Originally posted by Ghostavo
A black hole at the center of the galaxy would collapse the galaxy in the same way the sun at the center of the solar system would collapse it.
Hehe
For those of you who may not know, black holes don't "suck" surrounding matter into them. If you get beyond a few times their Schwartzchild radius (the "event horizon"), the only thing influencing orbits around them is the simple law of gravity. If the Sun were replaced by a black hole of the exact same mass, absolutely nothing would change about the orbits of the planets, other than the fact that we'd all freeze to death, of course.
Multiple black holes near the center of the galaxy? Do you have a reliable source for this? Last I heard, through detailed radio observations of the orbits stars at the very center of the galaxy, the object they were all orbiting was something on the order of two million solar masses, which could only be a supermassive black hole. Having multiple supermassive black holes would obviously make these orbits much more complex; from the images I've seen, they're pretty much regular. As far as I know, the only such supermassive black hole that's been detected at the center is Sagittarius A*; just recently, in fact, scientists observed X-ray bursts caused by matter falling into the black hole, backing up this theory.
As far as the solar system being in a perfect spot to view the rest of the universe, it's really not so perfect as you might think. Sure, we're in the arms of the galaxy instead of the core, which helps aid in visibility, but so are billions of other star systems out there; this doesn't necessarily make us special. Actually, if we're talking about viewing in the plane of the galaxy, we can't see visually more than a few hundred light years before the interstellar medium obscures our view; that's why we've had to use infrared and radio light to observe most of our own galaxy. I'm not saying that we have a bad view of the rest of the universe (a good telescope and a guide to the many visible galaxies in the night sky are testament to that), but it's not necessarily anything special.
As far as the orbit of stars in the galaxy goes, this is a subject that I just touched on in an astronomy class, so I can shed a little light on the subject. By using the Doppler effect to look at spectral shifts of galaxies edge-on to our own, we can determine the rotational speed at different distance out from the galaxy's center. The results obtained from these observations were far different than anyone expected. Here's an easy metaphor: in the case of our solar system, the orbital velocity of objects such as planets decreases in an inverse square relationship. This is to be expected due to Newton's law of universal gravitation, which states that the gravitational force drops off by one over the distance squared. Since over 99% of our solar system's mass is in the Sun at the center of our solar system, mass is taken out of the equation, and so the orbital velocity follows this simple law.
Astronomers were expecting that this same pattern would be present in galaxies; since they could estimate the number of stars in each part of the galaxy based on their apparent brightness, they knew that the majority of a galaxy's visible mass was concentrated in the core. Following from this, the orbital velocity of stars at the edge of the disk should be substantially less than that of stars near the core, since the distance increased with no substantial change in mass. Lo and behold, they were dead wrong. The orbital curves of every galaxy we've measured show a constant velocity as one moves further away from the core; in other words, the stars in the farthest reaches of the disk orbit at the same rate as those near the core. By Newton's gravitational law, this can only be true if the mass inside the orbit of these stars is increasing rapidly enough to offset the incresing distance. Based on the number of stars that we've observed, we know this can't be the case. Instead, according to current theories, there has to be some other form of matter, which accounts for about 90% of the galaxy's mass, that is causing this discrepancy. We call this dark matter, since we've never directly observed its presence. The objects orbiting in the Milky Way Galaxy aren't simply influenced by the two-million-mass black hole at its center, or the masses of the millions of star systems and gas clouds inside their orbits; that doesn't come close to accounting for their speed. Instead, this dark matter is responsible for keeping the stars in their regular orbits. Dark matter is pretty much on the cutting edge of physics; it's a subject that we know very little about. From what I've heard, there are papers on it out there that make general relativity look like kindergarten.
