Dimmest globular stars seen by Hubble
The Hubble space telescope has picked out the dimmest stars ever found in a globular star cluster, a concentration of hundreds of thousands of stars. The discovery gives important new insights into the formation and age of the universe. The cluster, known as NGC 6397, is one of the closest clusters to earth, and these final discoveries complete the survey of this cluster, with all the stars now positively identified. The team, led by Harvey Richer of the University of British Columbia, presented its findings on 17 August at the 2006 International Astronomical Union General Assembly in Prague, Czech Republic, and in the journal Science. 'We have run out of hydrogen-burning stars in this cluster. There are no fainter such stars waiting to be discovered. We have discovered the lowest-mass stars capable of supporting stable nuclear reactions in this cluster. Any less massive ones faded early in the cluster's history and by now are too faint to be observed,' said Dr Richer. Hubble is a joint European Space Agency (ESA)/National Aeronautics and Space Administration (NASA) project. According to information from ESA, the light emitted by these dimmest stars in the cluster is equivalent to looking at a birthday candle on the Moon from the Earth, if that were possible. The NGC 6397 cluster is 8,500 light-years from Earth. The information from the Hubble telescope has confirmed hypotheses predicted but never observed - that white dwarf stars have temperatures low enough to undergo chemical change. This makes the stars appear more blue than red as they cool. These dim stars would once have been up to eight times the size of our Sun, but long ago exhausted its fuel. Even larger stars would undergo supernovae, leaving neutron stars, black holes, or nothing. Logic dictates that the Universe must be at least as old as the oldest stars. These white dwarfs cool in a regular, predictable, way, making estimations of their age highly accurate. Using this technique, Dr Richer and his team estimate the age of the cluster NGC 6397 to be around 12 billion years. The Universe is currently estimated to be 13.5 billion years old. The Hubble telescope's Advanced Camera, installed only in 2002, helped Dr Richer's team to probe into the cluster for almost five days to capture the faint stars. The camera's ultra-high resolution enabled the team to isolate cluster stars, and to distinguish them from other stars in the same area of sky that are in front or behind the cluster.