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)…
Finding HDE 226868/Cygnus X-1
Cygnus X-1 (more correctly, HDE 226868) is found near η Cygni, the middle star in the neck of the swan. This is right in the middle of a rich portion of the Milky Way with lots of 9th magnitude stars. None of the common star atlases show Cygnus X-1. You may be able to find it using your favorite star atlas software (HDE 226868 is also known as AG+35 1910, BD+34 3815, HIP 98298, SAO 69181, and V1357 Cyg). But to print out charts that I can use in the field, I prefer to use the AAVSO Variable Star Plotter [3]. HDE 226868 is not one of AAVSO’s variable stars, so you’ll have to enter its coordinates directly to generate the charts (RA: 19h 58m 21.6756s, Dec: +35° 12′ 05.775″). I’ve generated a few of charts for anyone to download:
Cyg X-1 AAVSO “A” chart [4](15 deg FOV), with γ and η Cyg labeled.
Cyg X-1 AAVSO “B” chart [5] (3 deg FOV) with η Cyg labeled.
Cyg X-1 AAVSO “C” chart [6] (2 deg FOV) with η Cyg labeled.
With these charts, you’ll quickly find the pretty little blue star that is HDE 226868. It’s not much to look at, but just think of that black hole orbiting around it, sucking away at it like a leech. This is as close as we’ll get to a black hole, at least in our lifetimes.
The Bet
In 1975, Kip Thorne and Stephen Hawking made a famous wager regarding the nature of Cyg X-1. Hawking bet that it wasn’t a black hole; Thorne that it was. While I was a graduate student at Caltech in the early 1980s, Thorne had the wager framed and mounted outside his office (the wager involved subscriptions to Penthouse and Private Eye magazines). Hawking conceded the bet in the 1990s in the face of mounting observational evidence in favor of a black hole.