IT’S only a few hundred million years after the big bang. A star unlike anything we know is shining. It is so enormous it could swallow the present-day solar system with ease and so bright that it could outshine a galaxy of a trillion run-of-the-mill stars. Impressive as these characteristics are, they barely hint at the object’s peculiarities. For in its heart a dark secret is hiding. As the outside of the star finally cools, like a dying ember, its outer layers are suddenly blown away into space. And there, uncloaked for the first time, is a monstrous black hole.
It’s a scenario that flies in the face of conventional black hole thinking. Black holes are supposed to arise when a huge star’s core collapses under its own weight- and keeps on going until it is crushed to a point. So the last place you’d expect to find one lurking is inside a star-like object. Yet astrophysicist Mitchell Begelman at the University of Colorado in Boulder believes this scenario is not only possible, but also that it could explain one of the biggest mysteries in the cosmos.
Just a billion years after the big bang, supermassive black holes as much as 10 billion times the mass of the sun were making their presence felt in the universe. We know this because we see evidence for them in distant quasars- extremely bright light emissions given out when gas is drawn in by a supermassive black hole’s enormous gravity. This light has taken nearly 13 billion years to reach us. The question, says Begelman, is how did such monster black holes form and grow so soon after the big bang?
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Working with colleagues Elena Rossi and Philip Armitage, Begelman has shown could solve the mystery. Within a few years he thinks we could be staring at our first images of these exotic celestial objects, courtesy of NASA’s James Webb space telescope.
For years various teams have been trying to explain how supermassive black holes could have arisen in the infant universe. Several attempts have concentrated on the much smaller black holes which form when a star reaches the end of its life and collapses. In a superdense cluster of stars, several of these black holes might merge to create a huge one that continues to grow by feeding on gas.
Begelman points out that there is simply not enough time in the first billion years after the big bang for a stellar-mass black hole to grow to around 10 billion solar masses. Instead, he has revived a scenario first envisaged by Fred Hoyle and Willy Fowler in 1963, which involves the formation of a single, supermassive star.
Hoyle and Fowler were attempting to explain the prodigious radio energy pumped out by powerful galaxies such as Cygnus A. They proposed that Cygnus A and other active galaxies were powered by supermassive stars. The idea turned out to be wrong, but Hoyle and Fowler’s gigantic stars sounded just what Begelman needed to investigate the supermassive black hole problem.
Such a star, with a mass thousands of times that of the sun, would be unstable. Being so massive, the heat generated by nuclear fusion in its core could make only a forlorn attempt at opposing the gravity trying to crush the star. The whole thing would shrink in one fell swoop to make a supermassive black hole. Begelman soon realised, though, that Hoyle and Fowler had not considered how these stars would actually form. “The evolution of such an object might be crucially important to the final outcome,” says Begelman. What he discovered turned out to be weirder than anything he had imagined.
Begelman began studying the evolution of supermassive stars two years ago when he was on sabbatical at the University of Cambridge. He had been hoping to collaborate with his old friend Martin Rees, who is the UK’s stronomer Royal and president of the Royal Society. “Martin is a very busy man, so I needed to find a project that he would find utterly irresistible,” Begelman recalls. “The supermassive star route to seeding supermassive black holes was just the thing.”
All stars, ordinary or otherwise, form from a gas cloud shrinking under the pull of its own gravity. Begelman and Rees, together with Marta Volonteri, who was also at the University of Cambridge at the time, began studying in detail the equations that describe these processes ().
They soon realised that what happens to a shrinking gas cloud depends crucially on how fast the material falls inwards under gravity. If it is relatively slow, then it may be possible to form something like the monolithic supermassive star envisaged by Hoyle and Fowler. “However, if the infall rate is much higher – greater than about a few tenths of a solar mass a year – a radically different object is possible,” says Begelman: a black hole star.
The key is the violence of the infall, which is governed by gravity. Material rushing in creates a shock wave that heats the gas cloud. As the hot gas expands, it puffs up the outer regions of the cloud, which expand and cool down. The result is an object whose temperature falls so steeply from the centre to the outer edge that only the core is hot enough for nuclear fusion. “In effect you get two objects in one – a fully fledged star lurking at the centre of a much more massive envelope of gas, which continues to shrink like a protostar,” says Begelman (see Illustration).
As material from the massive envelope continues to rain down on the burning centre, the extra weight squeezes the star, making it ever hotter. Then, when the temperature reaches about 500 million °C, a catastrophic change is initiated.
The star is now hot enough for photons of light to spontaneously convert into pairs of electrons and positrons. These pairs quickly annihilate each other and in turn produce pairs of particles and antiparticles. Among these are neutrinos and antineutrinos – ghostly particles that barely interact with matter. This lack of interactivity allows the neutrinos and antineutrinos to zip away from the star, taking precious heat energy with them.
Robbed of heat – the only thing that was holding back gravity’s crushing force – the star collapses in on itself to form a black hole. Rather like a stone in the heart of a peach, what is left is a black hole embedded in a massive envelope of gas. As this gas is pulled into the black hole and consumed, super-bright light is emitted.
This process can lead to the growth of a truly huge black hole, according to calculations by Begelman and his colleagues. “When people think of a black hole, they usually imagine it surrounded by an accretion disc of in-swirling matter that is considerably less massive than the black hole itself,” he says. “In this case, the black hole is surrounded by an envelope as much as 100 times as big as the hole itself, which opens up the possibility of the black hole growing at an unprecedented rate.”
The researchers calculated that a black hole embedded in a massive envelope of gas would be able to gobble matter far faster than an ordinary black hole. It’s a surprising conclusion, because the radiation emitted by hot matter swirling into a black hole should blow gas away, or so standard physics says, and this places an upper limit on the amount of material a black hole can devour in one sitting. In a black hole star, however, the sheer weight of the surrounding gas overcomes the push of the radiation. “The result is that its black hole accretes at a rate as much as 10 to 100 times faster than a normal black hole,” says Begelman. The black hole star’s core starts off weighing a few hundred solar masses, but within a million years or so builds up to perhaps a hundred times that.
The black hole does not stay hidden in its cocoon forever. All the time it is growing, deep inside the star, it is giving out heat into space and cooling. Begelman’s team has calculated that when the outer edge of the gas envelope falls to a mere 4000 °C, a drop in ionisation in the cloud allows a sudden surge of radiation to escape from around the black hole. “It blows away the envelope, revealing the naked black hole of maybe 1000 to 10,000 times the mass of the sun, possibly even bigger.”
At this point the mass of the black hole is still well short of a supermassive behemoth. However, creating the seed is the hard part, Begelman says. Once formed, the black hole can grow by accreting gas from a surrounding galaxy, or by merging with another supermassive black hole when two galaxies collide. Both of these scenarios could produce large enough black holes within the first billion years after the big bang.
Missing links
Begelman envisages black hole stars of about 100 million to a billion solar masses. They would look like colossal stars with a girth greater than Pluto’s orbit, he says, and for much of their lives, would shine 10 times as brightly as the entire Milky Way.
The findings have some support from other researchers. They agree broadly, for instance, with a 1994 study by Abraham Loeb and Frederic Rasio, then at the Institute for Advanced Study in Princeton, New Jersey (). Rather than resolving equations like Begelman’s group, they used simulations to study giant clouds gas and also found they collapsed to form the seeds of supermassive black holes.
So where and when are black hole stars born? Begelman says that since the crucial requirement is an inflow rate of gas of at least a few tenths of a solar mass per year, this rules out the first few hundred million years after the big bang, when the first stars lit up.
The universe’s very first stars are believed to have formed in “minihaloes” – clouds of gas which later coalesced into large galaxies. Simulations of galaxy formation suggest that minihaloes weighed about a million solar masses and that their gravity would have been too weak to concentrate gas quickly enough. Once the galaxy fragments merged and became bigger, though, black hole stars could start forming. By Begelman’s reckoning, this was when the universe was around 6 to 9 per cent of its present size.
Black hole stars could even occasionally form in today’s universe. “All that is needed is an infall rate of gas that is sufficiently large,” he says. “Conceivably this could be created when galaxies merge and gas rains down on their mutual core.”
Not everyone, however, is convinced that black hole stars exist. “With [these] mechanisms, something unusual – even dramatic – has to happen to make them work,” says astrophysicist Fulvio Melia of the University of Arizona in Tucson. He is concerned about the timescale needed for the envelope of swirling gas to lose both its angular momentum and energy before it can collapse to seed the black hole. “Somehow this has to happen in a matter of only a few hundred million years,” he says, “whereas simulations with standard physics show that it should take billions.”
“Black hole stars could still be forming, even in today’s universe”
The best way to win round the sceptics would be to find a black hole star in the early universe. With the launch of the in 2013, we have a chance, says Begelman. Because the universe has expanded since the time of black hole stars, their light should be stretched, or red-shifted, to longer wavelengths. So black hole stars should appear as intensely bright star-like objects at near-infrared wavelengths of about 3 to 5 micrometres. “There should be about one in every field of view of the James Webb space telescope,” says Begelman.
Identifying them won’t be easy, however. The problem will be distinguishing them from clusters of highly red-shifted primordial stars, and from nearby objects such as brown dwarfs. All of these objects will show up at infrared wavelengths too.
There is another possibility. Begelman points out that black hole stars may have affected the universe in other ways that we may be able to detect. For the first 100,000 years of their lives, before the black hole actually forms, the stars may shine with fierce ultraviolet light capable of ionising any neutral hydrogen gas in the vicinity. Neutral hydrogen emits radio waves of a signature wavelength, whereas ionised hydrogen does not. So such stars may show up as “holes” in maps of the sky made by future radio telescopes such as the Square Kilometre Array, which begins construction in 2012 (91av, 16 May 2007, p 44). “It is possible they may have left their ‘fingerprint’ on this gas and we may be able to see it,” says Begelman.
It’s not often that anyone predicts an entirely new type of astronomical object. But black hole stars might be the missing link everyone has been looking for to explain how the monster black holes that litter our universe came into being. When the James Webb telescope opens its giant eye on the infrared of the baby universe, no one will be watching more closely than Begelman.
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