Could Giant Black Holes be Lurking Inside Supermassive Stars?

According to conventional thinking, a black hole is created when a very large star collapses into a point under its own weight. However, astrophysicist Mitchell Begelman at the University of Colorado in Boulder believes that black holes could lay hidden inside supermassive stars, revealing themselves when the outer layers of the star are blown away as the star cools.

Distant quasars – emissions of bright light given out when a black hole’s gravity draws in gas – with light that has taken almost three billion years to reach the Earth - are proof that supermassive black holes existed in the universe only a billion years after the big bang. The conventional theory of black hole formation does not explain how these black holes, ten billion times the mass of the sun, formed so quickly after the big bang.

Begelman says that according to conventional thinking,  there was not enough time in the first billion years after the big bang for black holes to grow this size. Instead, he believes that a scenario involving the formation of black holes inside supermassive stars is more likely to be correct.

This idea of supermassive starts was first developed  by Fred Hoyle and Willy Fowler in 1963, when they were trying to explain the enormous amount of radio energy emitted by powerful active galaxies such as Cygnus A. Thy proposed that these galaxies were powered by supermassive stars. While this theory turned out to be incorrect, Begelman believes that it could be the basis for finding out how black holes were created.

All stars form when a gas cloud shrinks under the pull of it own gravity.  Two years ago, Begelman, who was at the University of Cambridge, Marta Volonteri, also at the University of Cambridge, and Martin Rees, the UK’s Astronomer Royal began studying the equations that describe this process. They discovered that the speed at which the gaseous material falls inward affects the fate of the shrinking gas cloud.  If it is slow, it could form a supermassive star as Hoyle and Fowler envisaged. But it if it is higher, a black hole may be created.

Material rushing in creates a shock wave which heats the gas cloud. As the hot gas expands, the outer regions of the cloud expand and then cool down. This creates an object whose temperature falls so quickly from the centre to the outer edge that only the core is hot enough for nuclear fusion.  Effectively, two objects are created – a fully fledged star at the centre, surrounded by a massive gas envelope that continues to shrink. The weight of the shrinking gas envelope squeezes the star at the centre, and the star grows hotter than hotter. When it reaches around 500 million degrees Celsius, a reaction involving photons, which ultimately leads to the creation of neutrinos and antineutrinos occurs.  As neutrinos and antineutrinos barely interact with matter, they leave the star, taking heat with them.  As heat was the only thing holding back the weight of the gas envelope, the star now collapses in on itself to form a black hole, embedded within the envelope of gas.  As the gas is pulled into the black hole, bright light is emitted.

According to calculations by Begelman and his University of Colorado colleagues Elena Rossi and Philip Armitage, this process can lead to the creation of an enormous black hole. A black hole lying within a massive gas envelope would be able to consume matter at ten to one hundred times the rate of an ordinary black hole.  Begelman imagines that these black hole stars would be about one hundred million to one billion solar masses.

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