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If center of galaxy is supermassive blackhole

  1. Sep 27, 2011 #1
    than why center is the brightest part of galaxy? shouldn't black hole suck all the light in?

    and if so, the more close to center of galaxy, the more time flow slower right?

    ................................

    English is not my native language, forgive me If I'm wrong in spelling or gamma
     
  2. jcsd
  3. Sep 27, 2011 #2

    phyzguy

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    The diameter of the black hole at the center of the galaxy is extremely small compared to the size of the galaxy. We can't resolve the central black hole with even the most powerful telescopes. The reason the center of the galaxy is brighter is because there are a lot more stars there, but these stars are far outside the event horizon of the central black hole. In the same way, any GR-induced changes in the rate of time passing happen only very close to the black hole, far closer than we can see.
     
  4. Sep 27, 2011 #3

    Drakkith

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    For comparison, a black hole with 1 solar mass would have an event horizon with a radius of 2.95 km. So almost 6 kilometers in diameter. The Sun has a diameter of 1.392×10^6 km, or 1,392,000 km.
    Edit: Used the calculator here: http://hyperphysics.phy-astr.gsu.edu/hbase/astro/blkhol.html
     
    Last edited: Sep 27, 2011
  5. Sep 27, 2011 #4

    Drakkith

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    It will only "suck" in the light that passes the event horizon. Further away the light will simply be bent around the black hole and continue on its way.
     
  6. Sep 27, 2011 #5

    DaveC426913

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    While the black hole at the centre of our galaxy is huge - about 4 million solar masses, it is likely no more than about 6 light hours in radius - about the size of Uranus' orbit*. The galaxy core on the other hand is on the order of ten thousand light years across - about 120,000x larger - and that's all stars - millions of em.

    *http://en.wikipedia.org/wiki/Supermassive_black_hole#Doppler_measurements

    That radius will represent the event horizon. That means only light that passes within 6 light hours of the black hole itself will get consumed.

    Furthermore, the BH will surely have a huge accretion disc of superheated infalling matter that shines very bright (esp. in X-rays). This makes black holes counterintuitively very bright objects to behold.
     
    Last edited: Sep 27, 2011
  7. Sep 27, 2011 #6
    Also we have lots of observations of the Sagittarius A*

    http://en.wikipedia.org/wiki/Sagittarius_A*

    We can see the stars go around the black hole.

    The other thing is that relativistic effects don't matter unless you are really close.
     
  8. Sep 28, 2011 #7

    D H

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    That radius represents an upper bound on the event horizon, not the size of the event horizon. Whatever that object at the center of our galaxy is, it must have a radius smaller than 45 AU because astronomers have observed stars pass within 45 AU of the object and survive.

    If that object at the center of our galaxy truly is a black hole (and what else could it be?), its Schwarzschild radius is about 31 lunar distances, about 0.08 AU, or about 17 times the radius of the sun.
     
    Last edited: Sep 28, 2011
  9. Sep 28, 2011 #8

    Drakkith

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    Wow we have observed objects passing that close to it DH? That's cool.
    Was this using radio or infrared or what? (Or a mix)
     
  10. Sep 28, 2011 #9

    DaveC426913

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    ...which is why I said 'no more than'...

    Your numbers are more stringent. According to the wiki article I linked to:

     
  11. Sep 28, 2011 #10

    D H

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    The wiki article about observations only talks about observations. It doesn't mention the Schwarzschild radius. So I guess it is the 45 AU you are saying is more stringent. It's not. It's right there in the text you quoted:
    45 AU/c = 6.24 light-hours.


    Reference 13, referenced above: http://arxiv.org/abs/astro-ph/0306130
     
  12. Sep 28, 2011 #11

    Ken G

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    Indeed, with radio interferometry, the prospects for actually observing the event horizon of Sag A* are pretty good. It might be the only event horizon in the universe that we will ever directly resolve. (Or more correctly, we will resolve the light passing somewhat outside it, bent around by its gravity. The light is there because of the accretion disk, but it can be enough affected by the gravity itself that we are in effect seeing the EH.)
     
  13. Sep 28, 2011 #12

    phyzguy

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    I saw a talk on this, and the speaker was hopeful that VLBI will also be able to resolve the SMBH in the center of M87. It's about 2000 times further away than SgrA*, but about 1000 times larger.
     
  14. Sep 28, 2011 #13

    DaveC426913

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    And more importantly I suspect, it's viewed from above - not obscured like ours by the bulk of our own galaxy's dust and gas.
     
  15. Sep 28, 2011 #14

    George Jones

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  16. Sep 29, 2011 #15
    I see...

    Even super massive balck hole that strong enough to hold every star in galaxy still very small.


    by the way,

    How light bent when it pass near event horizon?
    Bend.jpg

    is it bent a bit and continue on their original way?
    or forever change their way?
     
    Last edited: Sep 29, 2011
  17. Sep 29, 2011 #16

    DaveC426913

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    No. Like this. (forgive the straightness of the lines, they should be smooth)

    Note the differences from your diagrams:
    - once the rays are "bent", they do not "unbend"; they proceed onward on their bent course.
    - as they "bend" they are brought closer to the BH, so they "bend" even more. But then, as they get past it, they are carried farther away, thus they "bend" less.
    - every light ray follows a smooth hyperbolic path past the BH (kind of like a parabola but more open)
    - every light ray is actually symmetrical from one side of the BH to the other - you could move every arrow to the other end of its line segment and the diagram would still be perfectly accurate.
     

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  18. Sep 29, 2011 #17

    Ken G

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    OK, then that is certainly the same ball park. Probably they'll both be resolved around the same time.
     
  19. Sep 29, 2011 #18

    DaveC426913

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    Last edited by a moderator: Apr 26, 2017
  20. Sep 30, 2011 #19

    cepheid

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    Last edited by a moderator: Apr 26, 2017
  21. Sep 30, 2011 #20

    phyzguy

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    I think Cepheid is correct. The VLBI observations of the galactic core are using wavelengths in the mm and sub-mm region, where I don't think dust obscuration is a problem. Here's a very nice presentation that I found online:

    http://www.tiara.sinica.edu.tw/activities/workshop/2006-4/presentation/Shen_workshop2006.pdf [Broken]
     
    Last edited by a moderator: May 5, 2017
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