The biggest black holes in the universe might have grown within the bellies of giant stars, a new study suggests. If these hole-bearing "quasistars" exist, then they might be bright enough to see from across the universe.
Quasistars are one attempt to explain the existence of supermassive black holes, which astronomers have detected at the hearts of most large galaxies, and whose origin is still unknown.
Smaller black holes are easier to account for - a massive star's core can sometimes collapse into a black hole with around 10 times the mass of the Sun. But their big brothers can be a billion times as massive.
It is possible that the smaller siblings can grow that big by eating stars and gas or by colliding with each other and merging. But they would have to grow up very quickly in cosmic terms, because some supermassive black holes were already around just a few hundred million years after the big bang.
Mitchell Begelman and colleagues at the University of Colorado in Boulder, US, have worked out how the big holes might have gotten a head start in life.
Large clouds of hydrogen and helium were common in the early universe. Begelman says that if such a cloud collapsed into a massive star, a dense knot of the gas could pile up so rapidly in its core that it would collapse into a small black hole.
When that happens in stars just a few times as massive as the Sun, the enormous energy released is enough to blast away the surrounding layers of gas, revealing a brilliant supernova explosion.
But as long as a quasistar is at least 1000 times the mass of the Sun, its great bulk could have absorbed all that energy, containing the supernova with no more than a shudder, becoming a black-hole sun.
The black hole embryo could then grow fast, nourished by the dense body of the quasistar. Gas falling onto the hole would heat up and release an immense amount of light, so much that its pressure would hold up the layers of the star above it.
That could lead to a potentially unstable situation, with dense gas sitting on a lighter layer. Begelman suspects that the pressure would be released as some of the light would escape in "photon bubbles", large blobs of radiation that would burst from the surface of the star. "My guess is it would have to be bubbly," Begelman told New Scientist.
Gestation would last about a million years, at which point the hole could reach at least 10,000 solar masses - not yet an adult supermassive black hole, but a pretty big baby. With such a head start, it would be relatively easy to reach a billion solar masses on a diet of stars and other black holes.
Astronomers may be able to test the idea by searching for the objects. A quasistar would be a little cooler than our Sun, Begelman calculates, but at more than 10 billion kilometres across, it would produce about as much light as a small galaxy.
Detecting them will be difficult, however. They are most likely to have existed in the early universe, when stars are thought to have been much more massive than today. The expansion of space since then would have stretched their light into a band of the infrared spectrum that is absorbed by Earth's atmosphere.