Project Cyclops, A Design... - Department of Earth and Planetary ...

Red Giant. The transition from main sequence star to
red giant is very short, as evidenced by the scarcity of
stars observed in this phase. Hydrogen burning in the
shell stabilizes the star as a red giant for a time, but as
the shell grows the rate of burning drops and the star
again contracts. This time the contraction raises the
temperature in the core to the point where helium
fusion can commence and the star, now obtaining energy
from both hydrogen and helium fusion, again expands.
Eventually, a core of elements heavier than helium forms
and the hydrogen and helium burning shells approach
each other. Here the interaction becomes complicated
and computer modeling has not as yet been successful
beyond this stage.
Advanced Age. When gravitational contraction can no
longer increase the core temperature and density sufficiently
to initiate the next series of reactions, the star
evolves away from the red giant region. Its path is to the
left in the HR diagram of Figure 2-3 at roughly constant
luminosity. During this time, the star may enter a
pulsating phase, return briefly to the red giant regime,
then conclude by slowly or violently ejecting its surface
layers. Most frequently, the end result is a white dwarf.
It is a configuration in which the material is almost
completely degenerate from center to surface. The
physical size of these objects is about the same as the
Earth, but. their mass is between 0.2 and 1.2 solar
masses. The surface temperature is typically 6,000 to
12,000 ° K. Because of the small radius, the luminosity is
very low compared with the Sun, even though the
temperature is higher [see eq. (1)].
A more violent, explosive end for a star is called a
supernova (Lma x typically 108 Lo) and produces large
quantities of heavy elements. These elements are ejected
with velocities of roughly 104 km/sec and so soon
become well mixed with the interstellar medium. The
star may not always be entirely destroyed by the
process: At the center of the expanding clouds, rotating
neutron stars (pulsars) have been discovered. Such
explosive fates await stars of 10 solar masses or larger.
They are the primary source of elements heavier than
neon and provide a large fraction of the elements
between helium and neon.
Clearly when a star leaves the main sequence the
effects on its planets are catastrophic. It is doubtful if
even very intelligent life could remain in the planetary
system and avert disaster. (Our Sun will not enter the
giant phase before approximately seven billion more
years.) So far as intelligent life is concerned we should
therefore confine our attention to main sequence stars,
and probably only to those cooler than F0. When we
come to consider atmospheric evolution and tidal effects
we will want to exclude M stars. Thus, the stars of
greatest interest are F, G, and K main sequence stars,
which amount to about 25 percent of all stars. If we
exclude binary and multiple star systems, because of
unstable planetary orbits, this still leaves over 20 billion
stars in the galaxy as possible suns for intelligent life.
The Solar
Neighborhood
ls ours in any way a special neighborhood? Photographs
of spiral galaxies give the impression that most
stars are concentrated in the nucleus and along the spiral
arms. Our Sun is located midway between two spiral
arms-actually, on the fringe of a spur extending off one
arm. We are about 33,000 light-years from the galactic
center or two-thirds of the way to the rim. The disk is
about 1500 light-years thick at this radius, and the Sun
is only a few tens of light-years off the midplane. At first
glance, it might appear that we are in a somewhat
isolated neighborhood but this is not the case.
It is true that the star density in the nucleus is
extremely high-a hundred or more times that in the
solar neighborhood. This means that the average separation
between stars is only 20 to 25 percent as great as
here. But only 10 percent of the total stellar mass is in
the nucleus.
Contrary to common belief, the spiral arms are not
regions of higher star density. They are regions of high
gas and dust density, the maternity wards of the Galaxy,
where new stars are being born at a higher rate. The gas
clouds fragment to form clusters of hundreds of thousands
of new stars including a large number of shortlived
O and B stars. These stars die before their motions
carry them very far from their birthplace. It is these very
bright stars scattered along tile spiral arms that give the
arms their extra brightness.
The rotation about the galactic center of the stars in
the disk is quite different from that of the spiral arm
pattern. The Sun, along with most other stars in our
neighborhood, circles the galactic center once each 240
million years and in one orbit will cross all components
of the spiral arms. Stars born in the spiral arms drift out
of the arms and, because of their velocity dispersion,
become thoroughly mixed with older stars. While we can
find the common age of stars in a cluster by noting the
mass and hence the age of those leaving the main
sequence, we can only estimate the general population
class of isolated stars. Thus, if we should decide that 4
billion years were needed to evolve a technological
civilization, we have no way of determining whether a
particular population ! star is too young or too old. The
thorough mixing of stars with time means that there is
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