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Why are the centers of galaxies so bright?

  1. Dec 15, 2012 #1
    If there is supposedly a super massive black hole in the center of each galaxy, wouldn't that eat all the light given off? Also, if the centers are so bright, why cant we see our galactic center from earth?
     
  2. jcsd
  3. Dec 16, 2012 #2

    Drakkith

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    The diameter of the black holes event horizon in the center of our galaxy is only a few dozen AU I believe. It is VERY VERY small compared with the overall size of the galaxy. On top of that it doesn't "eat light" unless the light goes into it. Since it is so small practically all the light emitted in the galaxy misses it completely.

    We can't see the center of our galaxy very well because there is a lot of dust in the arms of our galaxy that block visible light. But we can use cameras to see in the infrared and below, which DOES pass through dust. In the microwave range we have enough resolution to see where the event horizon is when the black hole eats up gas and dust and whatever else spirals into it by watching the gas and dust heat up and emit EM radiation before it gets swallowed up. It's been quiet so far and hasn't had anything spiraling into it, but I believe next year there is a giant gas cloud that will start to spiral in. It should be quite a show!
     
  4. Dec 16, 2012 #3

    Drakkith

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    Not true. You cannot see light unless it enters your eye or enters a sensor to be detected. Light does not bounce off of other light, so we wouldn't be able to see it.
     
  5. Dec 16, 2012 #4

    phinds

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    We not only CAN see the galactic center (although not if the visible spectrum), there is a detailed plot of the orbits of stars around (very close in to) the super-massive black hole that in fact is part of the evidence that it it IS a black hole.
     
  6. Dec 16, 2012 #5
    I'm sorry, I mispoke. Unless the star is very close to the black hole, it is likely that some of the light will come in our direction.
     
  7. Dec 16, 2012 #6

    Drakkith

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    As long as the star is outside of the event horizon its light will reach us. This is a gross simplification, and things like tidal effects would rip apart the star if it were too close, but basically as long as the light is emitted outside of the event horizon, and it doesn't pass too close to the black hole, it will reach us. Also, perhaps you aren't realizing that light is emitted in all directions from a star. Some of the light IS going to fall into the black hole, but that is a very small fraction of the total light.
     
  8. Dec 16, 2012 #7
    What did I say wrong then?
     
  9. Dec 16, 2012 #8

    phinds

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    You said
    Do you not see how Drakith's comment shows that to be incorrect? (Unless you define "very close" as meaning "inside the event horizon", but that would be an unusual interpretation since "inside the event horizon" is not "very close" it is IN)
     
  10. Dec 16, 2012 #9

    Drakkith

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    You weren't really "wrong", I just want to make sure you understand that unless the light is emitted inside of the black hole then it will probably be able to reach us. (Or someone else since light is emitted in all directions)
     
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