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planetsareformedin a secondfragmentation phase,
aftertheglobulethat was to become the star and its
planets had already fragmented from the main cloud and
had contracted by several orders of magnitude. We see
no reason for the statistics of the primary fragmentation
to carry over into this later phase.
Stellar
Evolution
Thanks to advances in nuclear physics we now have a
detailed understanding of stellar evolution. Stars in the
different regions of the HR diagram are stars in different
phases of their life histories. We may divide stellar
evolution into four principal phases:
Birth. The star begins as one of many globules about a
light-year in diameter into which a larger gas cloud has
fragmented. The globule contracts under its own gravity
compressing the gas. The star is born when the gas
becomes heated to incandescence. The luminosity steadily
increases as gravitational potential energy is converted
into heat. The contraction phase is very short-well
under 1 percent of the main sequence lifetime.
live long enough for intelligent life to evolve if the
evolution rates are comparable to that on Earth.
As the hydrogen fusion continues, a growing core of
almost pure helium is produced, inside which all energy
release has stopped. When this core reaches about
one-tenth solar mass it collapses, releasing a large
amount of gravitational energy in itself and in the
surrounding shell where hydrogen burning is still occurring.
The increased temperature increases the hydrogen
fusion rate with the result that the outer layers of the
star expand to absorb and eventually radiate the
increased energy output, The expansion increases the
surface area so much that the surface cools even though
the total luminosity is greater than in the main sequence
phase.
I00 i I n\l _ _ In I r u l l
\
MO
Main Sequence. When the internal temperature has
become high enough to initiate proton-proton fusion or
a carbon-nitrogen-oxygen cycle (or both), the contraction
stops. The temperature needed in the core to
maintain hydrostatic equilibrium and to supply the
radiation losses is now obtained from nuclear energy.
The star has now taken its place on the main sequence at
a point determined by its mass. The lifetime on the main
sequence is proportional to the amount of nuclear fuel
(i.e., to the mass) and inversely proportional to the
power radiated (i.e., to the luminosity). The Sun's
lifetime is about 12 billion years so the residence time of
any star on the main sequence is approximately:
% KO
p
x
tl-
o
K5
Gs
-._ I0 GO
5
,?,
tins _ (12X109) __M* __Lo (4)
M o L,
In view of the mass-luminosity relation (3) this can be
written
z
.5
GE 5
G_ _-N--_-T_-- \--
\
FO
tins _ (i 2× 10 9)
.Mo/5/2
A5
(5)
The larger the star, the shorter its life. Figure 2-7 is a
plot of (5) in the spectral range of interest to us. We see
from Figures 2-6 and 2-7 that the vast majority of stars
I I I 1 I I I 1 I
.I I IO
MASS/SOLAR STAR MASS
Figure 2-7. Stellar lifetimes versus mass.
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