Stars as Laboratories for Fundamental Physics - MPP Theory Group

Anomalous Stellar Energy Losses Bounded by Observations 25
medium. Mass loss in advanced stages of stellar evolution can take
on the benign form of a “stellar wind,” or it can occur in gigantic
“supernova explosions.” The end state of stellar evolution must be
complete disruption, or a compact object (white dwarf, neutron star, or
black hole), because a normal star, supported by thermal pressure, can
exist only as long as its nuclear fuel lasts (Sect. 1.2.2). The masses of
degenerate stars are constrained by their Chandrasekhar limit of about
1.4 M ⊙ for white dwarfs, and a similar value for neutron stars. Most
massive stars seem to return enough mass to the interstellar medium
that their typical end states are degenerate stars, not black holes. The
primordial material evolves chemically because it is cycled through one
generation of stars after another.
Today, the continuing birth and death of stars takes place mostly in
the disks of spiral galaxies. Such galaxies also have a population of old
halo stars and of globular clusters, about 150 in our Milky Way, each of
which is a gravitationally bound system of about 10 6 stars (Fig. 2.1).
Most of the globular clusters are in the galactic halo, far away from the
disk. The gravitational escape velocity for stars or gas from a cluster
is rather small, about 10 km s −1 . A supernova explosion, on the other
hand, ejects its material with typical velocities of several 10 3 km s −1 .
A single supernova is enough to sweep a globular cluster clean of all
Fig. 2.1. Globular cluster M3. (Image courtesy of Palomar/Caltech.)