Free Astronomy Magazine November-December 2016

SPACE CHRONICLES Rare fossil relic of early Milky Way discovered by NASA T erzan 5, 19,000 light-years from Earth, has been classified as a globular cluster for the forty- odd years since its detection. Now, an Italian-led team of astronomers have discovered that Terzan 5 is like no other globular cluster known. The team scoured data from the Ad- vanced Camera for Surveys and the Wide Field Camera 3 on board Hub- ble, as well as from a suite of other ground-based telescopes, such as the Multi-conjugate Adaptive Optics Demonstrator at ESO’s Very Large Telescope and the Near Infrared Camera 2 at the W. M. Keck Obser- vatory. They found compelling evi- dence that there are two distinct kinds of stars in Terzan 5 which not only differ in the elements they con- tain, but have an age-gap of roughly 7 billion years (the two detected stellar populations have ages of 12 billion years and 4.5 billion years re- spectively). The ages of the two populations in- dicate that the star formation pro- cess in Terzan 5 was not continuous, but was dominated by two distinct bursts of star formation. “This requires the Terzan 5 ancestor to have large amounts of gas for a second generation of stars and to be quite massive. At least 100 million times the mass of the Sun,” explains