‘Quiet’ black holes
Black holes form when very large stars collapse at the ends of their lives. And while they may seem exotic to us, cosmically speaking, these stellar-mass black holes should be extremely commonplace.
Scientists expect our Milky Way Galaxy should hold hundreds of millions of them. Yet, so far, just dozens have been discovered.
The stellar-mass black holes that astronomers have discovered were only found because they are acting like cosmic lighthouses, shooting off X-rays as they violently consume things like gas and stars. But these extreme objects are actually not representative of what astronomers expect to be out there. There should be a silent majority of black holes quietly lurking, not eating anything. Though HR 6819 is one of just a few covert black holes ever found, there should be countless more just like it.
“This is really only the tip of the iceberg,” said study co-author Marianne Heida, an astronomer with the European Southern Observatory.
And as astronomers seek out more and more hiding black holes, they expect to find them closer and closer to Earth. Hadrava estimates there should be black holes within a few dozen light-years of Earth, which would put them closer than some of the brightest stars in our night sky.
In comparison, the previous record holder for closest black hole is V616 Monocerotis, which is nestled in a binary system more than 3,000 light-years away.
“Just looking at the number of black holes that we expect in the Milky Way, I would be very surprised if there aren't a few of them still closer by than HR 6819,” Heida said.