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Riding a Black Hole

01.05.2003

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The center of our galaxy may well consist of a supermassive black hole – the name given to black holes whose mass is more than one million times that of the sun. Reported in Nature, this finding heralds a new epoch of high-precision black-hole astronomy and might help us better understand how galaxies are born and evolve.
 
Supermassive black holes can be found at the center of many galaxies. The pioneering study, which traced a racing star as it journeyed through the Milky Way, suggests that this may also be true of our galaxy.
 
The massive black holes are thought to develop 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 a supernova, which happens when suns use up all their nuclear fuel. The process results in one of the most powerful explosions in nature. Lacking the fuel to maintain the huge pressure required to counter gravity, the star first implodes, and then its outer layers rebound against its core and are violently ejected into space. Simultaneously, the massive core continues to collapse rapidly into itself, forming 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,” says theoretical astrophysicist Dr. Tal Alexander of the Weizmann Institute of Science’s Physics faculty, who participated in the study together with scientists from Germany’s Max Planck Institute for Extraterrestrial Physics and several institutions in France. “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 heated up just before it is swallowed forever by the black hole.” In a 10-year study, Alexander and his colleagues succeeded in tracking a star known as S2 as it orbited around a known unusual source of radiation (a black-hole candidate called Sagittarius A*) located at the center of our galaxy.
 
The team found that the S2 star does indeed orbit Sagittarius A*; moreover, it picks up speed as it gets closer and closer to its maw, reaching a peak velocity as it whizzes past at 5,000 kilometers (around 3,000 miles) per second. 
 
Some astrophysicists had previously suggested that the dark mass at 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 subatomic particles.
 
It now appears that these hypotheses are not viable. The new detailed analysis of the orbit, made possible by the techniques developed by the present team, is fully consistent with the view that the dark mass is a supermassive black hole.
 
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, the Observatoire de Paris, the Office National d’Etudes et de Recherches Aerospatiales and the Observatoire de Grenoble. American scientists participated in the observations.
 
The new techniques allow for precise observation of the center of the galaxy, overcoming the problem of interstellar dust that pervades space. “Such sightings could provide information on a point we know surprisingly little about: our own place in the universe,” Alexander concluded. “We currently do not even know the earth’s exact distance from the center of our own galaxy. Understanding stellar orbits of this kind might tell us where we are.”

 

What a black hole might look like

 

 

White dwarf -- red giant collision near a black hole

 

Dr. Tal Alexander’s research is supported by Sir Harry Djanogly, CBE, London, UK.

 

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