The beginning of the stellar era in the Universe is a singularly
fascinating phase in the history of the Cosmos. The baryonic material
filling the Universe at that time, having a composition inherited from
Big Bang nucleosynthesis, has its physical characteristics modified by
the very first stars. Indeed, the first stars will change the degree of
ionized material in their vicinity, and, through their winds and/or
supernova explosion, will inject energy, momentum, and newly-synthesized
elements. Some pockets of gas, enriched by the first stars, will in turn
form new stars, whose initial composition is inherited from the first
nucleosynthetic events. These low-mass stars can live sufficiently long
to be observed today in halo of the Galaxy, providing the opportunity to
obtain information about the very high redshift Universe by the study of
nearby stars.
How did these processes happen? How were these processes different from
similar ones occurring in the present Universe? What can they teach us
about the infancy of the Universe during the reionization era? These are
the questions that will be discussed during this workshop, questions
that are particularly topical at a time when new facilities, such
as SKA, JWST, and the next generation of extremely large telescopes have
as one of their prime objectives to probe this very period.
At present, observational constraints on these first stars are scarce
and/or indirect. Important constraints come from observations of the
elemental abundances of very, extremely, and ultra iron-poor stars that
likely formed from the ejecta of the first stellar generations. Of
course the full story can be complex, as some elements might be formed
by more than one source, and others may be somewhat altered by in-situ
processes that occurred in the star that is observed today.
Nevertheless, through the accumulation of observational data, progress
made in numerical simulations, and in our deeper understanding of the
physical processes involved, a much more complete and detailed story can
be told.
