Oct. 17, 2002 Rehovot, Israel – October 16, 2002--Supermassive black holes – the name given to black holes whose mass is more than 1,000,000 times the mass of the sun – can be found at the center of many galaxies. Scientists from the Weizmann Institute of Science, the Max Planck Institute for Extraterrestrial Physics, and several institutions in France have succeeded in tracking a star racing around a dark mass at the center of our galaxy. This achievement offers more support for the widely held view that the dark mass is a supermassive black hole. The findings were published in the current issue of Nature. The scientists tracked, for the first time, a star completing an orbit around a known unusual source of radiation (a black hole candidate) in the center of our galaxy. This discovery heralds a new epoch of high precision black hole astronomy and that might help us better understand how galaxies are born and evolve.
Supermassive black holes are thought to evolve when many smaller black holes merge at the center of a galaxy, and start swallowing everything that comes their way. Such a black hole is a remnant of an exploded sun much bigger than our own. The explosion is a rare celestial phenomenon called supernova, which happens when these developed suns use up all their nuclear fuel. Without fuel to maintain the huge pressure that is required to counter gravity, the star first implodes, and then the outer layers rebound against the sun's core and are violently ejected into space, in a process that is one of the most powerful explosions that occur in nature. Simultaneously, the massive core continues to cave in. It rapidly collapses into itself and forms a black hole.
The pull of this dark mass is so great that even light can't escape it, rendering it invisible. "Invisible - but not powerless," said Dr. Tal Alexander, a theoretical astrophysicist at the Weizmann Institute of Science's Physics Faculty. "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 at the Max Planck Institute for Astrophysics tracked the orbit of the closest known star to the black hole candidate Sagittarius A*, a dark mass 3,000,000 times the mass of the sun. Following the star for 10 years, they found that it does indeed orbit Sagittarius A*. Approaching the black hole's maw, the star reaches its highest velocity, whizzing past it at 5,000 kilometers per second.
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 rather a dense cluster of compact stars, or even a giant blob of mysterious sub-atomic 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 for Extraterrestrial Physics, Observatoire de Paris, Office National d'Etudes et de Recherches Aerospatiales, and Observatoire de Grenoble.
Such observations 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 – understanding such stellar orbits might tell us where we are."
The Weizmann Institute of Science, in Rehovot, Israel, is one of the world's foremost centers of scientific research and graduate study. Its 2,500 scientists, students, technicians and engineers pursue basic research in the quest for knowledge and to enhance the quality of human life. New ways of fighting disease and hunger, protecting the environment, and harnessing alternative sources of energy are high priorities at Weizmann.
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