It's big. It's bright. It's white. It's so intense. But what is at the center of the Milky Way?

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    $\begingroup$ Its not white and intense. You might get away with saying its really intense in the infrared, but we can't see it at optical wavelengths. $\endgroup$ – Rob Jeffries Jan 17 '15 at 21:56
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    $\begingroup$ God, if you believe a bad sci-fi movie. $\endgroup$ – Schwern Jan 18 '15 at 1:04
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    $\begingroup$ Super massive black hole sounds more realistic :P $\endgroup$ – Ero Sɘnnin Jan 18 '15 at 5:38

Photos of the galactic center aren't too bright because of all the gas and dust between us and it. For example (in infrared):

Galactic center

I'm guessing, though, that you're talking about other galaxies, because there are no views of the galactic center of the Milky Way face-on. Although the galactic center is pretty luminous, just not in the wavelengths we're used to.

Anyway, the galactic center has lots of stars - many massive, but a handful like our Sun. They're young, though, which is odd, as there isn't much star formation happening.

The main attraction, though, is Sagittarius A*, a radio source that is apparently a supermassive black hole. It's actually a subset of the more complex radio source Sagittarius A. So far, astronomers think that most galaxies have supermassive black holes in their centers.

We know that it has to be really massive because of how it perturbs the orbits of stars nearby:

Star orbits near Sagittarius A*

We know it's massive and we know it's a very strong radio source. A supermassive black hole is just about the only thing it could be.

Courtesy of WayfaringStranger: An awesome picture from NASA:

Awesome picture from NASA

In this paper by Ghez et al., the orbits of stars were monitored and a mass of the central object was determined to be $4.1 \pm 0.6 \times 10^6 M_{\odot}$. This paper discusses the properties of the star cluster itself.

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    $\begingroup$ APOD with X-ray emissions: apod.nasa.gov/apod/ap130906.html $\endgroup$ – Wayfaring Stranger Jan 17 '15 at 15:18
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    $\begingroup$ You could give the mass and a reference to a paper that determines it... $\endgroup$ – Rob Jeffries Jan 17 '15 at 21:37
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    $\begingroup$ Why isn't the supermassive black hole on the major axis (and on one of the two foci) of all those ellipses? $\endgroup$ – Peter Mortensen Jan 18 '15 at 0:54
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    $\begingroup$ @PeterMortensen I don't have a good answer, but I know that there's more matter in the area than just the black hole, so it's not the only influence. $\endgroup$ – HDE 226868 Jan 18 '15 at 1:04
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    $\begingroup$ @PeterMortensen: Perspective. When an ellipse is seen from a direction not perpendicular to its plane, the image is still an ellipse (in the picture plane), but the axes and foci of the image ellipse are not the image of the axes and foci of the original ellipse. $\endgroup$ – hmakholm left over Monica Jan 18 '15 at 5:20

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