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Second Black Hole at the Center of the Milky Way
Tsalg pastes "A second black hole lurks at the centre of our Galaxy, according to astronomers who have watched a cluster of stars spinning around it. Just three years ago, astronomers confirmed that the Milky Way revolves around a supermassive black hole, called Sagittarius A*, which is about 2.6 million times more massive than the Sun. But now a much smaller black hole, just 1,300 times our Sun's mass, has been found orbiting about three light years away from its supermassive cousin. placing it intermediate between the relatively small (stellar mass) black holes in the Milky way Galaxy and the supermassive black holes found in the nuclei of galaxies."
That would be a movie effect, not reality. The spiral arms of the galaxy are a density wave propagating through the stars and dust of the galaxy's disk. Think of how sound can be described as a density wave propagating through the air. Same thing.
Fascism trolls keeping me up every night. When I starts a preachin', he HITS ME WITH HIS REICH!
And some very interesting gravity waves will be generated!
Ben Hocking
Need a professional organizer?
What the other poster said. But even if things where like you said, this is a 1600 solar mass black hole orbiting a 2.6 million solar mass black hole at a distance of 3 light years. At galactic distances, they can be approximated as a single 2.6016 million solar mass object. It's just not big enough to matter in that respect.
Excessively low, I should think. Only 1300 solar masses. Even a tiny dwarf galaxy would be millions of solar masses. Also, they tend to get torn apart rather than crushed in the process of being "swallowed."
And here's a distributed computing project to detect such gravity waves: http://www.physics2005.org/events/einsteinathome/
From what I've read, astronomers suspect this does happen from time to time, but we haven't actually observed it. However, what it looks like isn't of as much interest as the fact that it is predicted to emit strong (or at least stronger than normal) gravity waves. Gravity waves are very weak and so they are very difficult to detect so they could get a lot more data from such a major event than they normally can by observing other phenomena.
That this is a surprise depends on whom you ask. The real issue here is to understand how those huge f**off multi-billion solar mass black holes form. And so far there had not been such high-quality evidence for anything in between a stellar-mass black hole formed by a single massive star collapse, and those monsters in the middle of galaxies.
So those who think that they come from mergers of solar-mass BHs are comforted. There's also those who say that in no way those monsters had enough time to form by such a slow process. Read for instance Spin, Accretion and the Cosmological Growth of Supermassive Black Holes. Formation of supermassive black holes in turn is likely to have an impact on star formation rate in galaxies, another highly speculative area.The other original thing here is that evidence for intermediate BHs in other galaxies comes from 1) luminosity measurements, which is a much more biased method than speed measurements of stars gravitating around it (to measure star velocities you have to be able to resolve them, which is only possible in relatively nearby objects) and 2) objects that were not in small clusters like here.
There is (according to current understanding) definately a density wave there. Read Binney and Tremaine's "Galactic Dynamics", for example. While it's certainly true that stars virtually never collide, they don't have to to propogate a wave. Their mutual gravity binds them together quite nicely. (We see the same sorts of behaviors in Saturn's rings, incidentally. The rings are also collisional, but self-gravity is what lets most of the waves propogate.)
If the arms rotated because of the stellar orbits, you can easily see that the arms would be wound up beyond recognition by now. So that clearly doesn't work. (It's referred to as the "winding problem" in astrophysics.)
By the way, I'm pretty sure that sound waves aren't consider "density waves". The latter are driven by gravity, sound waves are pressure waves.
Most - but not all - galaxies are thought to contain central balck holes.