# Black Holes and Galaxies

by dj1972
Tags: black, galaxies, holes
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 P: 24 Couple of questions here; I know they may sound dumb but here they go. I'm "new" with some questions, so be gentle. 1.) So, if a black hole has enough gravitational pull to pull in stars and what nots, and nothing can pull away from this force once it is to close, its consumed by the black hole. Why cant anything orbit this black hole? I know stars, matter, gas and such orbit while its being consumed, but what prevents it from being in a orbit around the black hole? 2.) If the center of our galaxy is a super massive black hole, then why are we staying in orbit around it and not being consumed into the black hole? I would assume that stars and matter and gas are being pulled in when they are close enough towards the center, but as an effect would that not be pulling in the rest of the galaxy into itself? I'm assuming galaxies can and do expand over time, how can one not effect the other? (If the black hole is consuming, how can we be expanding?)
 P: 15,319 Stuff does orbit a black hole. But, just like a collapsing protostellar disc, there is an amount of accretion where friction and pressure cause the orbits of much of the inner matter to decay.
 P: 103 I agree to Dave. If an object moves to the black hole exact head-on then it will collide matters around orbiting. If the object does not go head-on then it will move in an orbit like ellipic which could be stable until there is disturbance. Actually some stars orbit a black hole very fast, and astronomers can observe the oscillation of the red shift (forward and backword),-- which is interesting to me considering the massive blackhole mass and the extremely fast speed of the orbiting star.
 P: 351 Black Holes and Galaxies Black holes have the same gravitational pull as any other object of the same mass. If the Sun was collapsed into a black hole Earth's orbit wouldn't change. The weird stuff only happens when you start to get closer to the center of gravity than you would have been able to before (since you'd hit the surface, and then part of the mass would be behind you).
 P: 24 That's interesting. I always believed that a black hole had more gravitational effect then that. So its really no more then obviously the collapsed layers and core of a sun that is exposed, then i'm assuming it slowly turns into a singularity. I thought that a black hole was more powerful then that, I always imagined it would say like eat up our solar system if our sun collapsed and became one. So is it likely to think that this is how galaxies are formed if they have a black hole in the center? This is some pretty neat stuff. I kinda like it.
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 Quote by dj1972 That's interesting. I always believed that a black hole had more gravitational effect then that.
Oh. You hadn't mentioned you thought this or I would have corrected you. It's probably the single most common misconception about BHs.

Yes, there's nothing special about a BH in terms of its mass or its gravity. The only thing special about a BH is that you can get very close to it. If our sun collapsed into a BH, Earth would not change its orbit.

However, BHs do not from from stars smaller than 1.5 solar masses, and they do tend to accumulate mass, so that's why there's so much talk about massive BHs gobbling up stars.
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 Quote by dj1972 Couple of questions here; I know they may sound dumb but here they go. I'm "new" with some questions, so be gentle. 1.) So, if a black hole has enough gravitational pull to pull in stars and what nots, and nothing can pull away from this force once it is to close, its consumed by the black hole. Why cant anything orbit this black hole? I know stars, matter, gas and such orbit while its being consumed, but what prevents it from being in a orbit around the black hole?
Far away from a black hole, its gravitational field is like that of any other object with the same mass. If, for example, our Sun were replaced by a black hole of equal mass, there would be no difference as far as the orbits of solar system objects are concerned.

However, if you get very close to a black hole, it turns out that there are no stable orbits, and anything in orbit inside that distance will rather quickly fall into the black hole.

 Quote by dj1972 2.) If the center of our galaxy is a super massive black hole, then why are we staying in orbit around it and not being consumed into the black hole? I would assume that stars and matter and gas are being pulled in when they are close enough towards the center, but as an effect would that not be pulling in the rest of the galaxy into itself? I'm assuming galaxies can and do expand over time, how can one not effect the other? (If the black hole is consuming, how can we be expanding?)
The supermassive black hole at the center of our galaxy is only a tiny fraction of a percent of the mass of the stars in our galaxy. The mass of this black hole is a few million solar masses, while there are around 400 billion stars in our galaxy. And yet from the movements of these stars we know that the stars themselves are not orbiting one another, but are instead mostly orbiting the unseen dark matter that exists within our galaxy.

And galaxies don't expand over time. They're quite stable.
 P: 103 In our galaxy case, the size of black hole is several light hours, so anything approaching closer than that will be sucked in completely including photons. This size looks quite small in star-to-star distance scale. This size is called event horizon, and a little bit outside of it there can be photon(light)s orbiting -- kind of captured.
 P: 24 I'm sorry DaveC426913, mostly what I know is from what I read. And I fill in the gaps to make sense to myself. So this is mostly from my speculations because mostly what I read they leave gaps all over the place with no real explanation. Sorry, I should have said that. That's why some of my questions sound stupid. These are my thoughts from what I read, im just trying to clarify what I don't know. LOL And to Chalnoth, our galaxies don't expand? We stay the same size constantly? So if we don't expand, how did we become this size? I would "Assume" were expanding because if we started from a orbit from around a black hole per say, with stars and gases and debris orbiting we had to expand to become the size we are. So if another galaxy merges with ours (Hopefully much smaller) we wouldn't expand to accommodate the extra mass or volume? To v2kkim, Photons (Light) are orbiting the black holes in such cases like our galaxy, which is why when you view the center of galaxies it is bright. So bright it actually lights up the center which is the black hole but cant see it because of the dust and gases and debris in the way right, you just see a big blob of brightness? In some cases I have actually seen in pictures a bar shape in the center of galaxies. In another thread I was talking in, I said something to the effect that if two black holes came together, (My assumption) they would possibly orbit each other and not consume one another. I thought in my mind that they would have a tight knit orbit, (And by orbit I don't mean in a circular fashion, more of an elliptical) which in my mind I can see the bar shape appearing in the center of the galaxy. Maybe not from one or two of these but from several orbiting tightly. Again my speculation. See what happens when you read articles but have no clue. LOL
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 Quote by dj1972 And to Chalnoth, our galaxies don't expand? We stay the same size constantly? So if we don't expand, how did we become this size?
What happens is that galaxies collide with one another to make bigger galaxies. Our own Milky Way galaxy is currently undergoing collisions with two dwarf galaxies, the large and small Magellanic clouds. In a few billion years, it will undergo a collision with Andromeda, which is approximately the same size as our galaxy, a collision which will disrupt our spiral and produce a new, larger galaxy from the merger.

 Quote by dj1972 I would "Assume" were expanding because if we started from a orbit from around a black hole per say, with stars and gases and debris orbiting we had to expand to become the size we are. So if another galaxy merges with ours (Hopefully much smaller) we wouldn't expand to accommodate the extra mass or volume?
This isn't an expansion.
P: 15,319
 Quote by dj1972 I'm sorry DaveC426913, mostly what I know is from what I read. And I fill in the gaps to make sense to myself. So this is mostly from my speculations because mostly what I read they leave gaps all over the place with no real explanation. Sorry, I should have said that. That's why some of my questions sound stupid. These are my thoughts from what I read, im just trying to clarify what I don't know. LOL
I hope I didn't sound like I thought your questions were stupid!

 Quote by dj1972 And to Chalnoth, our galaxies don't expand? We stay the same size constantly? So if we don't expand, how did we become this size? I would "Assume" were expanding because if we started from a orbit from around a black hole per say, with stars and gases and debris orbiting we had to expand to become the size we are. So if another galaxy merges with ours (Hopefully much smaller) we wouldn't expand to accommodate the extra mass or volume?
Galaxies don't expand, they contract. They start as a giant collection of dust and gas that come together under gravity, concentrating in the centre, in the same way our solar system was formed. THe BH came after (or at least as) the galaxy was formed, or so is the prevailing opinion. THe BH is not the cause of the galaxy formation, it is the consequence of galaxy formation.

 Quote by dj1972 To v2kkim, Photons (Light) are orbiting the black holes in such cases like our galaxy, which is why when you view the center of galaxies it is bright. So bright it actually lights up the center which is the black hole but cant see it because of the dust and gases and debris in the way right, you just see a big blob of brightness?
Photons don't orbit BHs. (BTW, note the logic here: if photons were orbiting a BH, we would never see them, would we? They'd have to reach us.)

The brightness at the centre of our galaxy is, in fact, due to the high density of stars in the core of the galaxy; it is so packed with stars that it appears just one giant blob of light.

Nearer the BH, it's bit different. Infalling gas and dust (and even stars) form a disk of very hot, dense matter, which radiates its energy across the EM spectrum. Very close to the BH, the in-falling matter actually emits X-radiation as it falls. It's this X-radiation that is characteristic of BHs and that's what we detect to determine that we think we've spotted one.

 Quote by dj1972 In some cases I have actually seen in pictures a bar shape in the center of galaxies. In another thread I was talking in, I said something to the effect that if two black holes came together, (My assumption) they would possibly orbit each other and not consume one another. I thought in my mind that they would have a tight knit orbit, (And by orbit I don't mean in a circular fashion, more of an elliptical) which in my mind I can see the bar shape appearing in the center of the galaxy. Maybe not from one or two of these but from several orbiting ticghtly. Again my speculation. See what happens when ou read articles but have no clue. LOL
The bar is simply another effect of the pressure waves that cause the galactic arms, which are just like the radial waves you see in water spiraling down the drain.
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 Quote by DaveC426913 Galaxies don't expand, they contract. They start as a giant collection of dust and gas that come together under gravity, concentrating in the centre, in the same way our solar system was formed. THe BH came after (or at least as) the galaxy was formed, or so is the prevailing opinion. THe BH is not the cause of the galaxy formation, it is the consequence of galaxy formation.
Well, except that gravitational orbits are stable. The only way for galaxies to contract is for the stuff that makes them up to lose their orbital energy, which comes about through friction (for large objects) and through radiative cooling (for gas).
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 Quote by Chalnoth The only way for galaxies to contract is for the stuff that makes them up to lose their orbital energy, which comes about through friction (for large objects) and through radiative cooling (for gas).
Which happens. So galaxies over time contract (ignoring galactic collisions).
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 Quote by Nabeshin Which happens. So galaxies over time contract (ignoring galactic collisions).
True. But I like to point out that it isn't gravity that makes them do so: gravitational orbits are quite stable unless perturbed by external objects. It's the loss of energy through other means that causes this contraction.
 P: 24 No DaveC426913, that's not what I think, Its just my ignorance in this field against your experience. This is how I learn and I have learned a lot thanks to all of you. And as far as photons rotating around the black hole that makes sense. Gees why don't I think that far ahead before I write something? LOL OK, I think I get it now? So lets create a mess. Clouds of gas and debris and what nots starts to billow in the middle of nowhere. (Isnt this a nebula? And isnt a nebula an exploding star?) Well, in the center of this mess starts a so called galaxy we'll call dj020709 (LOL I made it so I can name it whatever) Stars are forming towards the center from these clouds of gas and debri and what nots from collapses of gas and gravity and friction packing itself into a ball of huge gas. (I know its more technical then this but hey) Well they start to orbit each other or whatever and over a few billion years one explodes. A massive one. Lets say over 10 SM. And wah lah a large BH is formed, starts to eat up stuff and at the same time more stars are being formed just packing the center because its the best place to get stars because of all the gases and debri and such, they want to be where the action is. Now that is a pretty far out there. I know its more technical then that. But that is the best my brain can come up with.
P: 103
 Quote by DaveC426913 I hope I didn't sound like I thought your questions were stupid! Galaxies don't expand, they contract. They start as a giant collection of dust and gas that come together under gravity, concentrating in the centre, in the same way our solar system was formed. THe BH came after (or at least as) the galaxy was formed, or so is the prevailing opinion. THe BH is not the cause of the galaxy formation, it is the consequence of galaxy formation. Photons don't orbit BHs. (BTW, note the logic here: if photons were orbiting a BH, we would never see them, would we? They'd have to reach us.) The brightness at the centre of our galaxy is, in fact, due to the high density of stars in the core of the galaxy; it is so packed with stars that it appears just one giant blob of light. The bar is simply another effect of the pressure waves that cause the galactic arms, which are just like the radial waves you see in water spiraling down the drain.
Regarding the bright bar in our Galaxy Dave is right. we are looking at our galaxy from side not top view. But some photons can orbit BH though unstable. Remember that the space time is so greatly bent near BH that the light can make a circular orbit, which also can be considered as an extreme case of gravitational lensing, but these photons does not have strong density and maybe theoretical calculation so far.
By the way, I do not understand what causes our galaxy contract ? due to gas and material collision etc ?
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 Quote by v2kkim Regarding the bright bar in our Galaxy Dave is right. we are looking at our galaxy from side not top view. But some photons can orbit BH though unstable. Remember that the space time is so greatly bent near BH that the light can make a circular orbit, which also can be considered as an extreme case of gravitational lensing, but these photons does not have strong density and maybe theoretical calculation so far. By the way, I do not understand what causes our galaxy contract ? due to gas and material collision etc ?
Yes, energy loss due to heating of gasses and collisions. Also, because of the simply massive amount of stars orbiting, the perturbations introduced by random close encounters tends to destabilize the orbits over time.

 Quote by dj1972 So lets create a mess. Clouds of gas and debris and what nots starts to billow in the middle of nowhere. (Isnt this a nebula? And isnt a nebula an exploding star?) Well, in the center of this mess starts a so called galaxy we'll call dj020709 (LOL I made it so I can name it whatever) Stars are forming towards the center from these clouds of gas and debri and what nots from collapses of gas and gravity and friction packing itself into a ball of huge gas. (I know its more technical then this but hey) Well they start to orbit each other or whatever and over a few billion years one explodes. A massive one. Lets say over 10 SM. And wah lah a large BH is formed, starts to eat up stuff and at the same time more stars are being formed just packing the center because its the best place to get stars because of all the gases and debri and such, they want to be where the action is.
There are a few things that would work to make this model not as simple as you've stated. Yes, stars would form towards the center of this dense cloud of gas and dust, but solar wind usually clears out the surrounding disk of material relatively soon. So, for all intents and purposes, the stellar systems (be they lone, binary, etc.) are essentially alone in their local neighborhoods, even in areas of relatively "high" density. The most that happens when one turns into a black hole is it begins to accrete matter from its companion star. It will not gobble up everything in the nucleus, for as we have said, it behaves (gravitationally, and non-locally) exactly the same as a star on the grand scale of things. And stars almost never collide or interact on more than light year scales.
 P: 24 Well, like I said. I know its more technical then what I stated. (on the other hand I don't need to know how many solar masses something has to be before it can do something else or a bunch of crazy math to have a simple understanding of it.) But I put it into terms I can understand. Remember, I'm not a cosmologist, so I need to make it easy. I'm trying to imagine how this is all coming together as I go. But you were definitely helpful with a clear explanation. Thank you. So another question, you stated "And stars almost never collide or interact on more than light year scales". Im also assuming your saying it happens, just not very often. So is there someplace I can go and see a picture if there is one cause I think that would be neat to see. So what would happen to these stars if that were to happen? Would they merge, explode? If they merge, what happens to the cores, do they merge as well? I cant even imagine what that would be like.

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