Why the black hole on our doorstep is nothing to worry about - By Emily Jones

Earlier this year, a black hole was discovered 1000 lightyears away from Earth. This may seem, well, lightyears away, but in astrological terms it’s basically next door - and it looks like we have a new celestial neighbour. Black holes have a gravitational pull strong enough that not even light can escape, so should we be concerned about the discovery that is closer to our Solar System than any other found to date?

The black hole was discovered by researchers using the 2.2m telescope at La Silla Observatory in Chile, part of the European Southern Observatory (ESO). They were originally observing a double star system called HR 6819 when they noticed the two stars seemed to be orbiting a third, invisible object. Observations showed the inner star rotated around it every 40 days, whereas the outer star had a much greater orbital period.

As the third body did not emit light, the researchers wondered whether it could be a black hole. Their suspicions were confirmed by analysing older data of the inner star’s orbit and brightness, in which they estimated that the inconspicuous object was 4.2 times the mass of our Sun. An invisible object this large could only be a black hole. Although it is of course black, the surrounding stars are actually close enough to be seen by the naked eye, if you are in the Southern Hemisphere.

Black holes are formed by the aftermath of a supernova - the death of a large star (25 times the mass of our Sun). Black holes consist of a huge amount of matter packed into a tiny space, and they grow by violently absorbing matter from their surroundings. This forms an accretion disc of gas and dust, and emits high-energy x-rays that can be detected by telescopes. This sounds daunting, but we should not worry about the HR 6819 black hole. Firstly, it is not close enough that its gravitational pull poses any threat to Earth.

It can also be considered ‘inactive’ as it is not growing in size. Its surrounding stars are not large enough to eject enough matter for the black hole to feed off, and it has likely been the same size since it formed. Furthermore, the HR 6819 black hole is relatively small, at four times the mass of our Sun. For comparison, the black hole at the centre of the Milky Way is 3.6 billion times the mass of our Sun. Dietrich Baade, an astronomer at ESO and co-author of the new study, said "Black holes are not vicious, they don't know about us, they're not after us."

What this discovery does mean, however, is that there are likely many more black holes just waiting to be discovered. Due to their dark nature, only a couple dozen have been discovered in the Milky Way to date. “Theoretical models suggest that in the Milky Way there are between 100 million and 1 billion black holes,” said Baade, “We are finding a tip of an iceberg.”

The HR 6819 discovery is novel, although it doesn’t mean it is unique. "Because the Sun is not in a special position in the Milky Way, black holes of this kind must be common because of how close this one is to us," Baade says. Now that scientists know what to look for, discoveries of this kind of black hole may become more frequent in years to come.