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It is believed (to my understanding) that at the center of all large galaxies are super massive black holes. Why is it then when you see photos of galaxies that the center is extremely bright if a black hole is at the center? You would think that a black hole which not even light can escape would be dark instead of bright. Why is this?

Edit - After reading the link provided by @dmckee it made me wonder why are Galaxies Centers so bright to begin with?

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Have you tried computing how big such a black hole would be in terms of the galactic radius? For comparison 1 solar mass leads to a 1.5ish km radius hole and radius scales linearly in the mass.. –  dmckee May 16 '12 at 19:03
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@dmckee - No. Feel free to share more detail => –  Lynda May 16 '12 at 19:05
    
The answer is actually in the link I edited into the previous comment. Now you need only compare it to the pixel resolution on the galactic image you're looking at... –  dmckee May 16 '12 at 19:10
    
@dmckee - Thanks for the link, I guess I never really comprehended the size of black holes, you always assume they are these massive bodies, and while super massive black holes can be very large I always assume larger. I edited question to reflect another question that came up. –  Lynda May 16 '12 at 19:50
    
@dmckee I think those comments could constitute an answer... –  David Z May 17 '12 at 0:20
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2 Answers 2

up vote 5 down vote accepted

Imagine a large sporting stadium (with glass walls) filled to the brim with LEDs. Now bury a dark marble in the middle. If you were in a neighboring city, would you see the marble?

The size of the marble is not too relevant here, it could be a basketball, truck, or even a house (although I think in the analogy marble is probably closest).

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Here is the WP image for the Sag* centre of the Milky Way where we can see depicted 6 orbits around what is beleived to be a central SMBH (Super Massive Black Hole). Here is a paper about the orbital parameters. I dont find any reference to any effect in the reddening of the light when the stars are in the nearby of the central region where a deep potential well is expected to be. Lets go ahead.

Here is the WP article about Keplerian orbital motion that constrains every body in a central force where we see that all the ellipses must have a commom focus.

The orbits are 3D curves and we see a 2D projection in the figure.
If it was a central force then the projection of the commom focus is a single point, depicted in the image by the cross of the vertical axis with the horizontal axis.

What we conclude from the image is that several of orbits are out of focus.

(elsewhere in this site I've already explained the origin of the gravitational vortex that subjects the motion of the galaxies)

Here is the above image after I've located, with two big black dots two aditional candidate focus that are completelly off centre.

Central force is the main cause of the orbits? No, it is impossible, and other effect (also a gravitational force field) is much more important.

Am I wrong ? Show me why!

three focus?

EDIT ADD:
There are not much physical simulations, that I'm aware, of the motion and light effects due to a BH. Almost always they are only "Artistic images" .
One is the light++ software package, freely accessible in the past, and the image bellow from European Southern Observatory
We can see that the simulated motions are almost keplerian and the motions of the stars are far from that. The curvature of the space is flat in the simulation.
I've never saw any reference in the paper that presents the orbital parameters (whatsoever in any other that concerns the Milky Way) any reference to a spatial curvature. THey only apply Newtonian mechanics, and never reported any light reddening even when the light was emmited in the vicinity of SgrA*.
If there is a deep potential well than some of the orbits can not be represented by ellipses. If they are represented always by ellipses then there is no chance to assign any curvature to the space.

One problem less known is the impossible youngness of the stars near the centre of the MilkyWay.
quoting Gezh[2005] and several

are unlikely solutions for the Sgr A* cluster stars. Unfortunately, alternative theories for producing young stars, or old stars that look young, in close proximity to a central supermassive black hole are all also somewhat problematic. Understanding the apparent youth of stars in the Sgr A* cluster, as well as the more distant He I emission line stars, has now become one of the major outstanding issues in the study of the Galactic center. In the vortex approximation (as I explained elsewhere in this site there is no need of a SMBH) the stars are obviously, young.

Almost Keplerian motions

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Hiding your---shall we say "non-standard"---ideas in a pile of roughly correct stuff doesn't make them any less nonsense. The ellipses are off center because space is not flat there. –  dmckee May 17 '12 at 0:00
    
Your model is wrong because the orbits around two black holes would not make closed shapes. Only an orbit around a single massive object is an ellipse. The fact that the orbits come back to themselves. You are missing the third direction, to see that the BH is the focus of the ellipse. –  Ron Maimon May 17 '12 at 0:32
    
@Ron : I'm not saying that there are two BHs. Not even one is necessary to justify galaxy motion. The dots belong to the (by eye) major axis of each ellipse and the focus by sure belongs to the the major axis. What I did was the intersection of several 2D projections of the major axis (by eye) to show that the star motions can not be described by a keplerian motion around a simple limited region. –  Helder Velez May 17 '12 at 1:18
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@HelderVelez: The one black hole is necessary, because the objects speed up as they get close to the center, and then when they are moving very fast, they swing around the center--- there must be something massive pulling them in there, and it must be compact, since it looks like a point. –  Ron Maimon May 17 '12 at 1:41
    
@Ron Maimon: The justification for the 'no need' of a SMBH is in two previous answers: the equations and images of how a void grows under the gravitational field and how the voids intersept giving a gravitational vortex. I will appreciate any comment on those answers, coming from you. –  Helder Velez May 17 '12 at 1:43
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