TPF-I SWG Report - Exoplanet Exploration Program - NASA

G ENERAL A STROPHYSICS
Figure 3-2. HST image of the giant galactic nebula NGC 3603 shows the various stages of the life
cycles of stars in one single view, from a starburst cluster of young hot Wolf-Rayet stars and early
O-type stars (center) to the evolved blue supergiant Sher 25 (upper right), which marks the end of the
life cycle.
Star formation is the process that determines the mass-spectrum (initial mass function —IMF; Kroupa
2000) of stars. Star formation determines how galaxies consume their interstellar media and how they
convert this material into long-lived low-mass stars, and star formation controls the rate and nature of
galactic evolution.
In the standard model of star formation (Figure 3-1), the inside-out gravitational collapse of a rotating
cloud core leads to the formation of a protostellar core on a time-scale of about 10 4 years. The infall of
high-angular-momentum gas forms a spinning, circumstellar disk through which most of the star’s final
mass spirals onto the protostar on a time-scale of ~10 5 years. Entrained and dynamo-generated magnetic
fields launch powerful jets and bipolar outflows along the rotation axis of the system for a period of order
10 5 to 10 6 years (Reipurth and Bally 2001). Planetary systems form and mature from remnants of the disk
in about 10 6 to 10 8 years; low-mass stars reach the main-sequence (MS) in 10 7 to > 10 8 years (see the
Grenoble stellar evolutionary tracks, Siess, Dofour, and Forestini 2000).
During the last decade, observations have shown that most stars form in highly over-dense, but shortlived
clusters that form from the collapsing and fragmenting cores of turbulent molecular clouds
(MacLow and Klessen 2004). Furthermore, the birth and early evolution of most stars occurs in the close
proximity of luminous, massive stars that irradiate the birth environments with intense UV radiation and
which explode as supernovae on time-scales ranging from 3 to 40 Myr, the same time-scale on which
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