Are There Any Black Holes Near Us?

Question posed by Amy, at the same time she asked this one.

The thing with black holes is that they’re difficult to see: They’re black, on a black background, and they don’t give out any energy that we can measure. We have to infer their existence from the effect they have on other things, rather than actually being able to image them directly.

Obviously, the bigger black holes have a bigger effect, and one beast that we’re pretty confident about resides at the centre of our galaxy in an astronomical feature known as Sagittarius-A. In the middle of the part of this feature known as ‘Sagittarius West’, there’s a bright, compact radio source known as ‘Sagittarius-A*’. Observations of stars in this region lead us to believe that Sgr A* (as it is known by its mates) is a supermassive black hole, weighing in at about 4 million times the mass of our Sun, and located around 25,000 light years away from us.

However, there are smaller, closer candidates. Here are a few of them:

• GRO J0422+32 is a black hole candidate discovered in 1992 and exists about 8,000 light years away, and can be found in the constellation Perseus. It is thought to have a mass somewhere between 3 and 5 solar masses, and if this is accurate, it makes it the smallest black hole ever found, and is just above the theoretical limit required (much smaller and it could only have become a neutron star).

• Cygnus X-1 is a bright X-ray source that is believed to be a black hole. It is part of a binary system with a blue supergiant star. They orbit around each other once every 5-6 days. Cygnus X-1 was the subject of a bet between physicists Stephen Hawking and Kip Thorne. Hawking, maintaining that it was not a black hole, lost the bet in 1990 when new observational data strengthened Thorne’s view. It is around 6,100 light years away from us, located in the constellation Cygnus, and has a mass around 20 to 40 times that of our Sun.

• A0620-00 is a binary system consisting of a Sun-like star and a black hole with a mass at least 3 (but possibly as much as 11) times that of our Sun. It resides around 3,000 light years away in the constellation Monoceros, and is the closest black hole candidate discovered so far.

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