A:6. How are stars born and how do they evolve?
How do stars form in collapsing clouds of interstellar gas?
This crucial question can be tackled through two complementary approaches. The first involves direct observation of stellar birth in dense molecular clouds within our own Galaxy and in neighbouring galaxies, whereas the second entails state-of-the art numerical simulations using supercomputers. Once born, stars spend time on the main-sequence, a reference to the famous Hertszsprung-Russell diagram, before following evolutionary tracks which lead for fairly massive stars to death in a supernova explosion or, for less massive stars, to a quieter end as a cooling white dwarf. One astrophysical consequence of the cycle of stellar birth, life and death is the chemical enrichment of the surroundings by heavy elements forged in nuclear processes in the stellar interiors and atmospheres. Galaxies are comprised of different populations of stars which reflect episodes of star formation through the period since their formation up until the present day.
By tracing these various stellar populations, astronomers have a tool for investigating how the chemical enrichment of the Universe has progressed over cosmic time.