The heaviest black hole formed from the collapse of a single star weighs as much as 33 Suns - double the previous record, new measurements confirm.

Black holes come in a range of sizes, from "supermassive" behemoths weighing billions of Suns to "stellar-mass" objects weighing a few times the Sun's mass.

These stellar-mass black holes form when a massive star dies, sending its outer layers exploding outwards in a supernova and collapsing its core into a black hole. There are limits to how massive the objects can become, based on the mass and chemical composition of the parent star.

The previous record holder is a black hole in the nearby galaxy M33. Called M33 X-7, it was measured to have 16 times the Sun's mass.

In late 2007, astronomers led by Andrea Prestwich of the Harvard-Smithsonian Center for Astrophysics in Cambridge, US, suggested a black hole called IC 10 X-1 was even heftier, with between 24 and 33 times the Sun's mass.

But the result, which was based on the apparent orbit of a nearby companion star, was uncertain. That's because the orbit was deduced from repeating dips in the brightness of X-rays coming from the black hole's vicinity. Astronomers suspected the dips occurred when the companion star periodically passed in front of a disc of hot, X-ray emitting matter spiralling into the black hole, but there was no evidence to prove it.

Now, astronomers Jeffrey Silverman and Alexei Filippenko at the University of California, Berkeley, US, have confirmed the result by measuring the companion star's motion using spectra taken by the 10-metre Keck I telescope in Hawaii, US.

Weight loss

Their observations suggest the black hole weighs between 23 and 33 solar masses (the figure is uncertain because the mass of the companion star is not known precisely).

Just how massive can such stellar-mass black holes get? Stars can be born with more than 100 times the mass of the Sun, but over time they shed some material into space. The amount they lose depends on their chemical makeup – even the biggest stars can barely produce a 16-solar-mass black hole if they are born with a composition similar to the Sun's.

Stars low in elements heavier than hydrogen and helium, however, cast off material less efficiently and could therefore have up to 42 solar masses available to form a black hole when they die, says astrophysicist Stanford Woosley of the University of California, Santa Cruz, who was not involved in either study.

As it turns out, the nearby dwarf galaxy IC 10, which hosts the black hole, contains very few "heavy" elements.

Puzzlingly bright

Roy Kilgard of Wesleyan University in Middletown, Connecticut, US, a member of the earlier study by Prestwich, says heavy stellar-mass black holes could explain some puzzlingly bright X-ray sources seen in other galaxies.

The "ultra-luminous" sources are 10 to 100 times brighter in X-rays than typical stellar-mass black holes, leading some to suggest they are actually a separate class of "intermediate mass black holes". The middleweight black holes are thought to weigh in at hundreds or thousands of times the mass of the Sun and give off X-rays as they swallow matter.

The X-ray sources could instead be hefty stellar-mass black holes – such as IC 10 X-1 – that are undergoing especially bright outbursts of X-rays, Kilgard says.

There are probably many other stellar-mass black holes in this mass range that have eluded detection until now because they are farther away and harder to study, Kilgard says. "I expect in the coming years we'll identify many more of these things," he told New Scientist.

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