Stellar archaeology

THE FIRST STARS MUST HAVE been a magnificent sight. Far brighter, hotter and more massive than most stars that currently light the sky, they emerged after a period of relative darkness — about 100 million years after the Big Bang, aided by the gravitation of countless halos of dark matter. No large galaxies existed yet, nor did elements heavier than helium, save for a trace of lithium. But when the first stars ended their lives as immense supernova explosions a few million years later, they released heavier elements that helped form the next generation of stars.

These heavy elements, such as carbon, calcium and iron, astronomers collectively call metals. Metals helped radiate away heat from collapsing clouds of hydrogen and helium gas, fostering the creation of less-massive and longer-lived stars. The smallest of these second-generation Population II (Pop II) stars still exist in the Milky Way and in nearby dwarf galaxies. Their outer layers harbour traces of metals produced in the as-yet undiscovered first stars, called (counterintuitively from a chronologist’s view) Population III.

This presents astronomers with an opportunity to do archaeology — stellar archaeology. No current telescopes can look back more than 13 billion years to directly study the first stars. But astronomers can study the abundances and relative proportions of metals left behind by the first stars in the outer layers of the oldest surviving Pop II stars, much as earthbound archaeologists learn about ancient cultures by studying the artifacts they left behind. Using this approach, astronomers hope to increase our understanding of how the first stars evolved, how they generated the first elements heavier than