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There's a Hole in the Middle of It All
Apparition writes "CNN is reporting that the star at the center of our galaxy is actually a super-massive black hole. The article then claims that it occupies a volume of space about 3 times that of our solar system. If my math is correct, about 230 million suns could fit into that same volume, so it doesn't impress me that the claimed mass of the black hole is only between 2.6 and 3.7 million times that of the sun. So what is up here? Since when do black holes occupy so much space (I thought they were points)? And how can something with a density only 1/100 of our Sun be called super-massive?" I think the article is talking about a maximum possible size of the object, due to limitations on the resolution of our instruments. Nature has a no-registration story about the research. Update: 10/16 23:44 GMT by M : There's an article with more information on space.com, and a press release from the European Southern Observatory.
Current theories in no way preclude the formation of a singularity. In fact it is pretty much the required outcome when a sufficiently massive start reaches the end of it's life. There is some discussion that when quantum theory and gravity are unified quantum effects may smear the singularity out of existence, but at this point it is all hand waving. Perhaps what you're thinking of is a naked singularity. A naked singularity is a singularity that is not cloaked by an event horizon, and is extraordinarily unlikely to occur.
Of course this is all based upon classical arguments, and without a theory of quantum gravity we can't be sure. However it hasn't stopped Hawking and Penrose arguing about "cosmic censorship principles" :)
Jon Erikson, IT guru
Theoretcially (we'll likely never have building materials struturally sound enough to test this) light should behave in almost exactly this manner close to a black hole. For example, say you've built a circular torus space station around a black hole. If you're within a certain radius to the singularity, but still outside the event horizon, light will bend towards the blackhole, allowing your vision to see 360 degrees around the torus. You can stand in one point and see your back an apparent distance equal to the circumfurence of your imaginary torus away. Closer than that radius means that the torus would appear to bend the wrong way.
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As such with a larger black hole (large M, smaller 1/r^2) the difference in gravitational effects over the size of say a person is fairly small because r^2 doesn't change an awful lot. However with a small hole (small M, large 1/r^2) the difference in strength of the gravitational field over the size of a person is a lot larger and so there are tidal forces which tend to cause things to be ripped apart.
Jon Erikson, IT guru
I've always thought it was obvious that super-massive blackholes lie at the center of galaxies. The intense gravity at the center should create one, and spiral galaxies are all just pinwheeling "down the drain".
Several things wrong in here. First, it's the huge density at the center of the galaxy that would lead you to think a black hole might form there. Yeah, the density is big there because it's way down in a gravitational potential well. But intense gravity doesn't create a black hole-- quite the other way around, in fact.
Second, spiral galaxies are *not* spiralling down the drain. Most of the stars in a spiral galaxy orbit the center approximately circularly; they aren't spiraling in any more than the earth is spiraling into the Sun. So why the spiral shape? Spiral shape can come from a couple of differnet things. In some galaxies, they are density waves. Think of them as a cosmic "traffic jam". In some places, the stars are closer together than other places; in those places, densities are higher, and gas clouds get compressed, and more stars form (which is why spiral arms are bluer). As the wave passes through those stars, they will spread back out. It's similar to sound waves (which are density waves), or, indeed, clumps of cars on freeways (which seem to maintain their identity even though they don't always have the same cars in them-- you pass through them, so for a while you're a part of the clump, but eventually you get past the clump).
Other theories of spiral structure formation are based on the differential rotation; when a big group of stars form, the differential rotation will tend to stretch it out into a little spiral arm segment. These theories are probably more responsible for spiral structure in galaxies where the arms are ratty and choppy. The density wave theory is probably more responsible in "grand design" spirals where you can trace one long arm all the way from the center out to the edge.
One thing spiral galaxies are definitely not however are stars spinning down the drain the way water spins down a drain. It may look obvious, but it's wrong. (Yes, there are ways to get material to sink down to the center of galaxies, but generally it's a whole lot easier with gas and dust than with stars. Gas and dust are viscous fluids, but stars are basically collisionless.)
-Rob
We haven't the foggiest idea what the universe was all the way back to time=0, but starting at at least time = 10^-43 seconds, the universe was a very large, quite possibly infinite, distribution of matter. It was not an explosion away from a point, but an expansion of matter "away". Space time expanded like a rubber sheet, with every point moving away from every other point.
Neat, eh?
Don't Bogart the fish sticks
Eh? Could you explain what you're talking about here? Because as far as I know, Hawking and Penrose's work has nothing to do with the likelihood of black holes forming. Indeed, one of the things about black hole formation in that no matter how unsymmetrical the initial state the end result is highly symmetrical, possessing no distinguishing features other than mass, charge and angular momentum... the "black holes have no hair" theorem.
Or are you talking about the recent results in M-theory proving Berkentstein's semi-classical formula for black hole entropy? If so, I'm still not sure what that's got to do with black hole formation... it strikes me you've got things confused...
Jon Erikson, IT guru
Are you posting this AC because you know it's false?
Schwartzschild radius scales as mass; density scales as mass divided by radius cubed; hence the density of black holes scales as 1/mass^2, i.e., as the inverse square of the mass.
Supermassive black holes are indeed quite un-dense. Taking the extreme limit of this relation, in fact, one finds that the observable universe is approximately the size of its own Schwartzschild radius, i.e., perhaps we are all living inside a giant black hole.
Damn! There I go again - it makes my head hurt every time I say that.
-renard
Is this true? Could you/someone explain to me what would prevent me from building a huge strong ring around the event horizon and lowering a probe from that ring through the event horizon? The ring could be stabilized by the gravity of the black hole itself and a counter-weight on the side oppossite to the probe. Would the force on the probe be so strong that no force is strong enough to pull it back? Or is it theoretically impossible to build a probe strong enough to withstand the gravity?
0x or or snor perron?!
Since we have no unified theory, it is not possible to prove anything mathematically with confidence. The current theory of gravitation, Einstein's general relativity, requires a singularity. But GR is presumed not to be valid at quantum scales of distance, and since a singularity is infinitely small in GR, all bets are off.