Record breakers: An artist's representation of M33 X-7, a binary system in the nearby galaxy M33. This system contains a massive blue star feeding materal to a black hole surrounded by a small accretion disk.

Credit: NASA/CXC/M. Weiss

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SYDNEY: In a galaxy not so far away, astronomers have located the biggest stellar black hole yet measured, and it is locked in a tight orbit with an even bigger star.

The size of the black hole, which tips the scales at 16 times the mass of our Sun, and its close proximity to an even bigger companion star may force a rethink about how the universe's biggest stars live and die. The research appears today in the British journal Nature.

Marriage made in the heavens

Black holes come in several flavours. Supermassive black holes weigh thousands to millions of Suns and are thought to reside at the heart of all galaxies, including the Milky Way. Stellar black holes, which result from the gravitational collapse of a single star, are more modest in weight, on the order of 10 solar masses.

Jerome Orosz, an astronomer from San Diego State University, USA, and his team pointed their telescope in the direction of an X-ray source in the spiral galaxy Messier 33, which is 3 million light years away.

What they found was a stellar black hole and a companion star that passes directly in front of it on a three-and-a-half-day orbit, eclipsing the black hole's X-ray emissions. This allowed the team to calculate the pair's masses more accurately than usual.

The black hole was found to weigh 15.7 solar masses, making it the biggest stellar black hole ever found. Dubbed M33 X-7, it still pales in comparison to the size of its companion star. At 70 solar masses it is the most massive star found in a binary system with a black hole.

"This is a huge star that is partnered with a huge black hole," said co-author Jeffrey McClintock of the Harvard-Smithsonian Centre for Astrophysics. The black hole find is even more remarkable, because current astronomical models have difficulty predicting anything greater than ten solar masses.

"Giant leap forwards"

In order for it to form before its companion star, M33 X-7's parent star must have been the bigger of the two stars. Such a massive star would have had a radius bigger than the current separation between the two celestial bodies, suggesting the stars once shared a common atmosphere, said the researchers.

This process usually leads to a large amount of mass being lost from the system, so much that a 16 solar-mass black hole should not have been able to form. Given M33 X-7's existence, the scientists deduced it must have lost gas 10 times slower than current models predict.

"Massive stars can be a lot less extravagant than people think," Orosz said, "hanging onto a lot more of their mass toward the end of their lives."

The study is "a giant leap forwards," said astronomer Tomasz Bulik, from the University of Warsaw, Poland, in an accompanying Nature commentary. "By making additional, more precise measurements of black holes in binaries, and extending the survey to other galaxies, we will gain a better understanding of how black holes form," he said.

Bulik added that the work may be pave the way for studies of double black hole binaries, which would be prime candidates for emitting the as yet undetected gravitational waves predicted by Einstein's theory of general relativity.