<p>Here comes positive feedback - if something is heavy it tends to attract more things so it is even heavier and heavier. At one point it will clean its surroundings. This is the process for those supermassive black holes to create. In middle of galaxies there is lot of stuff (stars) that can be "eaten" by these huge black holes. Plus they had a lot of time to do so.</p>

<p>EDIT (thanks for questioning the answer I was very sure with it but there are some problems):</p>

<p>As Rob Jeffries said, the mass could not be acquired this way (probably not all of it) because of radiation pressure: when stuff falls towards black hole in accretion disk there is lot of heat created which pushes the rest of gas away. See Eddington limit. <a href="https://en.wikipedia.org/wiki/Eddington_luminosity" rel="nofollow">https://en.wikipedia.org/wiki/Eddington_luminosity</a></p>

<p>So after checking some pages those are the theories I found:
<a href="http://science.nasa.gov/astrophysics/focus-areas/black-holes/" rel="nofollow">http://science.nasa.gov/astrophysics/focus-areas/black-holes/</a></p>

<p>"One possible mechanism for the formation of supermassive black holes involves a chain reaction of collisions of stars in compact star clusters that results in the buildup of extremely massive stars, which then collapse to form intermediate-mass black holes. The star clusters then sink to the center of the galaxy, where the intermediate-mass black holes merge to form a supermassive black hole."</p>

<p>Another source:
<a href="http://astronomy.swin.edu.au/cosmos/S/Supermassive+Black+Hole" rel="nofollow">http://astronomy.swin.edu.au/cosmos/S/Supermassive+Black+Hole</a></p>

<p>"Stellar black holes result from the collapse of massive stars, and some have suggested that supermassive black holes form out of the collapse of massive clouds of gas during the early stages of the formation of the galaxy. Another idea is that a stellar black hole consumes enormous amounts of material over millions of years, growing to supermassive black hole proportions. Yet another, is that a cluster of stellar black holes form and eventually merge into a supermassive black hole."</p>

CC BY-SA 3.0
2016-03-12T19:59:09.713Z
@David - What is wrong exactly?
Oto Shavadze
CC BY-SA 3.0
2016-03-12T21:27:32.320Z
This is unlikely to be main route that the biggest black holes grow, at least initially. It is difficult (a) to arrange for that much gas to be present near a black hole and (b) feed it fast enough because of radiation pressure.
ProfRob
CC BY-SA 3.0
2016-03-12T21:48:49.023Z
@Otariki I have rewritten it checking some more sources, hope now it makes sense and I am sorry to post something so wrong.
Vojta Klimes
CC BY-SA 3.0
2016-03-12T23:38:54.640Z
How supermassive black holes formed is not even close to settled science.  Gravitational wave detectors such as LIGO might change that.
David Hammen
CC BY-SA 3.0
2016-03-12T14:24:17.847Z
2016-03-12T21:47:11.243Z
2016-03-12T21:47:11.243Z
Vojta Klimes
<p>The mass of a object does not increase when it collapses into a black hole. So a supermassive black hole must have started off quite small, and then grown.</p>

<p>The formation and growth of supermassive black hole is not settled science. Supermassive black holes probably started as large stellar mass black holes (The very earliest stars could have been very large, and were almost entirely free of any heavier elements, and could have collapsed to form black holes with a mass of about 100 suns) The black holes then grew as more matter fell into them. </p>

<p>There is a strong correlation between the size of a SMBH and its galaxy. Its not clear why this is, but it suggests that the environment that drove galaxy growth, also contributed to the growth of the black hole. </p>

<p>This article on the <a href="http://arxiv.org/pdf/0903.0547.pdf">Growth of supermassive black holes across cosmic time</a> may interest you.</p>

CC BY-SA 3.0
2016-03-12T16:31:44.887Z
2016-03-12T16:38:28.920Z
2016-03-12T16:38:28.920Z
James K
<p>There are two theories of how super massive black holes formed.
One is that they started as several stellar massive black holes that fell into the center of galaxies (which were forming around them) and came together creating a super massive black hole we see now.
Another theory is that they started as "Dark stars", which are theoretical stars held together by dark matter, which can be much larger than a normal star. When the dark star collapses upon its self, it became a super massive black hole.</p>

CC BY-SA 3.0
2018-01-04T16:29:36.707Z
2018-01-04T16:29:36.707Z
Sofia Filardo
<p>Today known most massive star, <a href="http://space.about.com/od/stars/tp/The-Top-10-Most-Massive-Stars.htm" rel="nofollow">R136a1</a>  weighs approximately 256 times the mass of our sun, how did supermassive black holes, with a mass of 10 billion times the sun form?</p>

<p>I understand, that when a star becomes black hole, its density increases, but does its mass also grow?</p>

CC BY-SA 3.0
2016-03-12T14:55:59.643Z
Maybe plasma in the early dense universe formed gigantic stars, which couldn't possibly exist today, which then directly collapsed into SMBHs? Incredibly enough, within three years the James Webb space telescope will be able to see that happen, if it ever did happen. Or it might have something to do with dark matter which dominates the mass after all. Galaxies are gravitationally dominated by their dark matter halos, I think that has become the definition of a galaxy actually. Maybe that has to do with its SMBH too even if it's only of 1% of the mass of the dark matter halo.
LocalFluff
CC BY-SA 3.0
2016-03-12T16:10:55.427Z
No there is a limit of the size of star: http://astronomy.stackexchange.com/questions/328/is-there-a-theoretical-maximum-size-limit-for-a-star
Vojta Klimes
CC BY-SA 3.0
2016-03-12T17:10:59.550Z
Did you see https://en.wikipedia.org/wiki/Supermassive_black_hole#Formation?
HDE 226868
<star><black-hole><mass>
Why are black holes that massive?
Oto Shavadze
# Why are black holes that massive?

(Asked by: username_0 on 2016-03-12)

Today known most massive star, [R136a1](http://space.about.com/od/stars/tp/The-Top-10-Most-Massive-Stars.htm) weighs approximately 256 times the mass of our sun, how did supermassive black holes, with a mass of 10 billion times the sun form?

I understand, that when a star becomes black hole, its density increases, but does its mass also grow?

**Tags:** star, black-hole, mass
> **Comments:**
> **username_1**: Maybe plasma in the early dense universe formed gigantic stars, which couldn't possibly exist today, which then directly collapsed into SMBHs? Incredibly enough, within three years the James Webb space telescope will be able to see that happen, if it ever did happen. Or it might have something to do with dark matter which dominates the mass after all. Galaxies are gravitationally dominated by their dark matter halos, I think that has become the definition of a galaxy actually. Maybe that has to do with its SMBH too even if it's only of 1% of the mass of the dark matter halo.  
> **username_2**: No there is a limit of the size of star: http://astronomy.stackexchange.com/questions/328/is-there-a-theoretical-maximum-size-limit-for-a-star  
> **username_3**: Did you see https://en.wikipedia.org/wiki/Supermassive_black_hole#Formation?  

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## 3 answers

### Answer by username_2 on 2016-03-12. Score: 3 | Accepted answer

Here comes positive feedback - if something is heavy it tends to attract more things so it is even heavier and heavier. At one point it will clean its surroundings. This is the process for those supermassive black holes to create. In middle of galaxies there is lot of stuff (stars) that can be "eaten" by these huge black holes. Plus they had a lot of time to do so.

EDIT (thanks for questioning the answer I was very sure with it but there are some problems):

As Rob Jeffries said, the mass could not be acquired this way (probably not all of it) because of radiation pressure: when stuff falls towards black hole in accretion disk there is lot of heat created which pushes the rest of gas away. See Eddington limit. <https://en.wikipedia.org/wiki/Eddington_luminosity>

So after checking some pages those are the theories I found:
<http://science.nasa.gov/astrophysics/focus-areas/black-holes/>

"One possible mechanism for the formation of supermassive black holes involves a chain reaction of collisions of stars in compact star clusters that results in the buildup of extremely massive stars, which then collapse to form intermediate-mass black holes. The star clusters then sink to the center of the galaxy, where the intermediate-mass black holes merge to form a supermassive black hole."

Another source:
<http://astronomy.swin.edu.au/cosmos/S/Supermassive+Black+Hole>

"Stellar black holes result from the collapse of massive stars, and some have suggested that supermassive black holes form out of the collapse of massive clouds of gas during the early stages of the formation of the galaxy. Another idea is that a stellar black hole consumes enormous amounts of material over millions of years, growing to supermassive black hole proportions. Yet another, is that a cluster of stellar black holes form and eventually merge into a supermassive black hole."

> **Comments:**
> **username_0**: @David - What is wrong exactly?  
> **username_4**: This is unlikely to be main route that the biggest black holes grow, at least initially. It is difficult (a) to arrange for that much gas to be present near a black hole and (b) feed it fast enough because of radiation pressure.  
> **username_2**: @Otariki I have rewritten it checking some more sources, hope now it makes sense and I am sorry to post something so wrong.  
> **username_5**: How supermassive black holes formed is not even close to settled science.  Gravitational wave detectors such as LIGO might change that.  

---

### Answer by username_6 on 2016-03-12. Score: 7

The mass of a object does not increase when it collapses into a black hole. So a supermassive black hole must have started off quite small, and then grown.

The formation and growth of supermassive black hole is not settled science. Supermassive black holes probably started as large stellar mass black holes (The very earliest stars could have been very large, and were almost entirely free of any heavier elements, and could have collapsed to form black holes with a mass of about 100 suns) The black holes then grew as more matter fell into them.

There is a strong correlation between the size of a SMBH and its galaxy. Its not clear why this is, but it suggests that the environment that drove galaxy growth, also contributed to the growth of the black hole.

This article on the [Growth of supermassive black holes across cosmic time](http://arxiv.org/pdf/0903.0547.pdf) may interest you.


---

### Answer by username_7 on 2018-01-04. Score: 1

There are two theories of how super massive black holes formed.
One is that they started as several stellar massive black holes that fell into the center of galaxies (which were forming around them) and came together creating a super massive black hole we see now.
Another theory is that they started as "Dark stars", which are theoretical stars held together by dark matter, which can be much larger than a normal star. When the dark star collapses upon its self, it became a super massive black hole.


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