Supermassive black holes are found at the centre of galaxies.
Credit: NASA/JPL-Caltech
SYDNEY: Stars more than one million times as massive as the Sun may be more stable than astronomers thought, and have created seeds that grew into the largest supermassive black holes.
Supermassive black holes are found at the centre of most galaxies, including our own Milky Way. Theories about their formation range from collapsing clouds of gas to collisions between smaller black holes.
Astrophysicists have also suggested that supermassive black holes could have formed from the catastrophic collapse of incredibly large stars that were one million to one billion times the mass of the Sun.
Extreme instability
These hypothetical stars would be extremely unstable. The process of forming the black holes must have happened rapidly, say experts, as these stars could only exist for around four million years, accumulating matter at a rate exceeding about one solar mass per year. This is in contrast to the our roughly 4.5-billion-year-old Sun.
To find out if huge stars could create supermassive black holes, astrophysicist Mitchell Begelman from the University of Colorado in Boulder, USA, modelled the structure and evolution of stars more than a million times the mass of the Sun.
He found that, if they were stabilised by rotation or magnetic forces, the stars could have created seed black holes a few per cent of the star's mass; about 10,000 to 100,000 times the mass of the Sun.
"Bloated, turbulent envelope of gas"
Once these seeds form, the stars inflate into what Begelman calls a quasistar — a bloated, turbulent envelope of gas powered by black hole accretion at the centre of the star. His research will appear in the Monthly Notices of the Royal Astronomical Society.
"Because of the likely importance of rotation in the collapsing core, only a small fraction of the supermassive star collapses to a black hole initially. The energy liberated during the formation of the black hole inflates the remainder of the star into a bloated object that resembles a red giant, which we have previously termed a 'quasistar'," Begelman says.
At a certain point, radiation begins to leak out of the star and blows the remains of it apart. The black hole 'seed' then rapidly grows into a supermassive black hole.
The mechanism is probably responsible for the really big black holes more than 13 billion light-years away, Begelman says. Other processes, such as mergers and accretion onto smaller black holes, may have formed the black holes that populate local galaxies.
While the theory is valid, the problem is forming such enormous stars in the first place, commented astrophysicist Scott Croom at the University of Sydney in Australia.
"Whether you can form stars of this mass is a very open question. Gas tends to fragment and form medium-sized stars, and the typical accretion rate of material onto the cores of stars is much lower than is required to build [massive stars]."
But if supermassive stars do exist, Begelman's theory of their evolution makes sense, says Croom. "At the moment we don't have good evidence of merging small black holes in the early universe. This sort of work is useful in developing different routes [to supermassive black holes] and stretching the boundaries of star formation."
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