In a second model, the demise of these stars unfolds less directly. First, they explode as supernovas, hurling their outer layers into space and leaving behind a dense, burned-out remnant called a neutron star. Debris from the explosion then falls back onto the neutron star, turning it into a black hole.

Theorists have spotlighted the supernova scenario because it may explain the origin of mysterious flashes of high-energy radiation, or gamma-ray bursts (SN: 7/10/99, p. 28). Now, observations of a star that closely orbits a suspected black hole strongly support the supernova model of black hole formation, researchers report in the Sept. 9 Nature.

Rafael Rebolo of the Institute of Astrophysics of the Canary Islands in La Laguna, Spain, and his colleagues examined the composition of the atmosphere of an ordinary star, about twice the sun's mass, that circles a suspected black hole more closely than Mercury orbits our sun. Residing some 10,000 light-years from Earth, the pair of objects is known as GRO J1655-40 or Nova Scorpii 1994.

Using the Keck I Telescope on Mauna Kea in Hawaii, the team found that the star's outer layers contain oxygen, magnesium, silicon, and sulfur in abundances 6 to 10 times those found in the sun. That's a puzzle because the relatively lightweight star would never have reached the internal temperature, greater than 3 billion kelvins, required to forge high concentrations of these elements. In contrast, the star's massive companion could easily have generated them before it became a black hole.

If the companion had collapsed directly into a black hole, the material would have remained locked inside. However, if the heavyweight had first exploded as a supernova, ejecting the elements into space, its lower-mass partner could have captured them.

"This is, to our knowledge, the most direct evidence ever found for a link between a supernova and black hole formation," Rebolo and his colleagues assert.

"There's no other way I can think of that you could actually have an enhancement [of the four elements] except to say that the star that just became a black hole must have first blown up and injected that enriched material into the companion star," says John J. Cowan of the University of Oklahoma in Norman.

Another possibility, enrichment via a wind blown from the massive star, doesn't work, he says. A wind would contain a variety of materials, including iron, but the star shows enhancement by only the elements likely to be released in a supernova explosion, Cowan notes.

"These are fascinating observations," says Stan Woosley of the University of California, Santa Cruz, but he adds that he would like to see a detailed supernova model that explains the observed abundance pattern. Woosley has championed the supernova-black hole model to explain gamma-ray bursts.

One complication is the star's proximity to its partner. Before becoming a black hole about a million years ago, the latter was a star 25 to 40 times the mass of the sun. Its outer layers would have enveloped the smaller star.

The evidence suggests, however, that such direct contact played only a minor role in altering the star's atmosphere, Rebolo says. For example, nitrogen, which would have been plentiful in the outer layers of the black hole's predecessor, has relatively low abundance in the low-mass companion.

Rebolo holds that black holes may arise either through supernova explosions or direct gravitational collapse. His team plans to search for the supernova signature in other black hole systems.

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