Type

Stellar mass

Location

In the constellation Ursa Major

Finder Chart

Distance

6,000 light-years

Mass

7 times the mass of the Sun

Size

Diameter 26 miles (42 km), equal to the size of a large city.

Discovery Methods

• Measuring the motions of stars
• Measuring X-rays

XTE J1118+480

The orbiting Rossi X-Ray Timing Explorer discovered this system when it produced an outburst of X-rays in March 2000. Since then, other space-based observatories have helped astronomers piece together a relatively detailed story of its structure and history.

The system consists of a dense, compact object that's around seven times the mass of the Sun. Such an object is too heavy to be a neutron star, so astronomers are left to conclude that it must be a black hole. The system also contains a relatively low-mass "normal" star. The two objects are so close together that the black hole is pulling gas from the surface of the normal star. The gas forms a broad accretion disk that encircles the black hole.

In 2000, observations with four astronomy satellites provided evidence that the inner edge of the accretion disk is about 600 miles (1,000 km) from the black hole's event horizon. Although the disk produces large amounts of high-energy X-rays and ultraviolet energy, along with visible light, it does not produce the kind of X-rays that would indicate the inner edge of the disk is much closer to the horizon.

Observations made by telescopes on the ground and in space, combined with 43-year old pictures on which the system appeared, show that XTE J1118+480 follows a looping path that takes it not only through the disk of the Milky Way galaxy, but well outside the disk, too. This may indicate that the system was born billions of years ago inside a globular cluster -- a dense ball of hundreds of thousands of stars. The star that eventually collapsed to form the black hole was ejected from the cluster by gravitational interactions by other stars, and it may have "grabbed" its companion as it left the cluster. Today, it's traveling at 300,000 miles per hour (145 kilometers per second) with respect to our solar system.

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This document was last modified: November 19, 2009.