Space is littered with really hard-to-see things. Rogue planets drift darkly between stars. Lonely black holes drift away from their points of formation. Surveys looking for how their gravity bends light allowed us to learn they don’t exist in huge numbers, but like so many soda bottles littering a park, they are out there.
It’s a bit disconcerting to know there are black holes out there, just hanging out, unseen.
Sometimes, a chance companionship between a star and a dark object will allow them to get noticed. This is the case for a newly discovered tiny black hole just three times the mass of the Sun. The red giant star V723 Mon was observed to move in ways that indicate it is orbiting a high mass object, and changes in its brightness indicate that something isn’t exactly bright. In fact, the observed changes in brightness appear to indicate not the light of a companion but rather the distortion of the star. As near as researchers can tell, V723 Mon is stretched out into the shape of a teardrop, and the star appears brighter when we see that teardrop from the side, and thus see more surface area.
Located just 1,500 light-years or so away, this is both about the smallest and totally the closest black hole so far discovered. Lead study author, Tharindu Jayasinghe, notes, that the discovery “implies that there are many more [small black holes] that we might find if we increased the volume of space that we searched.” And, the finding “should create a push to find these systems.”
While black holes are theorized to have masses between about 2.8 solar masses and millions or billions of solar masses, we haven’t had a lot of luck finding the small ones. There have been hints from gravitational wave studies of small ones, but those studies can’t distinguish between black holes and neutron stars, so the question has been, “How do we know for certain that neutron stars can’t be bigger?” There have also been a couple cases of stars appearing to maybe have dark, high-mass companions, but the observations weren’t definitive.
This new object, with its well-defined darkness and mass, is being called a unicorn for its uniqueness. Here is to hoping that rather than being a unicorn it is more like a common horse painted in Vantablack that will make perfect sense once we look a bit longer.
The Ohio State University press release
Newfound black hole may be the closest to Earth (National Geographic)
“A unicorn in monoceros: the 3 M⊙ dark companion to the bright, nearby red giant V723 Mon is a non-interacting, mass-gap black hole candidate,” T Jayasinghe et al., 2021 May 1, Monthly Notices of the Royal Astronomical Society