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Black holes · the first

Cygnus X-1: the first black hole anyone was sure of

Cygnus X-1, a black hole ringed by a bright disc of gas, its light bent round it, drawn in Exo-View

Cygnus X-1, in the constellation Cygnus the Swan, 7,240 light-years away, was the first object widely accepted as a black hole. It is 21 times the Sun's mass, circled every 5.6 days by a blue supergiant whose wind it is dragging into a disc of gas heated to millions of degrees. Exo-View flies you to it.

Fly to Cygnus X-1See the Milky Way
Distance
7,240 light-years
Mass
About 21 Suns
Horizon
About 63 km in radius: a black hole of this mass is the size of a city
Companion
HDE 226868, a blue supergiant about 40 times the Sun's mass
Orbit
Every 5.6 days, at about a quarter of the Earth–Sun distance
Found
An X-ray source in 1964; accepted as a black hole by the early 1970s

How a black hole is seen

A black hole gives off no light of its own, but gas falling into one is squeezed and heated so much that it shines in X-rays. Cygnus X-1 was found in 1964 as a source of X-rays in a rocket flight above the atmosphere. In 1971 its position was matched with a blue supergiant, whose motion showed that it was circling something invisible, too heavy to be a neutron star. Nothing but a black hole fits.

A famous bet

In 1974 Stephen Hawking bet Kip Thorne that Cygnus X-1 was not a black hole, as insurance, he said, in case years of work on them came to nothing. By 1990 the evidence was in, and Hawking conceded. In 2021 a chain of radio telescopes across the United States fixed its distance and mass more tightly than ever, and showed that it is spinning very fast, close to the greatest rate a black hole can.

In Exo-View

The key below flies from the Sun to Cygnus X-1 and draws the hole with its disc, seen nearly face-on, the sky bent round it. A horizon only 63 km across is invisible at this scale, so the disc and the shadow are drawn enlarged, and the plate says so. For the fall itself, see the Simulations page.

Fly to Cygnus X-1See the Milky Way

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