A Black Hole's Dinner is Fast Approaching
VLT spots cloud being disrupted by black hole
14 December 2011
Astronomers using ESO’s Very Large Telescope have discovered a gas cloud with several times the mass of the Earth accelerating fast towards the black hole at the centre of the Milky Way. This is the first time ever that the approach of such a doomed cloud to a supermassive black hole has been observed. The results will be published in the 5 January 2012 issue of the journal Nature.
During a 20-year programme using ESO telescopes to monitor the movement of stars around the supermassive black hole at the centre of our galaxy (eso0846) , a team of astronomers led by Reinhard Genzel at the Max-Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, has discovered a unique new object fast approaching the black hole.
Over the last seven years, the speed of this object has nearly doubled, reaching more than 8 million km/h. It is on a very elongated orbit  and in mid-2013 it will pass at a distance of only about 40 billion kilometres from the event horizon of the black hole, a distance of about 36 light-hours . This is an extremely close encounter with a supermassive black hole in astronomical terms.
This object is much cooler than the surrounding stars (only about 280 degrees Celsius), and is composed mostly of hydrogen and helium. It is a dusty, ionised gas cloud with a mass roughly three times that of the Earth. The cloud is glowing under the strong ultraviolet radiation from the hot stars around it in the crowded heart of the Milky Way.
The current density of the cloud is much higher than the hot gas surrounding the black hole. But as the cloud gets ever closer to the hungry beast, increasing external pressure will compress the cloud. At the same time the huge gravitational pull from the black hole, which has a mass four million times that of the Sun, will continue to accelerate the inward motion and stretch the cloud out along its orbit.
“The idea of an astronaut close to a black hole being stretched out to resemble spaghetti is familiar from science fiction. But we can now see this happening for real to the newly discovered cloud. It is not going to survive the experience,” explains Stefan Gillessen (MPE) the lead author of the paper.
The cloud’s edges are already starting to shred and disrupt and it is expected to break up completely over the next few years . The astronomers can already see clear signs of increasing disruption of the cloud over the period between 2008 and 2011.
The material is also expected to get much hotter as it nears the black hole in 2013 and it will probably start to give off X-rays. There is currently little material close to the black hole so the newly-arrived meal will be the dominant fuel for the black hole over the next few years.
One explanation for the formation of the cloud is that its material may have come from nearby young massive stars that are rapidly losing mass due to strong stellar winds. Such stars literally blow their gas away. Colliding stellar winds from a known double star in orbit around the central black hole may have led to the formation of the cloud.
“The next two years will be very interesting and should provide us with extremely valuable information on the behaviour of matter around such remarkable massive objects,” concludes Reinhard Genzel.
 The black hole at the centre of the Milky Way is formally known as Sgr A* (pronounced Sagittarius A star). It is the closest supermassive black hole known by far and hence is the best place to study black holes in detail.
 The observations were made using the NACO infrared adaptive optics camera and the SINFONI infrared spectrograph, both attached to the ESO Very Large Telescope in Chile. The centre of the Milky Way lies behind thick dust clouds that scatter and absorb visible light and must be observed at infrared wavelengths where the clouds are more transparent.
 A light-hour is the distance that light travels in one hour. It is a little more than the distance from the Sun to the planet Jupiter in the Solar System. For comparison the distance between the Sun and the nearest star is more than four light-years. The cloud will pass at less than ten times the distance from the Sun to Neptune from the black hole
 This effect well known from the physics of fluids and can be seen when for example pouring syrup in a glass of water. The flow of syrup downwards through the water will be disrupted and the droplet will break apart — effectively diluting the syrup in the water.
This research was presented in a paper “A gas cloud on its way towards the super-massive black hole in the Galactic Centre”, by S. Gillessen et al., to appear in the 5 January 2012 issue of the journal Nature.
The team is composed of S. Gillessen (Max-Planck-Institut für extraterrestrische Physik [MPE], Germany), R. Genzel (MPE; Department of Physics, University of California [UC], USA), T. Fritz (MPE, Germany), E. Quataert (Department of Astronomy, UC, USA), C. Alig (Universitätssternwarte der Ludwig-Maximilians-Universität [LMU], Germany), A. Burkert (MPE; LMU), J. Cuadra (Departamento de Astronomía y Astrofísica, Pontificia Universidad Católica de Chile, Chile), F. Eisenhauer (MPE), O. Pfuhl (MPE), K. Dodds-Eden (MPE), C. Gammie (Center for Theoretical Astrophysics, University of Illinois, USA), T. Ott (MPE).
ESO, the European Southern Observatory, is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive astronomical observatory. It is supported by 15 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 40-metre-class European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.
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