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

GALACTICEVOLUTIONANDSTELLAR
POPULATIONS
OurGalaxy, the Milky Way, is essentially a fiat disk
with an oblate spheroidal nucleus. Ragged spiral arms
twist around in the plane of the disk between the
nucleus and the rim. The diameter of the disk is about
100,000 light-years and the thickness tapers smoothly
from thick near the nucleus to thin at the rim, its
thickness at the half radius point being about 1600
light-years. We believe that our Galaxy and the great
nebula (or galaxy) in Andromeda, M31, shown in Figure
2-1 are very similar. The nucleus and disk are surrounded
by a halo of stars and globular clusters of stars in highly
elliptical orbits about the galactic center, but by far the
largest part of the total mass is in the nucleus and the
disk.
The Milky Way was originally spheroidal and many
times its present size. Its evolution from spheroid to disk
as it underwent gravitational contraction was probably
due to conservation of angular momentum. We believe
this pattern is typical of galaxies having a comparable
amount of initial angular momentum. Most galaxies have
less spin and have retained their spheroidal shape.
The history of our Galaxy spans at least 12 billion
years and is traced through the study of the different
chemical compositions, kinematics, and ages of the
different stellar populations. Broadly speaking, there are
two major populations of stars: old (population II) and
young (population I). The oldest stars dominate in the
halo and in the nucleus; the young stars dominate in the
disk, and the very youngest stars are found in the spiral
arms closely associated with the gas clouds out of which
they recently formed. Elliptical galaxies seem primarily
composed of old (population I) stars and do not contain
the very bright young stars that mark the arms of the
spiral galaxies.
The roughly spherical, slowly rotating, pregalactic
clouds of hydrogen and helium collapsed under their
own gravity until this centripetal force was balanced by
gas pressure and the centrifugal force of the rotation. In
such a gas cloud, we might expect a great deal of initial
turbulence. But this turbulence would die out with time,
while the systematic rotation would be. maintained. The
rate of star formation is expected to be proportional to
the square of the gas density. Thus when the contraction
had proceeded sufficiently, stars began to form and the
rate of formation increased rapidly. Many of the larger
stars that were formed initially went through their
complete life cycle and vanished as supernovae while the
Galaxy was very young and still contracting.
The observed distributions of stellar populations
agree qualitatively with this general picture of contraction,
decreasing turbulence, and increasing spin and
flatness of the evolving galactic gas mass. The oldest stars
(the smaller mass stars, which have survived) are found
as expected in a spheroidal distribution, while progressively
younger generations are found in increasingly
flattened distributions. The youngest are confined to the
present disk. The orbits of the older generations are
highly eccentric and show greater velocity dispersion,
reflecting the greater gas turbulence during the early
phases. Young stars show much smaller velocity dispersion
and quietly orbit the galactic center along with their
neighbors in the disk.
Old (population II) stars have a much smaller heavy
element content than young (population I) stars. By
studying the elemental abundances in star clusters
belonging to different age groups, we can plot the
increasing abundance of heavy elements with time as
shown in Figure 2-2. Although the ratio of hydrogen to
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