The stars in the core of our galaxy are moving damned fast.
December 20, 2000 10:21 PM   Subscribe

Over 8 years, Hubble has been taking IR pictures of the core, and they've put together a timelapse sequence of it. this is a better view; more frames, smoother motion. I know that stars move, but I think of it as being something that takes tens of thousands of years; it's eerie to watch these stars move so far in just 8 years.
posted by Steven Den Beste at 10:24 PM on December 20, 2000

Hey Steven. Three front page posts in a matter of minutes? don't have to make up for MeFi being gone for over two days all on your own. :-)
posted by pnevares at 10:31 PM on December 20, 2000

Think of it as "pent up demand". Actually, there were four others which I discarded.
posted by Steven Den Beste at 10:44 PM on December 20, 2000

Where's a General Products Hull No. 2 when you need one?
posted by dhartung at 11:15 PM on December 20, 2000

The shape of our galaxy alone should prove that,
a. There is a black hole in the center of our galaxy, or
b. This galaxy was formed out of a black hole which absorbed as much as possible and then the big bang as some would call it.

The swirly shape of our galaxy supports this theory when thinking about it logically, and with the current info available.
posted by Zool at 3:54 PM on December 21, 2000

It's a pretty amazing sequence.

At first I wondered whether it was gravity lensing that was causing the apparent motion (ie. something dark, dense and moving distorting the paths of the light) but I guess not, that would also change their size and shape too.

Instead its seems that they are wizzing around a black hole or something.
posted by lagado at 4:00 PM on December 21, 2000

This galaxy was formed out of a black hole which absorbed as much as possible and then the big bang as some would call it.

Did I read you correctly, are you saying that the galaxy existed before the Big Bang?
posted by lagado at 4:08 PM on December 21, 2000

My comprehension of a black hole suggests that it absorbs everything around it, but it ( depending on size and strength ) has a limit of how much it can absorb.

Once it has absorbed all it can it will then explode, which is how i believe all new galaxy's are formed. It is entirely possible that this is a repeat process, which happens over and over which could explain our galaxy expanding at first and then rectracting as some believe it is or will.

There is talk that the expansion of our galaxy is slowing down and very likely because of this black hole in the middle. So i think it will start retracting towards that black hole sooner or later.
posted by Zool at 4:32 PM on December 21, 2000

Zool, your understanding is wrong. There is no theoretical limit on how large a black hole can be, and they don't go "boom" ever. The only way for mass to escape is by Hawking radiation, which is one sub-atomic particle at a time and too complicated to explain here.
posted by Steven Den Beste at 4:46 PM on December 21, 2000

Steven, if you're right than the universe is doomed to be engulfed by the numerous black hole's that supposedly exist.

My comments don't have scientific basis, i came to those conclusion using logic and the generally available knowledge of things like balck hole's and that the galaxy is slowing down it's expansion and quite possible is going to shrink. Now that there is evidence to support a black hole in the middle of our galaxy, it is what i conluded to be causing the soon to be shrinking of our galaxy.

Add to that the big bang theory and it all makes sense the way i described it.

Don't forget, everything we know and that the scientific community knows is just THEORY,
i like mine better.

posted by Zool at 6:07 PM on December 21, 2000

no no. black holes do not "engulf" anything. the so called point of no return (aka the schwarzschild radius if you want to look it up) is dependant on the black hole's mass.

also, the *galaxy* is not doing anything (well, it's doing stuff, but it's not expanding or contracting significantly). zool, I think you mean the universe. however, black holes absolutely are not causing the "shrinking" of anything. (in fact, it seems pretty doubtful now that the universe is going to collapse.)

steven, as I understand it which is probably not very well, black hole evaporation *could* end in a boom of some sort. I know some people consider them a possible explanation for the high-energy gamma bursts hst observed.
posted by rabi at 6:23 PM on December 21, 2000

Don't forget, everything we know and that the scientific community knows is just THEORY,
i like mine better.


You're view is, frankly, somewhat wrong. The current view is that the universe will continually expand forever. It's not so much that the whole universe will become absorbed in blackholes, it's more so that the universe will become so large with such a low density, that it will become essentially black. There's a lot to this though, and also the fact, as Dan has mentioned, that black holes do, albeit extremely slowly, 'evaporate.'

And you idea of the 'galaxy' contracting is rather vague. A black hole in the center doesn't mean everything is absorbed by it and falls towards it. The same principles that determine Earth's orbit around the sun, determine the solar system's orbit around the galaxy (though more complicated, yes).

If anything, our galaxy will collide and merge with the Andromeda galaxy before 'contracting' in on itself.
posted by mkn at 6:32 PM on December 21, 2000

Rabi and/or mkn, where and how does the big bang theory fit into your views of what's going on?

Which last i heard, has been accepted by the scientific community at large.
posted by Zool at 7:27 PM on December 21, 2000

Anyone who says "Just a theory" doesn't understand what a scientist means by "theory". It's a common mistake made by laymen. I fear I must self-link: here I explain that in far greater detail. (Forgive me my sins, fellow MeFiers.)

Black holes can grow smaller through Hawking radiation, but it's a very slow process, and in most cases where large black holes exist, they're adding mass far faster than they are losing it, usually many orders of magnitude faster. A black hole has to reach literally microscopic size before it can pop; this is also the result of Hawking's work. But that kind of thing won't be happening with the kind of black hole which is at the center of our galaxy; it would take trillions of years for Hawking radiation to reduce its size to the point where that could happen.

Our galaxy will collide with the Andromeda galaxy; we're on course for a direct hit, but what will happen is that the two will pass through each other; both will be changed gravitationally, but will then continue on their way. This has happened before elsewhere; there are astronomical pictures of it. It has to be remembered that both galaxies consist mainly of empty space. But you can't get rid of momentum that easily.

I'll add more later, but I've got a date. (YAY!)
posted by Steven Den Beste at 7:31 PM on December 21, 2000

I'm going to try and take it from another angle. Please excuse length and please correct factual errors; my black hole knowledge is mildly out of date.

Our universe started with the Big Bang. There was nothing "before" the Big Bang, because our perception of time started with it. Any kind of measurement of time that we're currently aware of, even the very concept of time, completely breaks down before the Big Bang. So while there may have been something before then, any explanation holds as much ground with me as a god who ate a bad steak and farted us out.

Okay, some are slightly more plausible (which I'll get into) but that ones my personal favorite. :-)

Boom, goes the Big bang. Matter and energy (which are the same thing, really) shoot out in all directions. Particles (or probabily waves, or waves, or however you feel like looking at it, they're all valid for different purposes) of energy start hanging out together due to various forces of attraction.

Those groups eventually get big enough to act as we expect matter to, using essentially Newtonian physicis, so it's handy to look at them and physical chunks. Those chunks bounce off of and into each other, forming bigger chunks.

The bouncing around causes energy (in the classical sense) to be collected or expelled in different ways. The way the energy particles act around each other eventually begins to be something on a grand scale, like fusion and fission. Nuclear reactions. You take a big ball of matter that's one big, ongoing nuclear reaction and you call it a star.

After a really, really freakin' long time the reaction begins to slow down. Some stars eventually burn out like a match and become a hunk of rock. Some end up losing cohesiveness and explode (Novae) and some, under extremely rare (though what's rare to infinity? :-) conditions collapse in on themselves. They implode.

All the energy and matter and everything that made up this ball (and much of the crap in the immediate vicinity) get sucked up into this... thing. This singular point in space-time - thus the term "singularity".

Now, the forces contained within this singularity, the mass, the energy, they're the same as if it were a star, except rather than being spread out in an area the size of, say, our sun, it's condensed to an area the size of something infinitly smaller than an electron.

To hold such an entity together (and this is related to the implosion that created it) requires massive gravitational force; such force that with a couple of noteable exceptions, anything that gets within a certain range cannot escape that force.

It's essentially the same as with earth. Bits and pieces of space guck get caught in our gravitational force all the time, but because of friction it burns up as it enters our atmosphere. If you apply counter-energy (a space ship's thrust, for instance) you can combat that gravitational force and break out.

So it's quite possible, and probable, that space guck going fast enough can pass into the gravitational field of a black hole without reaching the point of no escape.

The point of no escape being the event horizon.

But what happens when it gets beyond that event horizon? Well, that's where all the interesting stuff comes into play. I also think that fun stuff is what you've heard about Zool, but I think you're misinterpreting some of it, or haven't been given the whole story.

See, this thing is sucking matter in with such force that light, the fastest thing in the universe, can't even get out of it. Which means that the speed of light isn't fast enough to counteract the gravitational forces involved. That's some pretty serious force, it means that when stuff gets sucked into it, it starts accelerating at, at a bare minimum, the speed of light.

The laws of relativity being what they are, the faster you go the slower time passes for you.

So you're going the speed of light past the event horizon, so no time whatsoever passes for you. Suddenly, BOOMPF, you're back to the same situation whatever the universe was before the Big Bang happened. A might interesting cycle.

That cycle, combined with the fact that there's an infinite amount of energy sucked into this beast naturally led people to the theory that the Big Bang was the result of a singularity.

Or, perhaps, the Big Bang coming from a singularity clued people in to the possiblity of a black hole. I'm not sure if the chicken or the egg came first here, to be quite honest.

So, working from the theory that the Big Bang was a collapse (and explosion) of a singularity, it kind of stands to reason that just maybe the Universe before the Big Bang got sucked into a black hole. So it also stands to reason that everything that gets sucked into a black hole ultimately (either immediately or perhaps some time in the distant future) gets shot out into a brand new universe.

Brand new, of course, being a relative term.

While the current trend of belief is that the universe will expand forever (evidenced mostly through the years of documented astronomy) it still stands to reason that it's possible everything will get sucked into a black hole, including the other black holes, until we're right back where we started in a singularity, and we'll do it all again.

tangent: an interesting correlation to reincarnation; the universe is reborn, why aren't we?

The amount of energy released by the Big Bang sent a whole bunch of crap into a big empty nothingness. Space is empty so the black holes can, theoretically, not be sucking anything in for, well, infinite amounts of time. If they don't encounter matter they can't suck it in.

Also note that the gravitational force of a black hole is identical to that of a planet. Things can (and are, if I recall correctly - there are stars that do it, which was one of the clues to the existence of black holes) orbit around a black hole without being sucked into it, the same way the moon orbits around the earth without crashing into it.

Also of interest is the theory (reasonably sound) that space-time is curved, and if viewed from 4 dimensions can be bent and shaped and squshed.

If you pretend the surface of a blanket laying on the floor, for instance, is two dimensional, you as a 3 dimension entity can twist and shape the blanket however you like, even to the point of putting two very remote regions right beside each other by poking a hole in the surface and connecting the two.

It's a pretty poor analogy, but it's late and my fingers are getting sore. Think klein bottles if it helps. But the point of the analogy is that people think that black holes can act as those pinpoints, a singularity doesn't really have a surface, so if two (or one black hole existing in different 3 dimension space-time locations) black holes connect the "surface" of 3 dimensional space, then you can jump from one point in 3d space to another that would be extremely far away using regular travel. That's a wormhole.

Actually, I should've used the worm in an apple analogy, but I don't remember it accurately enough to even try.

Some interesting books (I'm too lazy to look for links) include A Brief History of Time, by Stephen Hawking, Asmiov on Numbers, by Isaac Asmiov, and the Dancing Wu Li somethings or other (masters?) by some author whose name I can never remember. Contact, by whatsisname, Carl Sagan, is a fiction book that explores some of these premises, though it's starting to get a mite out of date. There are many, many other good books that bring explain the physics behind it, the above are just books I've read in the past few months, so they're still fresh in my mind.
posted by cCranium at 7:49 PM on December 21, 2000

Zool, I think you are confusing "Galaxy" with "Universe". Galaxies are clusters of stars orbiting around a common point which may or may not have a black hole in it.

The Universe is the collection all matter, space and time and has been expanding ever since the "Big Bang" as the current theory goes. Whether the Universe contracts or not has been the subject of much debate and basically about whether there is enough mass in the Universe to cause it to contract. The current view is that it will expand forever.

As Steven says, black holes don't go boom. At least based on our present understanding of them.
posted by lagado at 8:07 PM on December 21, 2000

Good luck, Steven. Hope it goes well.

She wouldn't happen to be an anglo woman, by any chance?

cheers ;-j
posted by lagado at 8:09 PM on December 21, 2000

For a very readable and entertaining introduction to general relativity and black holes, I can thoroughly recommend:

Black Holes & Time Warps Einstein's Outrageous Legacy
by Kip S. Thorne (California Institute of Technology)

with a foreword by Stephen Hawking
"A vivid and readable account of general relativity and its impact on modern astrophysics and cosmology by one of the leading experts in this area." -Roger Penrose, Oxford University

"No one tells the greatest moments in the quest to decipher Einstein's intellectual legacy . . . and tells them more clearly, more authoritatively, in plain language, with a more human touch, than Kip Thorne in this book." -John A. Wheeler, Princeton University

ISBN 0393312763
posted by lagado at 8:25 PM on December 21, 2000

Hokay. This is going to take a while.

Stars about the size of our Sun, up to about thre times that size, burn for a long time, then after a few billion years they change rather rapidly and become Red Giants. This state only lasts a few million years; then they shrink again (sometimes shedding a lot of gas in the process) and become white dwarfs. This process is called "going Nova".

Stars a lot bigger than that near the end of their lives go through a much more complicated process and eventually end up becoming neutron stars (which begin as pulsars). They actually end rather violently, and we call them "Supernovae". A neutron star is a very strange object; it's one where gravitational forces are so strong that the electric charge on its component particles can't keep it from collapsing. Thus it keeps collapsing until all the particles are actually in physical contact, just as they would be in the nucleus of an atom. In a very real sense, a neutron star is a big atom. What holds it apart is aspects of the Strong force. It's now thought that if something massive (planetary size) hits a neutron star, the result is what we call "gamma ray bursters" as the kinetic energy is released. But that hasn't been proved yet.

The largest stars go through a similar process except that when the core collapses, the forces which try to stop it aren't powerful enough to fight the force of gravity, and they keep collapsing and form black holes.

When we shoot a satellite at Jupiter, it has to achieve what's known as "escape velocity" in order to leave the gravitational influence of the earth. But escape velocity is partially a function of the initial position of the object. An object already in orbit requires much less velocity change to escape than one which is on the surface of the planet.

A black hole has what's known as an "event horizon". What this means is that a particle inside the event horizon can't reach escape velocity without kinetic energy sufficiently high that its velocity would exceed the speed of light. Since that can't happen, no such particle is capable of escaping. A particle outside the event horizon, on the other hand, can achieve escape velocity without exceeding the speed of light. That's what defines the event horizon.

All mass changes the curvature of space. Light always travels in a straight line, but the curvature of space can fool us into thinking that it's changing direction. (That's because space is non-euclidian.) A black hole curves space enough so that light inside it travels in a straight line -- but never escapes. That also defines the event horizon.

Objects can fall into a black hole simply by crossing the event horizon. But if they do, they'll never come back out again. So a black hole has a tendency to grow. Generally, when this happens there are quantum effects and a lot of plain old kinetic energy is released, which tends to result in a lot of light and other kinds of radiation. If a star hits one, the resulting conflagration is really quite spectacular -- and more mass gets added to the black hole.

But a particle can cross the event horizon at any speed. It isn't automatically sped up to relativistic velocities.

The gravitational effects of a black hole extend well beyond the event horizon, and objects can form stable orbits around them. Indeed essentially our entire galaxy is mostly in such stable orbits around the one at the center of our galaxy. But occasionally, orbital interactions will cause some star near that black hole to change just enough to hit the event horizon, at which point it becomes part of the black hole and the black hole gets more massive. As its mass grows, the diameter of the event horizon grows as well.

As to what happens inside, I don't think anyone knows, and it's not obvious that it makes any difference to us since it can't effect us. The only thing that comes out of a black hole is gravity (and Hawking radiation).

A black hole will absorb anything which hits it, but there's nothing magic about it and it doesn't cause objects to violate Newtonian/Einsteinian mechanics; which means that objects don't instantly change directions and fall in. They only get absorbed if their normal motion brings them into contact with the event horizon. If that doesn't happen, then they don't get absorbed. They can spend an arbitrarily large amount of time arbitrarily near the event horizon without getting sucked in, if their orbit is correct (except for tidal effects; see below).

Now the actual event of a star getting eaten turns out to be more complex than you might think, because as the star approaches the black hole, tidal effects tear it apart and smear its gas into a disk around the black hole. It's thought that most galaxies have black holes in the center, and that they all have rather massive disks of gas and dust forming a ring around them, the remnants of stars which got close enough for tidal effects to tear them apart. For reasons which are not quite understood, it also appears that they tend to form jets which escape at EXTREMELY high speed perpendicular to the disk of this dust, at a speed high enough to achieve escape velocity from the entire galaxy. It's now thought that when one of those jets points straight at us, we see what we call a quasar. (If it doesn't point at us, then we don't see it.)

Early estimates about quasar energies were based on the assumption that the energy emitted by them was equal in all directions, and based on what was measured here, the resulting numbers are (ahem) astronomical. But the new theory is that in fact the total energies involved are far lower, because it's not being emitted equally in all directions.

In the meantime, such a black hole in the center of a galaxy continues slowly to accrete matter from the stars and gas and dust around it, and slowly grows.

There is no theoretical limit of which I'm aware to how large such a black hole can grow. There are probably practical limits due to the rate at which they normally absorb matter and the anticipated life of the universe. There's no reason to believe that everything will end up absorbed by black holes, either. There's nothing magic about them; they're not like magnets grabbing everything within reach. Their gravity is just like any other object's gravity, and they don't violate the conservation of kinetic energy or the conservation of momentum. It's possible to orbit one without being absorbed by it.

There is only one way known that a black hole can get smaller, and that's through Hawking radiation. But that's really, really complicated to explain, and it's an extraordinarily slow process. A black hole such as the one at the center of our galaxy is growing, not shrinking.

In general, a black hole can't explode, because that would require whatever escaped to have kinetic energy in excess of the theoretical limit imposed by Special Relativity. But Hawking has proposed the possibility that there exist what he calls "Quantum black holes" -- black holes with diameters on the order of the size of atoms. Such a black hole might shrink due to Hawking radiation to the point where the diameter of the event horizon reaches zero. Then, likely, the mass within blows up because it's no longer constrained by the theoretical limit on kinetic energy. I don't remember what kind of mass such an object would have, but his theory was that they are remnants of the Big Bang, and that they don't form any more. He also theorizes that they blow up occasionally when their event horizons collapse to zero.

But that's an entirely different kind of thing and not really related to the really BIG black holes like the one at the center of our Galaxy. Probably that one and the others like it will never decay.

There's also the question of "Open Universe" versus "Closed Universe" and it isn't settled yet. An open universe has one big bang, then grows forever. A closed universe has more mass; grows to a certain point, then collapses back again to a singularity. Maybe it explodes again at that point, maybe not.

Right now, all the best measurements say that there's nothing like enough mass in existence for our Universe to be closed, and so it will probably grow forever. Eventually it suffers a heat death and then the game is over.

We get one chance at it, guys and gals; enjoy it while it lasts.
posted by Steven Den Beste at 11:24 PM on December 21, 2000

Hey; what's the odds?

Steven and I have dates the same night.

[ No, silly; we live in different states... ]
posted by baylink at 11:21 AM on December 22, 2000

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