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The center of the galaxy

  1. Jan 16, 2010 #1

    Every picture I see on the internet depicts it brighter than the galaxy as a whole.
    Now I know that when a black hole is feeding and a galaxy is still in the progress of creation, thats true.
    But is it true for old galaxies aswell whose black hole is not feeding anymore? And if it is, then why?

    Thanks in advance,
  2. jcsd
  3. Jan 16, 2010 #2


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    Staff: Mentor

    "Feeding"? Not sure what you mean by that. The galaxy has a big black hole in the center, and it also has a lot of stars orbiting that black hole, which is why the center of our galaxy (and most galaxies) is so bright.

    A black hole isn't some kind of cosmic vacuum cleaner.
  4. Jan 16, 2010 #3
    Active Galactic Nucleii and Quasars seem to be galaxies in which the central black-hole is extremely active, probably due to more accretion than usual. But most spiral galaxy cores are bright because of the Galactic Bulge, which is a denser sphere of old stars, many of which are bright red giants.
  5. Jan 17, 2010 #4


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    Gold Member

    What you're seeing is just a lot more stars towards the center. That's how the stars are distributed. Our galaxy's core is not really active, the black hole doesn't engage in enough accretion to make it a Quasar.
  6. Jan 18, 2010 #5
    I never claimed anything like that.
  7. Jan 18, 2010 #6


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    Staff: Mentor

    Then I don't understand what you mean by "feeding"....though if you question is answered we can just let it go.
  8. Jan 18, 2010 #7
    I thought it was fairly clear.

    Feeding = stars are falling into the event horizon.

    Quiet = all the stars that would do this are already gone, so all the remaining stars have orbits that avoid the black hole.

    If something disrupts lots of stelar orbits, like for example another galaxy passing by, then the black hole would start feeding again.
  9. Jan 18, 2010 #8
    One thing that you do have to be careful about is that when someone shows a picture of a galaxy on the internet, it's as much art as science, and their main concern is to make a cool looking picture. Trying to get actual brightnesses from a picture on the internet can be very misleading.

    For example, the centers of spiral galaxies tend to be red, and the arms tend to be blue, so by adjusting the color balance to get a good picture, you can change the relative brightness of things.
  10. Jan 18, 2010 #9
    From what I've heard the matter entering a black hole becomes part of it. So I just thought "feeding" would be an appropriate word for it.

    If thats not true, enlighten me.
  11. Jan 18, 2010 #10


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    Homework Helper

    You don't have to look at pictures to tell that the center of a galaxy is brighter than its arms. If you look through a telescope, it's obvious; in fact, the contrast is usually larger than what photos would imply.
  12. Jan 28, 2010 #11
    A black hole goes through a period of what you would call in laymen's terms "feeding".

    However, the gravitational pull of a black hole is so strong that not all elements that approach it are "eaten" by it. If these elements do no cross the black hole's event horizon, then they can get "caught" in the gravitational pull of the black hole and thus orbit the black hole- hence the brightness that you perceive since although the black hole does not emit light, the galaxies/etc surrounding the black hole seem to orbit an empty space in the middle.
  13. Jan 29, 2010 #12
    Planethunter is right, but when stars, gas clouds and everything else that begins to orbit the black hole, the gravity is strong enough to slowly degrade the orbit so that the element previously orbiting the black hole will cross the event horizon.
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