Free Astronomy Magazine September-October 2015

SPACE CHRONICLES Best evidence of first generation stars in the universe by ESO A stronomers have long theo- rised the existence of a first generation of stars — known as Population III stars — that were born out of the primordial material from the Big Bang. The name Popula- tion III arose because astronomers had already classed the stars of the Milky Way as Population I (stars like the Sun, rich in heavier elements and forming the disc) and Population II (older stars, with a low heavy-element content, and found in the Milky Way bulge and halo, and globular star clusters). All the heavier chemical elements — such as oxygen, nitrogen, carbon and iron, which are essential to life — were forged in the bellies of stars. This means that the first stars must have formed out of the only elements to exist prior to stars: hydrogen, helium and trace amounts of lithium. These Population III stars would have been enormous — several hundred or even a thousand times more massive than the Sun—blazing hot, and tran- sient —exploding as supernovae after only about two million years. But until now the search for physical proof of their existence had been inconclusive. Finding these stars is very difficult: they would have been extremely short-lived, and would have shone at a time when the Universe was largely opaque to their light. Previous find- ings include: Nagao et al., 2008, where