and Cosmology

2.3 The Structure of the Galaxy
Fig. 2.10. Chemical shell structure of a massive
star at the end of its life. The elements
that have been formed in the various stages
of the nuclear burning are ordered in a structure
resembling that of an onion. This is the
initial condition for a supernova explosion
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involved, where successive nuclei in this chain are obtained
by adding an α-particle (or 4 He-nucleus), i.e.,
two protons and two neutrons. Such elements are therefore
called α-elements. The dominance of α-elements
in the chemical abundance of the interstellar medium
is thus a clear indication of nuclear fusion occurring in
the He-rich zones of stars where the hydrogen has been
burnt.
Supernovae Type Ia. SNe Ia are most likely the explosions
of white dwarfs (WDs). These compact stars
which form the final evolutionary stages of less massive
stars no longer maintain their internal pressure by
nuclear fusion. Rather, they are stabilized by the degeneracy
pressure of the electrons – a quantum mechanical
phenomenon. Such a white dwarf can only be stable
if its mass does not exceed a limiting mass, the Chandrasekhar
mass; it has a value of M Ch ≈ 1.44M ⊙ .For
M > M Ch , the degeneracy pressure can no longer balance
the gravitational force. If matter falls onto a WD
with mass below M Ch , as may happen by accretion
in close binary systems, its mass will slowly increase
and approach the limiting mass. At about M ≈ 1.3M ⊙ ,
carbon burning will ignite in its interior, transforming
about half of the star into iron-group elements, i.e.,
iron, cobalt, and nickel. The resulting explosion of the
star will enrich the ISM with ∼ 0.6M ⊙ of Fe, while
the WD itself will be torn apart completely, leaving no
remnant star.
Since the initial conditions are probably very homogeneous
for the class of SNe Ia (defined by the limiting
mass prior to the trigger of the explosion), they are good
candidates for standard candles: all SNe Ia have approximately
the same luminosity. As we will discuss later
(see Sect. 8.3.1), this is not really the case, but nevertheless
SNe Ia play an important role in the cosmological
distance determination, and thus in the determination of
cosmological parameters.
This interpretation of the different types of SNe explains
why one finds core-collapse SNe only in galaxies
in which star formation occurs. They are the final stages
of massive, i.e., young, stars which have a lifetime of
not more than 2 × 10 7 yr. By contrast, SNe Ia can occur
in all types of galaxies.
In addition to SNe, metal enrichment of the interstellar
medium (ISM) also takes place in other stages of
stellar evolution, by stellar winds or during phases in
which stars eject part of their envelope which is then
visible, e.g., as a planetary nebula. If the matter in the
star has been mixed by convection prior to such a phase,
so that the metals newly formed by nuclear fusion in the
interior have been transported towards the surface of the
star, these metals will then be released into the ISM.
Age–Metallicity Relation. Assuming that at the beginning
of its evolution the Milky Way had a chemical
composition with only low metal content, the metallicity
should be strongly related to the age of a stellar
population. With each new generation of stars, more
metals are produced and ejected into the ISM, partially
by stellar winds, but mainly by SN explosions. Stars that
are formed later should therefore have a higher metal
content than those that were formed in the early phase
of the Galaxy. One would therefore expect that a relation
should exists between the age of a star and its
metallicity.
For instance, under this assumption [Fe/H] can be
used as an age indicator for a stellar population, with the
iron predominantly being produced and ejected in SNe
of type Ia. Therefore, newly formed stars have a higher
fraction of iron when they are born than their predecessors,
and the youngest stars should have the highest