A black hole as heavy as almost 16 Suns has set a new weight record for black holes that form from collapsing stars. Its discovery suggests that there may be even heavier ones lurking out there, spawned in the death throes of the universe鈥檚 most massive stars.
When a very massive star ends its life, its outer layers explode outwards, forming a supernova, while its core collapses to form a black hole. There are limits to the size of the so-called stellar-mass black holes born this way, not least because there is only so much matter available from the parent star.
Until now, all the black holes formed this way whose mass has been precisely measured turn out to weigh in at 10 Suns or less. Some astronomers had proposed that this might be the upper limit via this route. Heavier black holes weighing millions of Suns can be found at the centres of galaxies, but these are probably formed in a different, if still mysterious, way.
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鈥淭ill now, all black holes formed this way turned out to weigh in at 10 Suns or less, which was thought to be the upper limit鈥�
At 15.7 times the Sun鈥檚 mass, the newly measured object, called M33 X-7, has smashed through that tentative limit. It is also the first black hole to be discovered in an eclipsing binary: that is, it is orbiting a companion star in such a way that when seen from Earth it sometimes passes in front of the black hole, and vice versa.
The companion star is itself a brute, some 70 times the Sun鈥檚 mass. Jerome Orosz of San Diego State University in California and colleagues used the 8.2-metre Gemini North telescope at Mauna Kea, Hawaii, to work out the orbit precisely and pin down the black hole鈥檚 mass (Nature, ).
For stars with a chemical composition similar to the Sun, so much mass is blown away during their lifetime that only a small fraction remains when they die, so even the biggest could barely produce a 16-solar-mass black hole. This slimming-down process would be less efficient for stars made from relatively pure hydrogen and helium, which could explain how M33 X-7 was born so large, says astrophysicist Stanford Woosley of the University of California, Santa Cruz. That explanation is plausible because the companion star appears to have just 10 per cent of the heavy-element impurities that the Sun has.
Team member Charles Bailyn of Yale University says purer stars that formed in the early universe may have spawned even larger black holes. 鈥淭hose things would send most of their mass into a black hole,鈥� he says, producing black holes with as much as 1000 times the Sun鈥檚 mass.
This raises the possibility that the supermassive black holes might have formed from the growth or merger of such massive collapsed stars. 鈥淭hey might make extremely massive black hole remnants that could be the seeds for these much more massive black holes,鈥� he says.
