Cygnus X-1 is almost certainly a black hole. It was discovered in 1964 as a strong X-ray source, and has ever since been the object of intense study. Cygnus X-1 turns out to be too compact to be any known kind of object besides a black hole. It has a mass of about 8.7 solar masses (which exceeds the theoretical maximum mass of a neutron star of about 3 solar masses), but based on how quickly its x-ray intensity fluctuates, Cygnus X-1 has to be less than about 60 km wide. Assuming Cygnus X-1 is a black hole, its event horizon is currently estimated to have a radius of about 26 km.
Most interestingly, Cygnus X-1 orbits the blue supergiant star HDE 226868 at a separation of about 0.2 AU. Cygnus X-1 has distorted HDE 226868 into a tear-drop shape and is eating it away (although whether X-1 is actively stripping away 226868’s outer layers or simply sucking up 226868’s solar wind is unclear; the edge of material gravitationally bound to 226868 is close to the star’s surface). For washed-out astronomers, the most interesting aspect is that HDE 226868 is a 9th magnitude star, making it an easy target for even small telescopes in urban environments. And while you can’t actually see Cygnus X-1 itself, it’s still pretty cool to be looking at a star that’s being eaten alive by a black hole. (ESA/Hubble illustration)
Here’s how to find HDE 226868 (and Cygnus X-1)…