Alternate Names

Nova Muscae 1991, GRS 1124-684

Type

Stellar mass

Location

In the constellation Musca

Distance

16,500 light-years

Mass

7 times the mass of the Sun

Size

Diameter of 25 miles (42 km)

Discovery Methods

• Detection of an accretion disk

GU Muscae

Two orbiting X-ray telescopes, Granat and Ginga, discovered GU Muscae when the system produced an outburst of X-rays on January 9, 1991.

It is one of several likely black hole systems that are classified as X-ray novae. Such a nova periodically produces bright outbursts of X-rays, along with visible light and other forms of energy.

In such a system, a black hole pulls gas from the surface of a companion star. The gas forms a thin disk around the black hole, known as an accretion disk. In an X-ray nova, the flow of gas is fairly thin and slow, so the accretion disk remains relatively cool, and little gas falls into the black hole.

But just as a slow trickle of water into a bucket eventually will cause it to overflow, over time enough hot gas builds up to cause an overflow in the accretion disk. Much of the gas quickly “falls” to the innermost region of the disk, where it is heated to hundreds of millions of degrees, producing an outburst of X-rays. Some of the gas falls from the disk into the black hole. The X-rays heat the outer regions of the disk along with the companion star, causing them to shine brighter in ultraviolet and visible light. Such an outburst can last for hours or days, and can repeat itself.

In the case of GU Muscae, the black hole is about seven times as massive as the Sun, while the companion is three-quarters as massive as the Sun. The companion is also cooler than the Sun, so its surface is redder, and the star's total luminosity is only one-third that of the Sun's. Its outer layers probably were blown away by the supernova explosion that gave birth to the black hole. The two stars orbit each other every 10.4 hours at a distance of roughly 2 million miles (3.2 million km).

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This document was last modified: November 19, 2009.