Stars as Laboratories for Fundamental Physics - MPP Theory Group

24 Chapter 2
stellar structure and evolution and their observational consequences
are found in the textbook and review literature. 3
How do stars form in the first place While a detailed understanding
of this process is still elusive, for the present discussion it is enough to
know that gravitationally bound clouds of gas ultimately fragment and
condense because of their continuous energy loss by electromagnetic
radiation. As discussed in Sect. 1.2.2, the negative specific heat of a
self-gravitating system enforces its contraction and heating. For the
fragmentation process a variety of parameters may be important such
as overall pressure, angular momentum, or magnetic fields.
Because the formation process of stars is poorly understood the
initial mass function (IMF) is not accounted for theoretically. Typically,
the number of stars per mass interval may be given by Salpeter’s IMF,
dN/dM ∝ (M/M ⊙ ) −1.35 , which means that there are a lot more small
than large stars. However, the overall range of stellar masses is quite
limited. The largest stars have masses of up to 100 M ⊙ ; a value beyond
this limit does not seem to allow for a stable configuration. At the
small mass end, stars with M < 0.08 M ⊙ never become hot enough to
ignite hydrogen; such “brown dwarfs” have never been unambiguously
detected. It has been speculated that they could make up the dark
matter of spiral galaxies. A search in our galaxy by the gravitational
microlensing technique has yielded first candidates (Alcock et al. 1993,
1995; Aubourg et al. 1993, 1995).
The first stars consist of a mixture of X ≈ 0.75 (mass fraction of
hydrogen) and Y ≈ 0.25 (mass fraction of helium), the material left over
from the big bang of the universe. Subsequent generations also contain
a small amount of “metals” which in astronomers’ language is anything
heavier than helium; our Sun has a metallicity Z ≈ 0.02 (mass fraction
of metals). These heavier elements were bred by nuclear fusion in earlier
generations of stars which returned some of their mass to the interstellar
3 An excellent starting point for the nonexpert is Shu (1982). The classics are
Chandrasekhar (1939), Schwarzschild (1958), Clayton (1968), and Cox and Giuli
(1968). A recent general textbook is Kippenhahn and Weigert (1990). Recent monographs
specializing on the Sun in general are Stix (1989), and on solar neutrinos
Bahcall (1989). The theory of stellar pulsation is covered in Cox (1980). A classic on
the physics of compact stars is Shapiro and Teukolsky (1983). A recent monograph
on neutron stars is Lipunov (1992) and on pulsars Mészáros (1992). Recent collections
of papers or conference proceedings are available on the physics of red giants
(Iben and Renzini 1981), white dwarfs (Barstow 1993), pulsating stars (Schmidt
1989), neutron stars (Pines, Tamagaki, and Tsuruta 1992), and supernovae (Brown
1988; Petschek 1990). Many excellent reviews are found in the Annual Review of
Astronomy and Astrophysics.