An international group of astronomers has determined
the closest black hole to Earth. And, it is orbited by a star similar to our
Sun, the first time such a binary system has been discovered in our galaxy. The
object, known as Gaia BH1, is located 1,600 light-years away, and it has a mass
about 10 times that of our Sun.
Black holes such as this are known as stellar-mass
black holes. Their masses are at most 100 times that of our Sun and they were
created in catastrophic supernova explosions. About 100 million of these
objects should exist in the Milky Way, but only a few have been discovered. Among
them, almost all are actively feeding, stealing material from a companion star,
and releasing a large amount of X-rays.
But Gaia BH1 is different. It is not stealing
anything from its stellar companion, so it is considered dormant. The object
was originally spotted by the European Space Agency’s Gaia spacecraft. Gaia
captured peculiar irregularities in the companion star. Follow-up observations
with the International Gemini Observatory expanded on the readings of the
star's motions and confirmed that it is likely to orbit a black hole.
“Take the Solar System, put a black hole where the
Sun is, and the Sun where the Earth is, and you get this system,” lead author
Kareem El-Badry, an astrophysicist at the Center for Astrophysics, Harvard
& Smithsonian, and the Max Planck Institute for Astronomy, said in a
statement. “While there have been many claimed detections of systems like this,
almost all these discoveries have subsequently been refuted. This is the first
unambiguous detection of a Sun-like star in a wide orbit around a stellar-mass
black hole in our galaxy.”
The team considers this detection unambiguous after
years of hunting for dormant black holes. And while the data appears solid, it
is not clear exactly how this system came to be. The binary system ought to
have been made by a massive star about 20 times the size of our Sun, orbited by
the Sun-like companion we can still observe.
That massive star would have evolved into a super
red giant star in a matter of millions of years. The outer layer of that star
would have engulfed its companion, before the fateful collapse and subsequent
explosion of the supernova. How the companion survived all that is a mystery,
suggesting that we may not have a complete understanding of how black hole
binaries form and evolve.
“It is interesting that this system is not easily
accommodated by standard binary evolution models,” concluded El-Badry. “It poses
many questions about how this binary system was formed, as well as how many of
these dormant black holes there are out there.”
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