Free Astronomy Magazine July-August 2014

STELLAR EVOLUTION O ne of the best images of Wd1, taken with the MPG/ESO 2.2-metre tele- scope of the La Silla Observatory in Chile, showing the positions of the magnetar J1647-45 (invisi- ble) and its for- mer companion Wd1-5. [ESO] “new” celestial object was named Wester- lund 1, in honour of its discoverer, often shortened to Wd1. For a deducible selection generated by the physical properties of the Wd1 stars, all those categorized so far with sufficient ac- curacy are titans with masses 30-40 times greater than that of the Sun. Since stars of that size have a short life, measurable in a few million years (or a few tens of millions of years at most), it is self-evident that Wd1 must be a very young cluster, and in fact, astronomers estimate its age to be 3.5 to 5 million years, depending on the dif- ferent evolutionary models applied to the different types of stars that populate it. We are therefore in the presence of one of the youngest open clusters of our galaxy, an object that did not yet exist when our remote ancestors appeared on Earth. Since as far as is known the distribution of the initial masses of the stars in Wd1 should not be dissimilar to that typically observed in other open clusters, there must therefore certainly have existed stars even more massive than those observable today, and the absence of any trace of them means that they have already reach- ed the end of their lives by exploding as supernovae. It is estimated that the stars so far exploded in Wd1 could be between 50 to 150, with an average in the last mil- lion years of 1 supernova every 10,000 years. It is not surprising, therefore, that