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Today known most massive star, R136a1 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?

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  • $\begingroup$ 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. $\endgroup$
    – LocalFluff
    Mar 12, 2016 at 14:55
  • $\begingroup$ No there is a limit of the size of star: astronomy.stackexchange.com/questions/328/… $\endgroup$
    – Vojta Klimes
    Mar 12, 2016 at 16:10
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    $\begingroup$ Did you see en.wikipedia.org/wiki/Supermassive_black_hole#Formation? $\endgroup$
    – HDE 226868 ♦
    Mar 12, 2016 at 17:10

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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 may interest you.

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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."

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  • $\begingroup$ @David - What is wrong exactly? $\endgroup$
    – Oto Shavadze
    Mar 12, 2016 at 19:59
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    $\begingroup$ 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. $\endgroup$
    – ProfRob
    Mar 12, 2016 at 21:27
  • $\begingroup$ @Otariki I have rewritten it checking some more sources, hope now it makes sense and I am sorry to post something so wrong. $\endgroup$
    – Vojta Klimes
    Mar 12, 2016 at 21:48
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    $\begingroup$ How supermassive black holes formed is not even close to settled science. Gravitational wave detectors such as LIGO might change that. $\endgroup$
    – David Hammen
    Mar 12, 2016 at 23:38
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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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