What is a safe distance to maintain between you and a black hole? A hundred miles? A solar system? A light year? Would you feel comfortable getting close enough to peer into one with your naked eye? Guess what? That’s where you are right now. Astronomers have determined that a tri-stellar system just 1,119 light years from Earth is made up of a B3-type star (hydrogen-burning), a black hole and a supergiant Be star orbiting the unusual binary pair. That distance makes it the closest black hole to Earth and the brightness makes the cluster visible without even binoculars from the southern hemisphere. Too close for comfort?
Dr. Petr Hadrava, Emeritus Scientist at the Academy of Sciences of the Czech Republic, is co-author of the research paper, published in the journal Astronomy & Physics, entitled “A naked-eye triple system with a nonaccreting black hole in the inner binary” (in case you wanted to impress your friends by using the proper name). Hadrava tells Sci-News that the system HR 6819 was well-known to astronomers, but they had no idea it contained a black hole until astronomers using the 2.2m telescope at La Silla Observatory in Chile blinking in a way that could only be explained by it orbiting another object – the same strategy used to find exoplanets. What could be big enough to black out an entire star?
“We were totally surprised when we realised that this is the first stellar system with a black hole that can be seen with the unaided eye.”
The black hole in HR 6819 is truly invisible because it’s truly black. Other black holes have been discovered by the light and x-rays emitted as they consume everything around them. This one is not hungry (good news for nearby solar systems, right?), so its presence was made known by the periodic (every 40 days) disappearance of the orbiting star. That was enough proof for co-author Dr. Thomas Rivinius, an astronomer at ESO (European Organisation for Astronomical Research in the Southern Hemisphere).
“An invisible object with a mass at least 4.2 times that of the Sun can only be a black hole.”
While the HR 6819 system can be seen with the naked eye in the Southern Hemisphere, now is not a good time to look because it’s just emerging from behind the Sun. (Here's a video re-creation.) That gives us some time to ponder another observation made to the BBC by co-author Marianne Heida, a postdoctoral fellow at ESO.
"In the Milky Way, the idea is that there should be about 100 million black holes. So there should be perhaps a couple more that are closer by still."
One-thousand-and-change light years wasn’t sounding so bad, especially since the black hole in HR 6819 isn’t of the planet-gorging kind. But closer? How close? Proxima Centauri, the closest star, is only 4.244 light-years away – close enough that more than a few people believe we’ve been visited by beings from one of its planets. There are a lot of stars between 4 and 1,000 light years from the Sun. That’s a lot of chances to find more “truly black” black holes that aren’t eating their way towards us. However, not much is known about these black holes. How long will they stay calm and non-dangerous?
In 1957, a team of psychologists in California began a seven-year interview with an eight-handed alien from Alpha Centauri who spoke to them through a young woman under hypnosis. The story of ‘Hands’ was told in "Hands: The True Account. A Hypnotic Subject Reports on Outer Space." Perhaps we might try to contact this alien again.
Hey Hands! Are you still around? Know any nearby black holes? Are you familiar with Zoom?