Black holes — regions in space where gravity is so powerful that nothing can escape — are tricky to spot in the inky darkness of space. They are also the most terrifying things in the universe: they are so dark that even light cannot escape and falling into one would easily be the worst way to die as its enormous gravity would pull you apart, tearing your bones, muscles, and even molecules. Now, astronomers may have discovered a giant black hole that is 1,550 lightyears distant and has a mass comparable to 12 suns. And it’s “practically in our backyard”.
The science and other stuff to know
Interacting black holes often make themselves known by throwing off flares of light as they devour stars, chow down on dust and gases, or collide with each other. Non-interacting black holes, on the other hand, are harder to find.
To find the “monster” black hole, a recent study examined data from almost 200,000 binary star systems identified by the sky-scanning Gaia mission. “We searched for objects that were reported to have large companion masses but whose brightness could be attributed to a single visible star,” said Dr. Sukanya Chakrabarti, a physics professor at The University of Alabama and the lead author of the study, in a press release. “Thus, you have a good reason to think that the companion is dark.”
In the research, Chakrabarti collaborated with astronomers from the Observatories of the Carnegie Institution for Science, the Rochester Institute of Technology, the SETI Institute Carl Sagan Center, UC Santa Cruz, UC Berkeley, the University of Notre Dame, Wisconsin-Milwaukee, Hawaii, and Yale. The team followed up on sources of interest by consulting spectrographic measurements from other telescopes. The measurements revealed a main sequence star subject to a powerful gravitational force.
“The pull of the black hole on the visible sun-like star can be determined from these spectroscopic measurements… They give us a line-of-sight velocity due to a Doppler shift,” Chakrabarti explained. Doppler shift is a wave’s frequency change in relation to an observer; imagine how the pitch of a siren’s sound changes as an emergency vehicle passes to better understand it. The team also explained that analyzing the line-of-sight velocities of the visible star could help explain how enormous the black hole is. It can also give the period of rotation and the orbit.
Interacting black holes are typically easier to observe in visible light. This is because they’re in tighter orbits and pull material from their stellar companions. This material forms a torus-shaped accretion disk around the black hole that’s accelerated to relativistic velocities.
Since non-interacting black holes have wider orbits and don’t form these disks, their presence has to be inferred from analyzing the motions of the visible star.
“The majority of black holes in binary systems are in X-ray binaries. In other words, they are bright in X-rays due to some interaction with the black hole, often due to the black hole devouring the other star. As the stuff from the other star falls down this deep gravitational potential well, we can see X-rays,” Chakrabart explained. “In this case, we’re looking at a monster black hole. But it’s on a long-period orbit of 185 days, or about half a year. It’s pretty far from the visible star and not making any advances toward it.”
However, there are still unresolved questions regarding the role noninteracting black holes play in galactic evolution.
“It is not yet clear how these noninteracting black holes affect galactic dynamics in the Milky Way,” Chakrabarti noted. “If they are numerous, they may well affect the formation of our galaxy and its internal dynamics.”
The techniques Dr. Chakrabarti and her colleagues employed could lead to the discovery of many more non-interacting systems. According to current estimates, there could be a million visible stars in our galaxy that have massive black hole companions.
“This is a new population that we’re just starting to learn about and will tell us about the formation channel of black holes, so it’s been very exciting to work on this,” Peter Craig, a doctoral candidate at the Rochester Institute of Technology who is advised on his thesis by Chakrabarti, said.