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Black Holes Encyclopedia

GRO J1655-40

Printable version

Alternate Names

Nova Scorpii 1994


Stellar mass


In the constellation Scorpius

Finder Chart


6,000 to 10,000 light-years


7 times the mass of the Sun


Roughly 25 miles (42 km)

Discovery Methods


GRO J1655-40

One of the paradoxes of black holes is that although they are completely dark -- they release no detectable energy of any kind -- they produce some of the brightest and most powerful phenomena in the universe. Many are surrounded by disks of superhot gas known as "accretion disks." And many of those disks produce "jets" of electrically charged particles that squirt into space at almost the speed of light.

One such black hole is GRO J1655-40. It's classified as a "microquasar" because it produces a large amount of energy from a small volume of space. Full-size quasars are disks of hot gas surrounding supermassive black holes in the cores of distant galaxies. Microquasars are much smaller and less powerful, but they are powered by the same mechanism and they produce large amounts of high-energy gamma-ray and X-ray energy.

In the case of GRO J1655-40, an aging, bloated star and a heavy, unseen companion orbit each other once every 2.6 days. The dark companion is "stealing" gas from the surface of the visible star. This material forms a superhot accretion disk that spirals around the black hole. Gas in the disk is heated to millions of degrees, so it produces X-rays and gamma-rays. From the orbit of the two bodies, and the presence of the accretion disk, astronomers calculate that the dark companion is almost certainly a black hole that's several times the mass of the Sun.

GRO J1655-40 produces several interesting phenomena.

First, it has two strong jets of material flowing away from its poles. The jets consist of material in the accretion disk that is accelerated to almost the speed of light by the disk's strong magnetic field. In fact, the first observations of the jet suggested that it was "superluminal," meaning that it's material moves faster than lightspeed. Further observations shows that the material is actually moving at just less than lightspeed, but it looks faster because of the viewing angle.

Second, GRO J1655-40 and its companion are moving through the galaxy at about 250,000 miles per hour (400,000 km per hour). This supports the idea that the black hole formed from the collapse of the core of a massive star. As the core collapsed, its outer layers exploded as a supernova. The explosion was a little off-centered, though, so it sent the system racing through the galaxy.

And third, there is some evidence that the black hole is rotating. Its rotation pulls along the interior of the accretion disk -- and even the fabric of space itself. Using the orbiting Rossi X-Ray Timing Explorer, Tod Strohmayer of NASA's Goddard Space Flight Center, determined that the black hole is spinning at about 450 times per second. This high-speed rotation pulls the space around the black hole, allowing superhot gas to spiral closer before it's sucked in by the black hole's gravity.



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This document was last modified: February 12, 2012.


Black Hole Devouring its Companion Star
Artist's Rendering


No animations available for this black hole.