and Cosmology

2. The Milky Way as a Galaxy
48
a luminosity of 10 9 L ⊙ , which is a considerable fraction
of the total luminosity of a galaxy; after that the luminosity
decreases again with a time-scale of weeks. In the
explosion, a star is disrupted and (most of) the matter of
the star is driven into the interstellar medium, enriching
it with metals that were produced in the course of stellar
evolution or in the process of the supernova explosion.
Classification of Supernovae. Depending on their
spectral properties, SNe are divided into several classes.
SNe of Type I do not show any Balmer lines of hydrogen
in their spectrum, in contrast to those of Type II.
A further subdivision of Type I SNe is based on spectral
properties: SNe Ia show strong emission of SiII λ
6150 Å whereas no SiII at all is visible in spectra of
Type Ib,c. Our current understanding of the supernova
phenomenon differs from this spectral classification. 5
Following various observational results and also theoretical
analyses, we are confident today that SNe Ia
are a phenomenon which is intrinsically different from
the other supernova types. For this interpretation, it is
of particular importance that SNe Ia are found in all
types of galaxies, whereas we observe SNe II and SNe
Ib,c only in spiral and irregular galaxies, and here only
in those regions in which blue stars predominate. As
we will see in Chap. 3, the stellar population in elliptical
galaxies consists almost exclusively of old stars,
while spirals also contain young stars. From this observational
fact it is concluded that the phenomenon of
SNe II and SNe Ib,c is linked to a young stellar population,
whereas SNe Ia occur in older stellar populations.
We shall discuss the two classes of supernovae next.
Core-Collapse Supernovae. SNe II and SNe Ib,c are
the final stages in the evolution of massive ( 8M ⊙ )
stars. Inside these stars, ever heavier elements are generated
by fusion: once all the hydrogen is used up, helium
will be burned, then carbon, oxygen, etc. This chain
comes to an end when the iron nucleus is reached, the
atomic nucleus with the highest binding energy per nu-
5 This notation scheme (Type Ia, Type II, and so on) is characteristic for
phenomena that one wishes to classify upon discovery, but for which
no physical interpretation is available at that time. Other examples are
the spectral classes of stars, which are not named in alphabetical order
according to their mass on the main sequence, or the division of Seyfert
galaxies into Type 1 and Type 2. Once such a notation is established,
it often becomes permanent even if a later physical understanding of
the phenomenon suggests a more meaningful classification.
cleon. After this no more energy can be gained from
fusion to heavier elements so that the pressure, which
is normally balancing the gravitational force in the star,
can no longer be maintained. The star will thus collapse
under its own gravity. This gravitational collapse will
proceed until the innermost region reaches a density
about three times the density of an atomic nucleus. At
this point the so-called rebounce occurs: a shock wave
runs towards the surface, thereby heating the infalling
material, and the star explodes. In the center, a compact
object probably remains – a neutron star or, possibly,
depending on the mass of the iron core, a black hole.
Such neutron stars are visible as pulsars 6 at the location
of some historically observed SNe, the most famous of
which is the Crab pulsar which has been identified with
a supernovae explosion seen by Chinese astronomers in
1054. Presumably all neutron stars have been formed in
such core-collapse supernovae.
The major fraction of the binding energy released
in the formation of the compact object is emitted in
the form of neutrinos: about 3 × 10 53 erg. Underground
neutrino detectors were able to trace about 10 neutrinos
originating from SN 1987A in the Large Magellanic
Cloud. Due to the high density inside the star after
the collapse, even neutrinos, despite their very small
cross-section, are absorbed and scattered, so that part
of their outward-directed momentum contributes to the
explosion of the stellar envelope. This shell expands at
v ∼ 10 000 km/s, corresponding to a kinetic energy of
E kin ∼ 10 51 erg. Of this, only about 10 49 erg is converted
into photons in the hot envelope and then emitted – the
energy of a SN that is visible in photons is thus only
a small fraction of the total energy produced.
Owing to the various stages of nuclear fusion in the
progenitor star, the chemical elements are arranged in
shells: the light elements (H, He) in the outer shells, and
the heavier elements (C, O, Ne, Mg, Si, Ar, Ca, Fe, Ni) in
the inner ones – see Fig. 2.10. The explosion ejects them
into the interstellar medium which is thus chemically
enriched. It is important to note that mainly nuclei with
an even number of protons and neutrons are formed.
This is a consequence of the nuclear reaction chains
6 Pulsars are sources which show a very regular periodic radiation,
most often seen at radio frequencies. Their periods lie in the range
from ∼ 10 −3 s (millisecond pulsars) to ∼ 5 s. Their pulse period is
identified as the rotational period of the neutron star – an object with
about one Solar mass and a radius of ∼ 10 km. The matter density in
neutron stars is about the same as that in atomic nuclei.