Do we know whats at the center of galaxies yet?

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The discussion centers on the nature of the core of galaxies, particularly the Milky Way, which is believed to contain a supermassive black hole (SMBH). Observations of stars in close orbit around this central object provide constraints on its mass and size, supporting the black hole theory. The brightness of the galactic core is attributed to a dense cluster of millions of stars surrounding the SMBH, rather than characteristics typical of black holes. There is a distinction made between the overall brightness of the core and the dark object at its center, which is inferred through the orbits of nearby stars. Current understanding strongly supports the existence of an SMBH at the center of galaxies, despite some confusion about the term "core."
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Probably a stupid question as I'm sure we don't but stargazing gets me thinking. The last prevailing theory that I heard of was a massive black hole at the center of the milky way (and all other galaxies). However, the core of the galaxy is the brightest part by far and other than extremely large amounts of gravity, it doesn't appear to have any other characteristics of a black hole (as far as I know anyway). Are there any other creditable theories out there?
 
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Topher925 said:
...it doesn't appear to have any other characteristics of a black hole...
Such as?

We're pretty sure it's a SMBH. It has a lower constraint on its mass, based on objects in orbit around it. It also has an upper constraint on its size - also based on objects in orbit around it. The only object we know of with that mass and that radius is a black hole.

Here's a cool animation showing the orbits of some of the stars in VERY close orbit.
http://chargedmagazine.org/2012/05/supermassive-black-hole-causing-a-stir-in-the-milky-way/
SO-16 virtually grazes it, defining at least one upper limit on its size.

Based on orbits like these, calculations show the object can't be larger than our solar system, in fact, not larger than Jupiter's orbit I think.

There's a thread here on PF somewhere that goes into more detail.
As for brightness, our SMBH is closely surrounded by a very dense group of many, many millions of stars. That's why the core as a whole is so bright.
 
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Topher925 said:
However, the core of the galaxy is the brightest part by far and other than extremely large amounts of gravity, it doesn't appear to have any other characteristics of a black hole (as far as I know anyway). Are there any other creditable theories out there?

Read this article: http://en.wikipedia.org/wiki/Supermassive_black_hole
 
Topher925 said:
However, the core of the galaxy is the brightest part by far and other than extremely large amounts of gravity, it doesn't appear to have any other characteristics of a black hole (as far as I know anyway).

I think there is confusion about what is mean by "core". There is indeed a cluster of stars about 1000 light years across, but at the center of that, there is this mysterious dark object and we can tell what it's doing by seeing the nearby stars orbits around it.
 

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