Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: bizerl on 28/10/2013 02:28:00
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Is it possible that one might form in a spiral arm for instance? or that a galaxy might have a multitude of "super-massive" black holes?
And while we're talking about it, is there even a scientific description of what one is?
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I think you could say no to the supermassive black hole in the the spiral arm. It would cause far too much destabilizing of the orbits of nearby stars, and should be detectable.
That isn't to say that there aren't potentially many smaller black holes scattered around the galaxy.
Apparently Markarian 739 or NGC 3758 (http://www.nasa.gov/mission_pages/swift/bursts/monster-black-holes.html) has a pair of supermassive black holes causing essentially twin galactic centers.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.nasa.gov%2Fimages%2Fcontent%2F558583main1_Mrk0739_close-226.jpg&hash=a6cd786d367e13b91300404d8b6dac0d)
And, if Andromeda and the Milky Way collide in 4 billion years, the resulting merged galaxy could end up with two supermassive black holes.
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Some recent observations suggest that at least in some cases the "supermassive black holes" may consist of clutches of smaller black holes orbiting very rapidly and closely together.
Think of the true scale of things. The event horizons of normal stellar mass black holes are only a few miles across no bigger than a small asteroid and even a monster one billion solar mass black hole is only about the size of the solar system out to the orbit of Saturn or Uranus.
This means that it is quite difficult for black holes to bump into each other and merge and this could take quite some time unless there is lots of gas and stuff around to slow them down in their orbits so once they have collectively swept up all the rubbish things could settle down without them merging for a very long time.
This also means that in in interacting with each other black holes could occasionally be flung way out into intergalactic space like some stars in interacting galaxies.
Remember also that stars are much bigger than black holes, around one billion times bigger in fact, and when galaxies collide which they doo quite regularly, stars almost never collide but just interact gravitationally with orbit and path changes to produce very unusual patterns seen in the galaxy zoo
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When black holes are in close orbits, radiation of gravitational waves will cause them to move into even closer orbits, and eventually coalesce.
See: http://en.wikipedia.org/wiki/Gravitational_wave#Orbital_lifetime_limits_from_gravitational_radiation
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I do not dispute this, but again consider the scale of the effect. The orbits have to be very close and short period for gravitational energy loss to be significant for most orbits that we observe, the lifetime will be vastly longer than the current age of the universe. There are very few orbits where gravitational energy loss is observable.
It has been observed in a pair of binary pulsars orbiting each other in a period of eight hours around 20 times linger than the 18 minute orbit of the cited pair with a lifetime of half a million years in the observed case the binary lifetime would be very many tens of millions of years because the relationship is a lot sharper than a simple linear one.
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Ok, here are some random thoughts about the Markarian 739/NGC 3758 Galaxy mentioned above. I'm sure there are better explanations.
There may be some pretty extreme tidal effects with the supermassive black holes. Similar to the theory that Earth is slowly pushing the moon away, and the moon slowing down the Earth's rotation due to tidal effects.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.nasa.gov%2Fimages%2Fcontent%2F558583main1_Mrk0739_close-226.jpg&hash=a6cd786d367e13b91300404d8b6dac0d)
The galaxy has two galactic centers with stars presumably generally orbiting each black hole. However, the spiral arms, and the rest of the galaxy orbits around both galactic centers.
With classic orbital mechanics, the spiral arms should be orbiting at a speed slower than the galactic centers.
However, the two galactic centers should create massive tides in the spiral arms and outer orbiting stars. This would speed up the stars, or add energy to their orbits, and take energy away from the binary orbit of the galactic centers and slowly push the centers towards each other. If each orbit takes millions of years, then it could take quite some time to finish.
However, that was ignoring the rotation of the black holes, and the stars orbiting around the twin galactic centers. One should have the opposite effect, of transferring energy from the stars orbiting the galactic centers to the binary centers. This may in fact stabilize the binary orbits of the black holes, at the same time slowly forcing the stars around the galactic centers into decaying orbits.
Anyway, that galaxy should last for some time.