The chemical elements are created in nuclear fusion processes in the hot and
dense cores of stars. The energy generated through nucleosynthesis allows stars
to shine for billions of years. When these stars explode as massive supernovae,
the newly made elements are expelled, chemically enriching the surrounding
regions. Subsequent generations of stars are formed from gas that is slightly
more element enriched than that from which previous stars formed. This chemical
evolution can be traced back to its beginning soon after the Big Bang by
studying the oldest and most metal-poor stars still observable in the Milky Way
today. Through chemical analysis, they provide the only available tool for
gaining information about the nature of the short-lived first stars and their
supernova explosions more than thirteen billion years ago. These events set in
motion the transformation of the pristine universe into a rich cosmos of
chemically diverse planets, stars, and galaxies.Comment: 13 pages, 2 figures. To appear in "From Atoms to the Stars", a
special issue of Daedalus (Fall 2014, vol. 143, no. 4