Cygnus X-1 is a strong X-ray source in the constellation Cygnus. It was discovered in 1964 during a rocket flight and is one of the strongest X-ray sources seen from Earth, producing a peak X-ray flux of 2.3×10−23 Wm-2Hz-1. Cygnus X-1 belongs to a high-mass X-ray binary system about 6,000 light years from the Sun that includes a blue supergiant variable star designated HDE 226868. A stellar wind from the star provides material for an accretion disk around the X-ray source. Matter in the inner disk is heated to millions of Kelvin (K), generating the observed X-rays. This system may belong to a stellar association called Cygnus OB3, which would mean that Cygnus X-1 is about five million years old and formed from a progenitor star that had more than 40 solar masses. The majority of the star's mass was shed, most likely as a stellar wind. If this star had then exploded as a supernova, the resulting force would most likely have ejected the remnant from the system. Hence the star may have instead collapsed directly into a black hole. Cygnus X-1 was the subject of a friendly scientific wager between physicists Stephen Hawking and Kip Thorne in 1974, with Professor Hawking betting that it was not a black hole. He conceded the bet in 1990 after observational data had strengthened the case for a gravitational singularity in the system. (more...)
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