signal detected, but it has enormous peaks in the gamma and x-ray part of the spectrum as well.
Earlier this month, the European Southern Observatory released the first actual photograph of Sagittarius A*:
How could something that enormous form? We have a pretty good idea about how massive stars (over ten times the mass of the Sun) become black holes; when their cores run out of fuel, the gravitational pull of its mass collapses it to the point that the escape velocity at its surface exceeds the speed of light. At that point everything that falls within its event horizon is there to stay.
But we're not talking about ten times more massive than the Sun; this thing is four million times more massive. Where did all that matter come from -- and how did it end up at the center of not only our galaxy, but every spiral galaxy studied?
A step was taken in our understanding of galactic black hole formation by a team of astronomers at the University of North Carolina - Chapel Hill, in a paper that appeared this week in The Astrophysical Journal. It's long been known that most large galaxies are attended by an array of dwarf galaxies, such as the Milky Way's Small and Large Magellanic Clouds. (Which, unfortunately, are only visible in the Southern Hemisphere. This is why they're named after Magellan. Typical of the Eurocentric approach to naming stuff; clearly indigenous people knew about the Magellanic Clouds long before Magellan ever saw them.) It's also known that because of the gravitational pull of the larger galaxies, the smaller ones eventually collide with them and merge into a single galaxy. In fact, that even happens to big galaxies; gravity has a way of winning, given enough time. The Milky Way and the Andromeda Galaxy, which are about the same size, will eventually come together into a single blob of stars, but what its final shape will be is impossible to predict.
As an aside, there's no need to worry about this. First, it's not going to happen for another four and a half billion years. Second, when galaxies (of any size) collide, there are relatively few actual stellar collisions. Galaxies are mostly empty space, and when they merge the stars that comprise them mostly just pass each other without incident.
But not the black holes at their centers. Those, being the center of mass of the entire aggregation, eventually slam together in a collision with a magnitude that's impossible to imagine. And the team at UNC found out that this is one of the ways that galactic black holes become so large; they discovered that even dwarf galaxies have central black holes, and when they get swallowed up, that mass gets added to the central black hole of the larger galaxy.
Sagittarius A* sits in the middle of the whirling vortex of stars, like the sea monster Charybdis in Greek mythology, sucking down anything that comes close enough -- including, apparently, other black holes. The celestial fireworks with a collision between two large black holes, such as the ones in the Milky Way and Andromeda, must release a fantastic amount of energy.
Wouldn't that be something to see?
From a safe distance, of course.