Cygnus X-1

Cygnus X-1 is a high-mass X-ray binary in the constellation Cygnus, containing a stellar-mass black hole that accretes matter from a massive companion star.

Accretion disk: A rotating structure of gas and dust that spirals inward around a massive object. Gas captured from the companion forms a hot disk that radiates X-rays.
U-2 rocket: A high-altitude research rocket used to carry scientific instruments above most of Earth’s atmosphere. A 1964 U-2 flight detected Cygnus X-1 as a bright X-ray source.
Stephen Hawking: A British theoretical physicist known for work on black holes, cosmology, and gravitation. He wagered against Cygnus X-1 being a black hole, then conceded after evidence accumulated.
Hawking–Thorne wager: A 1974 wager between Stephen Hawking and Kip Thorne over whether Cygnus X-1 contained a black hole. The wager captured the uncertainty surrounding the system’s early evidence.
V404 Cygni: A low-mass X-ray binary containing a black hole and a donor star that undergoes bright outbursts. Its episodic accretion contrasts with Cygnus X-1’s persistent wind-fed emission.
Stellar wind: A continuous outflow of charged particles from a star’s atmosphere. The black hole captures part of the companion’s powerful stellar wind.
Riccardo Giacconi: An Italian-American astrophysicist who pioneered observational X-ray astronomy. His research group helped discover and study the source in the 1960s.
Kip Thorne: An American theoretical physicist whose work focuses on gravitation and astrophysics. He argued that Cygnus X-1 contained a black hole in the famous Hawking–Thorne wager.
Cygnus X-1 distance: The distance from Earth to the Cygnus X-1 binary system, measured using radio parallax. A revised distance changed estimates of the system’s luminosity, mass, and size.
LMC X-1: A persistent high-mass X-ray binary in the Large Magellanic Cloud containing a black hole and an O-type companion. It offers a comparable massive-star system in a different galactic environment.

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