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C H A P T E R 10
e Evolution of Galaxies
A galaxy is not a “thing.” Rather, it is the sum of all its parts—individual stars, star clusters,
gas, and dust—all held together by gravity. And a galaxy is a dynamic structure; each piece must
move according to the sum of gravitational forces acting upon it. But also, the individual stars,
clusters, and nebulae within a galaxy evolve with time according to their own local conditions.
And so for a galaxy, both the individual pieces, and how those pieces are arranged in space must
change as time passes. A casual view through a telescope of a spiral galaxy similar to our Milky
Way shows only the overall effect of this history of change—the individual stars, for example,
are usually too faint to see.
10.1 FORMATION AND EVOLUTION OF THE MILKY WAY
e structure of our Galaxy provides clues to its formation. In particular, there are separate
populations of stars. A full description is more complex, but the two most extreme opposites
have the following properties.
in disk population (extreme Population I):
• Consists of hot upper-main-sequence stars, young open clusters of stars, and clouds
of gas and dust.
• Travel on circular orbits narrowly confined to the disk of the Galaxy, all orbiting in
the same sense.
• Relatively high metallicity.
Halo population (extreme Population II):
• Consists of older individual stars and the globular clusters.
• Orbits of high inclination to the disk of the Galaxy, and in essentially random direc-
tions.
• Relatively low metallicity.
e most obvious conclusion from these observations is that the halo population of stars
formed long before the thin disk population. e still-present gas and dust in the disk of the
Galaxy allows for continuous star formation. And so a recently formed open star cluster is the
result of many generations of star formation and supernova explosions. is process enriches the
140 10. THE EVOLUTION OF GALAXIES
interstellar medium (ISM) with metals. And so the ongoing processes of stellar evolution and
nucleosynthesis creates a stellar nursery that is enriched in metals.
e halo seems to be devoid of the relatively dense concentrations of gas and dust necessary
for the formation of stars. And so perhaps the globular clusters and the individual stars in the
halo formed very early on in the history of the Galaxy. And so the gas and dust they formed
from was much nearer to primordial—the mix of nearly pure hydrogen and helium provided by
the Big Bang itself.
In the next section I outline two scenarios for the formation of the Milky Way; an excellent
discussion can be found in Binney and Merrifield [1998, Section 10.7]; my description largely
follows theirs.
10.1.1 TOP-DOWN SCENARIO
e overall shape of a disk galaxy like the Milky Way bears a striking resemblance to the solar
system, but on a vastly larger scale. Both have a disk-like structure, the parts of which orbit
together in the same direction, confined to a narrow plane. And they both have a concentration
of matter in the center—the central bulge in the case of the Milky Way and the Sun in the
case of the solar system. Furthermore, each has a spheroidal population of objects that orbit in
random directions—the globular clusters and halo stars for our Milky Way and the Oort cloud
comets for the solar system.
ese similarities naturally lead to the idea that the Milky Way may have formed in a
manner similar to how the solar system is thought to have formed. A large cloud of gas contracts
due to self-gravity, but a small amount of initial rotation leads to the formation of a thin disk,
most-concentrated in the center.
e idea is that the globular clusters and the first generations of stars formed during that
initial gravitational contraction, and they retain the orbits of that initial motion—plunging to-
ward the center of the Galaxy.
Note that when a cloud of gas contracts due to its self-gravity, the gas is compressed, and
this is what allows it to form a disk; gas does not pass through gas. But already-formed stars do
pass easily through gas. And so the halo stars can pass through the disk of the Galaxy in their
plunging orbits.
After the original gas cloud had collapsed into a disk, stars could no longer form in the
halo of the Galaxy. And so stars that formed later formed in the disk itself, and they bear the
circular orbits of the disk gas they formed from. is top-down scenario was first proposed in
detail by Eggen et al. [1962].
10.1.2 BOTTOM-UP SCENARIO
e top-down scenario of Eggen et al. [1962] is simple and attractive, but it has been called into
question in light of more recent observations. It is unrealistic, at least in detail, for two reasons.
First, galaxies formed in an expanding universe, and this greatly changes the dynamics of a
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