Astronomers have discovered the nearest black hole to Earth and also the first to orbit a star visible with the naked eye.
The star, named HR 6819, is located in the constellation of Telescopium and visible from the southern U.S. and points south. At magnitude 5.4 skywatchers can spot it from a dark, rural sky with the naked eye (but I’d bring along binoculars just in case.) The team originally observed the star as part of a double star study but when they analyzed their observations they discovered a third body in orbit with the pair. Observations with the 2.2-meter MPR/ESO telescope in Chile showed that the inner of the two visible stars orbits an unseen object every 40 days.
Most of the time we heard about supermassive black holes, the kind that lurk in the cores of many galaxies including our own Milky Way. That black hole has a mass of 4 million suns! The hidden black hole in HR 6819 is a stellar-mass black hole — an object formed from the collapse of a single very massive star. It’s also one of the few known that do not interact violently with their environment and, therefore, appear truly black. Black holes can draw in nearby gases or even steal material from a closely orbiting companion star. As the gas swirls down into the hole it’s heated to extreme temperatures and radiates powerful X-rays. Astronomers have spotted only a couple dozen black holes in the Milky Way to date and even fewer “silent types” like this one.
“There must be hundreds of millions of black holes out there, but we know about only very few. Knowing what to look for should put us in a better position to find them,” says ESO scientist Thomas Rivinius, who led the study.
HD 6819’s black hole is invisible, but makes its presence known by its gravitational pull, which forces the luminous inner star to orbit what appears to be absolutely nothing. By studying the orbit of the star in the inner pair the team determined that the hole possesses at least 4 times the mass of our sun. Most black holes form when a star with a mass about 8 times that of the sun runs out of fuel to burn (via nuclear fusion) in its core. With nothing to hold back gravity’s incessant pull the star collapses in on itself and then explodes as a supernova that leaves behind a black hole.
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Black holes are black because their matter is so tightly compressed into such a tiny space that the gravity it exerts is enough to prevent light itself from leaving the star. No light means nothing to see. Astronomers typically infer a black hole’s presence by how its gravity affects things near it, in this case the HD 6819 double star system.
When it comes to observing the sky there are lots of things we can’t directly see such as the 4,264 known extrasolar planets, but I like knowing they’re there just the same. It adds another dimension to viewing the myriad points of light that dot the night sky.
