Super Massive Black Holes

4/22/07 – 4/28/07
by C. Zaitz

One of the more eyebrow-raising bits of gossip heard in astronomy circles is that most galaxies, even our own, contain a super massive black hole at their cores. A super massive black hole is much heavier than a garden variety stellar black hole, which can weigh as little as one and a half suns or as much as 14 suns. That may not sound spectacular, but a black hole with the mass of 10 suns could fit into the city of Detroit. The mass of a black hole is directly related to its size, so the heavier it is, the bigger it is. But how massive is super massive?

The monster black holes we find in the centers of galaxies tend to range in mass from a hundred thousands suns to tens of billions of suns. Some scientists suggest that they started out the same way stellar-sized holes do, but over long periods of time grew larger and larger from consuming the available material in the center of the galaxy. It seems more likely that these black holes, like the one in the middle of our galaxy, formed from a large cloud of collapsing gas, creating a massive central star, some hundreds of thousands of solar masses, which then collapsed (with no supernova) to form a gigantic black hole. Since that time it has been eating everything close enough to be drawn in. Don’t worry, though; we are very, very far from the center of our galaxy, and not in the least affected by it.

We noted that the mass of the hole is directly related to its size, but it turns out that the density of a black hole is inversely related to its mass. The bigger the original star, the less dense it needs to be to become a black hole. Super massive black holes can actually be about as dense as water, since they are so very massive. And the event horizon, the place beyond which we lose sight of you as you swirl in, is so far from the singularity at the center that a trip into the super massive black hole would take enough time to allow you to ponder your fate. In fact, scientists think that the tidal forces normally so very strong near a black hole, strong enough to “spaghettify” you (your atoms are ripped into a long strand of you as you twirl into the hole) are not so strong near the really massive black holes. So your trip into it might be somewhat non-eventful, if not pleasant. That is, if you were to be so foolish to be near such a black hole. Lest we forget, there are many dangerous things about black holes, not the least of which is the torrent of X-rays and gamma rays flooding out of the accretion disk. This is the plate of material feeding into the black hole, and it really does look like a big, gassy plate, serving up the special of the day.

If you’d like to try to see the black hole at the center of our galaxy, you will have to imagine it, for it’s shrouded by millions of stars, clouds of gas and dark nebula forming a curtain in front of it. Even if there was no curtain, you’d still be hard pressed to see it, since no light can escape their gravitational pull, making them earn their nefarious reputations of the invisible gas-eating monsters in space.

Until next week, my friends, enjoy the view.