Alternate Names

Cygnus XR-1

Type

Stellar mass

Location

In the constellation Cygnus

Finder Chart

Distance

6,000 to 10,000 light-years (240 megaparsecs)

Mass

5 to 10 times the mass of the Sun

Size

Diameter 20-40 miles (30-60 km), equal to the size of a large American city

Discovery Methods

• Measuring the motions of gas

Cygnus X-1

Several thousand light-years away, near the "heart" of Cygnus, the swan, two stars are locked in a gravitational embrace. One star is a blue supergiant, known as HDE 226868. It is about 30 times as massive as the Sun and 400,000 times brighter. The other star is 5 to 10 times the mass of the Sun, but it's extremely small. The object must be the collapsed core of a star. Its mass is too great to be a white dwarf or a neutron star, though, so it must be a black hole -- the corpse of a star that once resembled the supergiant.

The system is called Cygnus X-1, indicating it was the first source of X-rays discovered in the constellation Cygnus. Discovered by the Uhuru X-ray satellite in the early 1970s, it was also one of the first suspected black holes.

The X-rays come from a disk of gas that's spiraling into the black hole. As the two stars orbit each other once every 5.6 days, the black hole's gravitational pull causes the blue supergiant to "bulge" toward it. In profile, the supergiant would resemble an egg, with the small end aimed at the black hole. But this egg doesn't have a smooth edge. Instead, hot gas flows away from the star toward the black hole. The gas forms a wide, flat accretion disk that encircles the black hole. Friction heats the gas to a billion degrees or more, causing it to emit a torrent of X-rays -- enough to fry any living thing within millions of miles.

But the X-ray glow isn't steady. Instead, it flickers, which is one bit of evidence that identifies the dark member of the binary as a black hole. Gas enters the outer edge of the accretion disk then spirals closer to the star. If the center of the disk contained a normal star, or even a superdense neutron star, then the disk would get hotter and brighter all the way in to its center, with the brightest X-rays coming from the middle. Instead, the X-ray glow cuts off well outside the center of the disk. Observations with Hubble Space Telescope reveal that the central region occasionally flares up as blobs of gas break off the inner edge of the disk and spiral into the black hole.

These blobs are accelerated to a large fraction of the speed of light, so they circle the black hole hundreds of times per second. This causes the system's X-rays to "flicker." If the blobs of gas were orbiting a larger object, they would not move as fast, so their high-speed revolution is one bit of circumstantial evidence that identifies the dark companion as a black hole.

The black hole's strong gravitational field "redshifts" the energy emitted by this gas to longer and longer wavelengths. Eventually, as the gas approaches the event horizon, the redshift becomes so great that the material disappears from view -- just before it spirals into the black hole.

News

Black Hole Forges Invisible Bubble (Space.com)
Astronomers may have to revise their notion of how much energy stellar-mass black holes put back into the space around them after discovering a huge invisible bubble of energetic gas surrounding the black hole known as Cygnus X-1.

References

NASA's Black Holes

'Death Spiral' Around a Black Hole Yields Tantalizing Evidence of an Event Horizon

The Hundred Greatest Stars, by James Kaler (2003)

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This document was last modified: November 19, 2009.