How big are the black holes created by the collapse of massive stars? Astronomers estimate that there are 100 million black holes in our Galaxy (they had to count them all). They thought that the holes were rather small: not more than 15 times the mass of the Sun. But now, an international team of scientists including Prof. Joel Bregman (U-M Astronomy) and led by Prof. Jifeng Liu (National Astronomical Observatory of China, Chinese Academy of Sciences) has spotted a stellar black hole with a mass 70 times higher than the Sun.
This came as a big surprise. “Black holes of such mass should not even exist in our Galaxy, according to most of the current models of stellar evolution", explains Prof. Liu; “now theorists will have to take up the challenge to explain its formation in a solar metalicity environment." The newly discovered monster is located 15 thousand light years from us and has been named LB-1 by the discovering team.
Until just a few years ago, black holes of stellar origin could only be discovered when they gobbled up gas from a companion star. This process creates powerful emissions of X-rays, detectable from Earth, which reveal the presence of the collapsed object. However, the vast majority of black holes in our Galaxy are not engaged in a cosmic banquet right now, and remain in complete darkness. That is why only about two dozen Galactic black holes out of 100 million have been well identified and measured.
Prof. Bregman and collaborators used a different technique to find new black holes. They surveyed the sky with the LAMOST optical telescope in north-east China to look for stars that orbit around an invisible object, pulled by its gravity. This search is by no means an easy task. Only one star in a thousand may be going round a black hole: it is like trying to find a needle in a haystack. Recent technological developments of telescopes and detectors have finally made this quest possible, more than 200 years after this technique was first proposed by the visionary English scientist John Michell. LB-1 is the result of such a search: a star 8 times bigger than the Sun was seen orbiting around a 70-solar-mass black hole, every 79 days.
Prof. Bregman explains that “previously, such black holes were found only when they were eating a companion star, creating tremendously bright signals. Here, we discover black hole binaries in more ordinary circumstances, by surveying massive stars to see if they are in orbit around a black hole.”
The discovery of LB-1 fits nicely with another breakthrough in astrophysics. Recently, the LIGO and Virgo gravitational wave detectors have begun to catch the ripples in space-time caused by the collisions of black holes in distant galaxies. Intriguingly, the black holes involved in such collisions are also much bigger than what was considered the typical mass of a stellar black hole. The direct sighting of LB-1 proves that this population of over-weight stellar black holes exists even in our own backyard. “This discovery forces us to re-examine our models of how stellar-mass black holes form," comments LIGO Director Prof. David Reitze (University of Florida). “This remarkable result, along with the LIGO-Virgo detections of binary black hole collisions during the past four years, really points towards a renaissance in our understanding of black hole astrophysics."
Meanwhile, the team is already conducting a new observational campaign to measure the properties of LB-1 more precisely, and discover other systems like it. They are hopeful in their aim to discover several dozen black holes and shed light on the life cycle and final stages of massive stars. “This new approach has a great future and will expand our understanding of the mass range and diversity of stellar mass black holes,” said Prof. Bregman. “It will also allow us to understand the final stages of massive stars in orbit around black holes."
More information: Joel Bregman
Link to published Nature article: A wide star–black-hole binary system from radial-velocity measurements