Black holes, as frighteningly extreme as they may be, are relatively commonplace across the universe. Like most large galaxies, our own Milky Way packs a supermassive black hole at its core, a lurking monster some four million times as massive as the sun.

But our own neighborhood bully appears relatively tame next to a distant quasar, or bright galactic center, recently spotted by astronomers Todd Boroson and Tod Lauer of the National Optical Astronomy Observatory (NOAO) in Tucson, Ariz. The quasar, known as SDSS J153636.22+044127.0, appears to host a pair of black holes, bound together in a tight orbital relationship, circling each other every 100 or so years. The finding appears today in Nature.

Both of the black holes in the pairing dwarf that of the Milky Way: the smaller of the two, Boroson and Lauer write, appears to have a mass some 20 million times that of the sun, whereas the larger black hole boasts an estimated mass of nearly 800 million suns. According to the NOAO team's analysis, the pair is separated by a mere third of a light-year—for comparison, the sun's nearest stellar neighbor is about 13 times as distant.

Galaxies are believed to occasionally merge with their neighbors, bringing along a meeting of their respective black holes that may coalesce into even more massive galactic gobblers. But astronomers have been hard pressed to catch black holes in the act—even the new results are presented as a hypothesis requiring further examination.

In an accompanying commentary, astronomer Jon Miller of the University of Michigan at Ann Arbor calls the new study "the first compelling example of a tightly bound black-hole binary system." Previous candidate systems, Miller writes, hosted black holes separated by much greater distances or, as in the case of a quasar dubbed OJ 287, provided less conclusive evidence for a black hole duo.

Image credit: P. Marenfeld/NOAO