If center of galaxy is supermassive blackhole

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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?

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English is not my native language, forgive me If I'm wrong in spelling or gamma
 

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  • #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.
 
  • #3
Drakkith
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  • #4
Drakkith
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A good tool. I think the black hole may suck all the light in where is nearby it. But just i guess. Wait to give advice or comments.
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.
 
  • #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.
 
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  • #7
D H
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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*.

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

That radius will represent the event horizon.
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.
 
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  • #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)
 
  • #9
DaveC426913
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That radius represents an upper bound on the event horizon, not the size of the event horizon.
...which is why I said 'no more than'...

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

Astronomers are confident that our own Milky Way galaxy has a supermassive black hole at its center, 26,000 light-years from the Solar System, in a region called Sagittarius A*[10] because:

The star S2 follows an elliptical orbit with a period of 15.2 years and a pericenter (closest distance) of 17 light hours (1.8×1013 m or 120 AU) from the center of the central object.[11]
From the motion of star S2, the object's mass can be estimated as 4.1 million solar masses.[12]
The radius of the central object must be significantly less than 17 light hours, because otherwise, S2 would either collide with it or be ripped apart by tidal forces. In fact, recent observations[13] indicate that the radius is no more than 6.25 light-hours, about the diameter of Uranus' orbit.
Only a black hole is dense enough to contain 4.1 million solar masses in this volume of space.
 
  • #10
D H
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Your numbers are more stringent.
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:
According to the wiki article I linked to:
In fact, recent observations[13] indicate that the radius is no more than 6.25 light-hours, about the diameter of Uranus' orbit.
45 AU/c = 6.24 light-hours.


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)
Reference 13, referenced above: http://arxiv.org/abs/astro-ph/0306130
 
  • #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.)
 
  • #12
phyzguy
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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.)
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.
 
  • #13
DaveC426913
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...about 2000 times further away than SgrA*, but about 1000 times larger.
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
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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?
 
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  • #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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  • #17
Ken G
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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.
OK, then that is certainly the same ball park. Probably they'll both be resolved around the same time.
 
  • #20
phyzguy
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But dust obscuration due to viewing angle is not a problem for radio interferometry, right?
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]
 
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  • #21
cepheid
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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]
Right, I remember seeing a paper about the use of interferometry in the submm recently. If it's mm and submm, then obscuration by dust will not be a problem, however emission by the dust will be. The dust temperature is such that it emits thermally at these wavelengths, and so the Galactic plane is quite bright in these bands. I do wonder about that...

EDIT: Looking at the slides, it's more mm than submm. Maybe the wavelengths are long enough for dust not to be as bright.
 
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  • #22
DaveC426913
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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,
Point. Conceded.
 
  • #24
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can we use black hole gravity to help increase speed of spacecraft?

I use to watch documentary about idea to explore solar system, it seem that swing ship across planet and it's gravity can help increase speed of the ship.

can we use black hole in same theory?
 
  • #25
DaveC426913
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can we use black hole gravity to help increase speed of spacecraft?

I use to watch documentary about idea to explore solar system, it seem that swing ship across planet and it's gravity can help increase speed of the ship.

can we use black hole in same theory?
Sure, but I'm preeeeetty sure we'd find a BH if there were one in our solar system. :rolleyes:
 

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