quick question on black holes

[Cmac22]

there is a lot about black holes that i dont understand but heres a few questions i was just thinking about:

 

Are there back holes that arnt in the center of galaxies?

 

i understand that black holes form from collapsed stars (i think). but is this proven? has someone seen a star collapse into a black hole? and if a star did do this wouldnt its gravitation eventualy create a galaxy around it?

 

another similar question: are there galaxies without black holes at their centers?

 

i dunno if there is a difference between supermassive black holes and just normal ones besides i suppose that one is "supermassive". are supermassive black holes formed differently?

 

it just seems to me that if black holes are the center of all galaxies then are the galaxies themselves producing the black holes somehow? are the black holes a product of galaxies? or are galaxies a product of black holes? or is it neither?

 

any comments/ideas would be great!

[ajb]

there is a lot about black holes that i dont understand but heres a few questions i was just thinking about:

 

Are there back holes that arnt in the center of galaxies?

Yes, presumably.

 

i understand that black holes form from collapsed stars (i think). but is this proven?

Hawking and Penrose in the 60's showed that (in the context of general relativity) than if a body collapses enough a black hole (event horizon) is always formed before the singularity is reached. This has lead to the idea of cosmological censorship; all singularities are hidden behind an event horizon.

 

People have constructed space-times with extra fields that break this.

 

has someone seen a star collapse into a black hole? and if a star did do this wouldnt its gravitation eventualy create a galaxy around it?

 

I don't think anyone has seen a black hole form. All you would see is a star getting fainter and fainter and the redder and redder until it becomes so dim you would not see it any more.

 

Your second question is a common misconception. The gravity around a black hole (beyond the event horizon) is the same as the body before the black hole was formed. They don't "suck any stronger" or anything like that.

 

another similar question: are there galaxies without black holes at their centers?

I don't know. That is a question for an astrophysicist.

 

i dunno if there is a difference between supermassive black holes and just normal ones besides i suppose that one is "supermassive". are supermassive black holes formed differently?

 

They are all essentially the same. Formed by the gravitational collapse of matter. The only difference is that they are classified by there mass.

 

You should also be aware of the no-hair theorem which states that black holes are classified by there mass, angular momentum and electric charge and nothing else. People have also constructed black-holes that break this, but I believe they are not very natural, for example non-minimally coupled fields may be needed.

 

it just seems to me that if black holes are the center of all galaxies then are the galaxies themselves producing the black holes somehow? are the black holes a product of galaxies? or are galaxies a product of black holes? or is it neither?

 

I would think that the supermassive black holes grew out of the collapsing matter that eventually formed the galaxies. But I am no astrophysicists so don't take my word for it.

 

any comments/ideas would be great!

 

No problem.

[Klaynos]

Some additions etc....

 

Yes, presumably.

 

Most of them infact. There's a supermassive one at the centre of most galaxies it seems, but there are alos lots and lots and lots of stella sized black holes that are just around... Although they have to be in regions where the stars are the correct age to have allowed them to be created.

 

i understand that black holes form from collapsed stars (i think). but is this proven? has someone seen a star collapse into a black hole? and if a star did do this wouldnt its gravitation eventualy create a galaxy around it?

 

 

We've lots of evidence for supernovae -> black hole creation, including observational evidence of ones that appear to have happened previously, but we have no observed evidence of it actually happening.

 

They are all essentially the same. Formed by the gravitational collapse of matter. The only difference is that they are classified by there mass.

 

As I understand it supermassive's don't currently have a well understood formation method, whereas stellar sized ones do.

[Cmac22]

Thanks for the replys, they helped some. but i still am confused.

 

the reason i am so curious is because i was just thinking about supermassive black holes in the center of galaxies in specific. im sure i have a lot of misconceptions here so i wanted some clearing up. i was thinking of gravity as a straigt line force kinda, which is probably wrong in the first place. but i was thinking that at the center of a galaxy all the gravity would be pulling strongest from that point, with all the gravity overlapping at that point and what not. but if you think of it as all the gravity pushing in the other direction then that means you have all the gravity of the galaxy pushing onto one point. so all this gravity would create so much...?pressure? ,or gravitational force, that it would cause a black hole to form at that point. perhaps out of a star that might have been pulled there or something but maybe a black hole made of energy or light or something else?... i dunno. does this make any sence?

 

i mean, if Black holes are at the centers of all galaxies, i would think that black holes are a result of the galaxy being there, and not the other way around. or rather that as a result of a galaxy forming, a black hole was formed

 

i dunno if any of that made much sence. but heres another question. does a galaxy need a central black hole to orbit around, or spin around. can galaxies just spin around a central point with nothing in the center? how would this work?

[Klaynos]

As I understand it we only know about supermassive black holes because we observed them, so they are not required for a galaxy to orbit around.

 

The gravity at the centre of a galaxy due to the mass of that galaxy is 0 because it all cancells out.

 

It's very similar to be inside a hollow sphere you feel no gravity from the sphere, you can't use exactly the same idea here because galaxies are not spherically symmetric.

[Cmac22]

hmm, i guess that makes some sence. but then again, if theres a supermassive black hole in the center, the gravity wouldnt be zero would it? so what kind of a difference would it make between the two? the galaxy would be denser or somethin? i dunno

[iNow]

Try to remember that a blackhole's gravity is not distinguishable from a star's gravity with the same mass. In a blackhole, it just tends to be concentrated around a smaller radius (does the term radius still apply for 3D spheres, or is there better terminology?).

[Cmac22]

yea i get that. but then again, supermassive black holes tend to be larger, some as large as millions or billions of solar masses.... so i would say that is quite different then stars gravity. and yes i understand that it has a smallar radius and is more dense. but when objects come too close to black holes, and then get pulled in passed the event horizen, the objects mass is added to the black hole entirely...i think? i mean if not even light can escape then i wouldnt think any matter could.

 

im not really sure where i am going with all of this, i was just interested in the formation of supermassive black holes. i looked on the internet but there wasnt really that much i could find. as far as i know, there supermassive black holes are not fully understood, if even half understood.

 

one thing i was thinking of was that, if anywhere, the most likely (however unlikely it is) for planets and stars and such to collide or come in close contact would be at the center of the galaxy. with many objects being close together and moving at high speeds, collisions or distortion of stars by gravity would keep feeding the black holes to get larger and larger.

 

but that still doesnt really answer my question of why there are super massive black holes at the center of galaxys in the first place. it certainly cant be called coincidence. and it must have something to do with the galaxy itself or its formation i feel.... i dunno

[Dark matter]

I like your theory. I believe that a black hole acts almost as a paradox upon itself by gaining objects, which then gain other objects and so forth.

[molecule man]

maybe if there is more than one solar system in a galaxy then that would be saying that not every galaxy has a black hole at the exact centre of it.

[willawoga]

One theory i heard of for supermassive black holes is that they used to be galaxies with a normal balck hole in the center, that ended up sucking up the whole galaxy.

 

And about a galaxy without a black hole, it is theoretical, one possibility is that the gravitationaly pull of millions or billions of stars holds them together instead of the gravitational force of a black hole

[Dark matter]

And about a galaxy without a black hole, it is theoretical, one possibility is that the gravitationaly pull of millions or billions of stars holds them together instead of the gravitational force of a black hole

 

I don't understand what you're trying to say.