Cygnus X-1

<a href="/facts/Celestial_coordinate_system/23HhqvHj">Coordinates</a>: <a href="http://www.wikisky.org/?ra=19.972687666667&de=35.201604166667&zoom=5&show_grid=1&show_constellation_lines=1&show_constellation_boundaries=1&show_const_names=1&show_galaxies=1&img_source=IMG_all">19h 58m 21.6756s, +35° 12′ 05.775″</a>

Cygnus X-1 (abbreviated Cyg X-1) is a galactic <a href="/facts/Astrophysical_X-ray_source/LdWyPnzA">X-ray source</a> in the <a href="/facts/Constellation/nBQqgW8A">constellation</a> <a href="/facts/Cygnus_(constellation)/L1fYe414">Cygnus</a> and was the first such source widely accepted to be a <a href="/facts/Black_hole/1r2lmkcF">black hole</a>. It was discovered in 1964 during a <a href="/facts/Sub-orbital_spaceflight/NdLRlGIq">rocket flight</a> and is one of the <a href="/facts/Signal_strength/7DwVuERF">strongest</a> X-ray sources detectable from Earth, producing a peak X-ray <a href="/facts/Flux_density/Aw3P5Bvb">flux density</a> of 2.3×10−23 <a href="/facts/Watt/VK8oqRrm">W</a>/(<a href="/facts/Meter/hPhzc1CV">m</a>2⋅<a href="/facts/Hertz/6Wshj5qm">Hz</a>) (2.3×103 <a href="/facts/Jansky/8m8DQU5F">jansky</a>). It remains among the most studied <a href="/facts/Astronomical_object/BCHLa0Q2">astronomical objects</a> in its class. The compact object is now estimated to have a mass about 21.2 times the <a href="/facts/Solar_mass/j6Auhku8">mass of the Sun</a> and has been shown to be too small to be any known kind of normal star or other likely object besides a black hole. If so, the radius of its <a href="/facts/Event_horizon/KXH2ptnI">event horizon</a> has 300 <a href="/facts/Kilometre/hdQp79QE">km</a> "as upper bound to the linear dimension of the source region" of occasional X-ray bursts lasting only for about 1 ms.
Cygnus X-1 belongs to a high-mass <a href="/facts/X-ray_binary/fpvJvImP">X-ray binary</a> system, located about 2.22 kilo<a href="/facts/Parsec/SwSsk57J">parsecs</a> from the <a href="/facts/Sun/CadPCVHh">Sun</a>, that includes a blue <a href="/facts/Supergiant_star/y7w8WEcm">supergiant</a> <a href="/facts/Variable_star/Ac8mp2gy">variable star</a> designated HDE 226868, which it orbits at about 0.2 AU, or 20% of the distance from Earth to the Sun. A <a href="/facts/Stellar_wind/bn5J6vpx">stellar wind</a> from the star provides material for an <a href="/facts/Accretion_disk/8zJXshat">accretion disk</a> around the X-ray source. Matter in the inner disk is heated to millions of degrees, generating the observed X-rays. A pair of <a href="/facts/Relativistic_jet/QYerB8R3">relativistic jets</a>, arranged <a href="/facts/Perpendicular/u0k8xqx4">perpendicularly</a> to the disk, are carrying part of the energy of the infalling material away into interstellar space.
This system may belong to a <a href="/facts/Stellar_association/urBqjfYg">stellar association</a> called Cygnus OB3, which would mean that Cygnus X-1 is about 5 million years old and formed from a progenitor star that had more than 40 <a href="/facts/Solar_mass/j6Auhku8">solar masses</a>. The majority of the star's mass was shed, most likely as a stellar wind. If this star had then exploded as a <a href="/facts/Supernova/Pfo2J23l">supernova</a>, 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 <a href="/facts/Stephen_Hawking/b7dcce7n">Stephen Hawking</a> and <a href="/facts/Kip_Thorne/o6wkjTtP">Kip Thorne</a> in 1975, with Hawking—betting that it was not a black hole—hoping to lose. Hawking conceded the bet in 1990 after observational data had strengthened the case that there was indeed a <a href="/facts/Black_hole/1r2lmkcF">black hole</a> in the system. As of 2004<a href="https://en.wikipedia.org/w/index.php?title=Cygnus_X-1&action=edit">[update]</a>, this hypothesis lacked direct empirical evidence but was generally accepted based on indirect evidence.

Cygnus X-1 open-in-new

Cygnus X-1