by The black hole record books have just been rewritten.
A black hole roughly 10 times more massive than our sun lurks just 1,560 light-years from Earth, a new study reports. This is about twice as close as the previous proximity champion.
The newly discovered object, a stellar-mass black hole called Gaia BH1, is in a binary system whose other member is a helium-like star. This star is as far from its companion black hole as Earth is from the sun, which makes Gaia BH1 very special.
“While there have been many claimed detections of systems like this, almost all of these discoveries have subsequently been disproved,” lead study author Kareem El-Badry, of the Harvard-Smithsonian Center for Astrophysics in Massachusetts and the Max Planck Institute for Astronomy in Germany. , said in a statement (opens in new tab). “This is the first clear detection of a sun-like star in a wide orbit around a stellar black hole in our galaxy.”
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Astronomers believe that our Milky Way is home to about 100 million stellar-mass black holes, light-eating objects that are five to 100 times more massive than the sun.
Their small size makes these bodies relatively difficult to detect, however, especially with a telescope. (Gravitational wave detectors have had more success recently, finding evidence of mergers with these objects.) And the ones scientists see tend to be “X-ray binaries,” black holes that pull material from a companion star into an accretion disk around the themselves. This rapidly rotating dust and gas emits X-rays, high-energy light that some powerful telescopes can observe.
However, not all stellar-mass black holes that reside in binary systems are actively fueled. Finding these inactive items is even more difficult and requires different strategies.
The researchers used such an alternative technique in the new study. They looked at data collected by the European Space Agency’s (ESA) Gaia spacecraft, which precisely maps the positions, velocities and orbits of the Milky Way’s roughly 2 billion stars.
One of these stars is Gaia’s companion BH1. Its motion exhibits tiny irregularities—a sign that something huge and invisible is gravitationally pulling on it.
Gaia measurements suggested that a black hole could be that pull, but scientists needed more data to know for sure. So they studied the star with a series of ground-based instruments, including the Gemini North and Keck 1 telescopes in Hawaii and the Magellan Clay and MPG/ESO telescopes in Chile.
These follow-up observations, combined with the Gaia data, allowed the team to measure the system in detail. The unseen object contains the mass of 10 Suns, they determined, and orbits the system’s center of mass about once every 186 Earth days. And it must be a black hole.
“Our follow-up observations of Gemini confirmed beyond reasonable doubt that the binary system contains a normal star and at least one quiescent black hole,” El-Badry said. “We have been unable to find any plausible astrophysical scenario that can explain the observed orbit of the system that does not include at least one black hole.”
If the invisible object in Gaia BH1 were a star, for example, it would be much brighter than its companion, and therefore easier to see. But none of the team’s observations revealed a hint of a second star in the system.
The Gaia BH1 system is interesting, and not just because it’s relatively close to us. (Close in the cosmic scheme of things, anyway; the Milky Way’s famous spiral disk is about 100,000 light-years across.) The study team isn’t sure how the star and black hole ended up in their current positions.
The mass of Gaia BH1 indicates that the star that died and created it must have been massive — at least 20 solar masses or so. Such giants only live for a few million years and swell tremendously before giving up the ghost.
Modeling work suggests that such a bulge would likely have destroyed the companion before it had a chance to evolve into a sun-like star (if both were born at the same time). Or, if it survived, it should have ended up in a much tighter orbit than the one it currently occupies, the researchers said.
“Interestingly, this system is not easily accommodated by standard models of binary evolution,” El-Badry said. “It raises a lot of questions about how this binary system formed, as well as how many of these dormant black holes are out there.”
The new study (opens in new tab) was published online today (November 4) in the journal Monthly Notices of the Royal Astronomical Society.
Mike Wall is the author of “Out there (opens in new tab)” (Grand Central Publishing, 2018, illustrated by Karl Tate), a book about the search for extraterrestrial life. Follow him on Twitter @michaeldwall (opens in new tab). Follow us on Twitter @Spacedotcom (opens in new tab) or up Facebook (opens in new tab).
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