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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5$\begingroup$ 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. $\endgroup$– ProfRobCommented Jan 17, 2015 at 21:56
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1$\begingroup$ God, if you believe a bad sci-fi movie. $\endgroup$– SchwernCommented Jan 18, 2015 at 1:04
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1$\begingroup$ Super massive black hole sounds more realistic :P $\endgroup$– Ero SɘnninCommented Jan 18, 2015 at 5:38
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1 Answer
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Photos of the galactic center aren't too bright because of all the gas and dust between us and it. For example (in infrared):
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:
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:
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.
answered Jan 17, 2015 at 15:17
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1$\begingroup$ APOD with X-ray emissions: apod.nasa.gov/apod/ap130906.html $\endgroup$ Commented Jan 17, 2015 at 15:18
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1$\begingroup$ You could give the mass and a reference to a paper that determines it... $\endgroup$– ProfRobCommented Jan 17, 2015 at 21:37
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1$\begingroup$ Why isn't the supermassive black hole on the major axis (and on one of the two foci) of all those ellipses? $\endgroup$ Commented Jan 18, 2015 at 0:54
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1$\begingroup$ @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 ♦Commented Jan 18, 2015 at 1:04
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4$\begingroup$ @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. $\endgroup$ Commented Jan 18, 2015 at 5:20