October 21, 2002, Updated September 14, 2012

Black holes result from supernovas, which happen when stars explode after using up all their nuclear fuel. Weizmann Institute astrophysicists have managed to track a star, called S2, racing around a dark mass at the center of the Milky Way, a pioneering achievement that offers additional support for the widely held view that the dark mass is a “supermassive” black hole.

The discovery, published in the current issue of Nature, is said to herald a new epoch of high-precision black-hole astronomy and might help scientists better understand how galaxies are born and evolve.

“Supermassive” black holes, the term for black holes whose mass is more than one million times that of the sun, can be found at the center of many galaxies. Dr. Tal Alexander, a theoretical astrophysicist, and colleagues at the Weizmann Institute, the Max Planck Institute for Extraterrestrial Physics in Munich, and several institutions in France, made the discovery.

The pull of this dark mass is so great that even light can’t escape it, rendering it “invisible, but not powerless,” Alexander said. “The black hole’s presence is felt by its immense gravitational pull. A star that happens to be close to a supermassive black hole will orbit very rapidly around a point of seemingly empty space.”

Another clue is the radiation emitted by gas that is heated up just before it is swallowed forever by the black hole.

Alexander and his colleagues tracked the orbit of the closest known star to the black-hole candidate Sagittarius A*, a dark mass 3 million times the sun’s mass. After following the star for 10 years, they found that it does indeed orbit Sagittarius A*.

The scientists documented the star’s completion of an orbit around a known unusual source of radiation in the middle of our galaxy.

Supermassive black holes are thought to evolve when many smaller black holes merge like smaller bubbles into a big one at the center of a galaxy and begin swallowing everything that comes their way. Such a black hole is what remains from an exploded sun much bigger than our own. The explosion is a rare celestial phenomenon called supernova, which happen when these developed suns use up all their nuclear fuel.

Without fuel to maintain the huge pressure required to counter gravity, the star first implodes before the outer layers rebound against the its core and are violently ejected into space, in a process that is one of the most powerful explosions in nature. Simultaneously, the massive core continues to cave in and quickly collapses into itself to form a black hole.

“In two years or so, I expect to see evidence from others that will prove our findings,” Alexander said.

Some astrophysicists have suggested in the past that perhaps the dark mass in the center of the Milky Way is not a black hole, but a dense cluster of compact stars or even a giant blob of mysterious subatomic particles. It now appears that these are not viable alternatives.

The new detailed analysis of the orbit, made possible by the techniques developed by the team, is fully consistent with the view that the dark mass is a supermassive black hole.

Their technique allowed precise observation of the center of the galaxy, overcoming the problem of interstellar dust permeating space. The observations were made with the new European Very Large Telescope in Chile, whose detectors were developed by scientists from the Max Planck Institute, Paris Observatory, National Office of Aerospace Study and Research, and the Grenoble Observatory. Some American scientists also participated in the observations.

“Such sightings could provide information on a point we know surprisingly little about: our own place in the universe,” Alexander said. “We currently do not even know the Earth’s exact distance from the center of our own galaxy and understanding stellar orbits of this kind might tell us where we are.”

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