A Trinary Star System with a Black Hole ↦

Jennifer Ouellette reporting for Ars Technica (emphasis added):

The [European Southern Observatory (ESO)] team had been conducting a study of double-star systems, and HR 6819 was included as part of their observational data-gathering since it appeared to be just such a system. But while reviewing their data, the astronomers found clear evidence of an unexpected third object in the system: a black hole that had previously eluded detection.

This black hole is the closest ever found at only 1,000 light years away. It’s also relatively small — 4 solar masses compared to an estimated 4.28±0.1 million solar masses for the black hole at the center of our galaxy. The proximity of this black hole suggests that these smaller black holes may be quite common throughout the Universe. And more specifically, these previously-unknown trinary-with-a-black-hole systems may be somewhat common as well:

“If such a system happens to be in the immediate neighborhood, it is likely common in other regions of the galaxy as well,” said Rivinius. His back-of-the-envelope calculation suggests that there could be 2,500 such systems. That’s not going to clear up the large discrepancy between the black holes we’ve discovered and the number astronomers believe could be out there. “But considering so far we were not aware any such triple could exist, it is quite a step,” he added. The ESO team has already identified a second star system that might also be a trinary with a black hole, although more observational data is needed to confirm this.