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Spotlight | L w 6 ive from the Labs cnrs I international magazine Astronomy Recent observations have shed new light on black holes, enigmatic objects whose gravity is so strong that nothing escapes from them. Black Holes Revisited by Julien Bourdet Black holes, whose existence has yet to be fully confirmed, are some of the universe’s most mysterious objects. They are believed to let nothing escape, not even light. Some of them, known as supermassive black holes, are highly voracious, gobbling up any matter that gets too close. According to astronomers, most galaxies harbor one of these giants, the largest of which can reach several billion solar masses. “Before ‘falling’ into a black hole, matter whirls around it in the form of a huge disc,” explains Delphine Porquet from the Strasbourg Astronomical Observatory.1 “As a result, it heats up considerably and emits intense radiation. Seen from Earth, the object, called a quasar, appears brighter than all the stars in the galaxy put together.” Low-Activity Bla ck hole However, nothing of the kind can be observed around our very own supermassive black hole at the heart of the Milky Way, called Sagittarius A* (pronounced Sagittarius A-star). Indeed, it appears to have been on a strict diet for the past hundred years. “And yet it is orbited by a disc of massive stars that supply it with gas in the form of stellar winds,” explains Nicolas Grosso, also from the Strasbourg Astronomical Observatory. “As Sagittarius A* swallows up this matter, it should shine far more brightly than it currently does.” Why is it that our black hole has such little appetite? This is what a team of astrophysicists including the two Strasbourg researchers tried to find out, using the US satellite Chandra to observe Sagittarius A*’s X-ray emissions for more than a month. This enabled 01 Artist rendering of the US satellite Chandra. 02 Image of the Milky Way showing X-rays from Chandra (blue) and infrared emission from the Hubble Space Telescope (red and yellow). The inset shows a close-up view of the supermassive black hole Sagittarius A* only in X-rays, covering a region half a light-year wide. 02 01 © NASA/CXC/NGST


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