How many known black holes are there?

There are so many black holes in the Universe that it is impossible to count them. It's like asking how many grains of sand are on the beach. Fortunately, the Universe is enormous and none of its known black holes are close enough to pose any danger to Earth. read more

The nearest one is some 1,600 lightyears from Earth. In the region of the Universe visible from Earth, there are perhaps 100 billion galaxies. Each one has about 100 million stellar-mass black holes. And somewhere out there, a new stellar-mass black hole is born in a supernova every second. read more

The black holes involved in the binary black hole merger that produced the gravitational wave signal GW151226. Note that, as for GW150914, this detection confirmed the existence of three black holes: two initial black holes of mass 14 and 8 solar masses that merged into a new, more massive black hole of 21 solar masses. read more

By that I mean Cygnus x-1, the compact mass that started the "black hole" mania, back in 1964, is not a black hole, in the same manner that the Sun is not a black. read more