(1)
| An interstellar cloud collapses forming a
generation of stars. The masses of the
stars are spread over a range from maybe 0.01 M or less to maybe 100M
or so. The distribution function of stellar masses at birth,
known as the ``initial
mass function'' (IMF), has more small stars than big ones.
|
Fig. 1.6. HR diagram for nearby stars with
well-known paraflaxes (i.e. distances) from the
HIPPARCOS astrometric satellite mission, after M.A.C. Perryman et
al., Astr. Astrophys., 304,
69 (1995). The abscissa, B (blue) - V (visual) magnitude is a
measure of ``redness'' or inverse
surface temperature ranging from about 2 x 104 K on the left to
about 3000 K on the right, while
the ordinate measures luminosity in the visual band expressed by
absolute magnitude MHp, in
the HIPPARCOS photometric system. Luminosity decreases by a
factor of 100 for every 5 units
increase in M and the Sun's MV MHp
is 4.8. The main sequence forms a band going from top
left to bottom right in the diagram, with the ZAMS forming a
lower boundary. Subgiants and
red giants go upward and to the right from the MS and a few white
dwarfs can be seen at the lower left. Courtesy Michael Perryman.
|
|
(2)
| The young stars, which appear as variable stars
with emission lines known as
T Tauri stars and related classes, initially derive their energy
from gravitational contraction, which leads to a steady increase in
their internal temperature (see
Chapter 5). Eventually the central temperature becomes high
enough (~ 107
K or 1 keV) to switch on hydrogen burning and the star lies on
the ``zero-age
main sequence'' (ZAMS) of the Hertzsprung-Russell (HR) diagram in
which luminosity is plotted against surface temperature
(Fig. 1.6). Stars spend most
of their lives (about 80 per cent) in a main-sequence band stretching slightly
upwards from the ZAMS; the corresponding time is short (a few x
106 years)
for the most massive and luminous stars and very long (> 1010
years) for stars smaller then the Sun, because the luminosity varies as a high
power of the mass and so bigger stars use up their nuclear fuel supplies
faster.
|
(3)
| When hydrogen in a central core occupying about 10
per cent of the total mass is
exhausted, there is an energy crisis. The core, now consisting of
helium, contracts gravitationally, heating a surrounding hydrogen shell, which
consequently ignites to form helium and gradually eats its way outwards
(speaking in terms of the mass coordinate). At the same time, the
envelope expands, making the star a
red giant in the upper right part of the diagram.
|
(4)
| Eventually the contracting core becomes hot enough
to ignite helium (3 -> 12C;
12C + 4He -> 16O) and the core
contraction is halted.
|
(5)
| In sufficiently big stars (>~ 10M ) this process repeats;
successive stages of gravitational contraction and heating permit the
ashes of the previous burning stage to be ignited leading to C, Ne, O
and Si burning in the centre with less
advanced burning stages in surrounding shells leading to an
onion-like structure with hydrogen-rich material on the outside. Silicon
burning leads to a core rich in iron-group elements and with a
temperature of the order of 109 K, i.e. about 100 keV.
|
(6)
| The next stage of contraction is catastrophic,
partly because all nuclear energy
supplies have been used up when the iron group is reached, and
partly because
the core, having reached nearly the Chandrasekhar limiting mass for a white
dwarf supported by electron degeneracy pressure, is close to
instability and
also suffers loss of pressure due to neutronization by inverse
-decays. Further
contraction leads to photodisintegration, which absorbs energy,
and this leads to dynamical collapse of the core which continues until
it reaches nuclear density
and forms a neutron star. (If the mass of collapsing material is
too large, then
a black hole will probably form instead.) The stiff equation of
state of nuclear
matter leads to a bounce which sends a shock out into the
surrounding layers.
This heats them momentarily to high temperatures, maybe 5 x 109 K
in the silicon layer, leading to explosive nucleosynthesis of iron-peak
elements, mainly 56Ni (which later decays by electron capture
and + to
56Fe); more external
layers are heated to lower temperatures resulting in milder
changes. Assisted by high-energy neutrinos, the shock expels the outer
layers in a supernova explosion
(Type II and related classes); the ejecta eventually feed the
products into the
ISM which thus becomes enriched in ``metals'' in course of time.
This scenario was first put forward in essentials by Hoyle (1946), and modern
versions give a fairly good fit to the local abundances of elements from oxygen
to calcium. The iron yield is uncertain because it depends on the mass cut
between expelled and infalling material in the silicon layer, but can be
parameterized to fit observational data, e.g. for SN 1987A in the Large Magellanic Cloud (LMC).
The upshot is that iron-group elements are probably underproduced relative to
local abundances, but the deficit is plausibly made up by
contributions from supernovae of Type Ia. A subset of Type II supernovae
may also be the site of the r-process (see Chapter 6).
|
(7)
| For intermediate mass stars (IMS), ~ 1M M ~ 8M , stages (i)
to (iii) are much as before, bitt these never reach the stage of carbon
burning because
the carbon-oxygen core becomes degenerate first by virtue of high
density, and
later evolution is limited by extensive mass loss from the
surface. After core
helium exhaustion, these stars re-ascend the giant branch along
the so-called
asymptotic giant branch (AGB) track (see Fig. 5.14) with a double shell source:
helium-burning outside the CO core and hydrogen-burning outside
the He core. This is an unstable situation giving rise to thermal pulses or
``helium shell flashes''
in which the two sources alternately switch on and off driving inner and outer
convection zones (in which mixing takes place) during their active phases (see
Chapter 5). The helium-burning shell generates 12C and neutrons,
either from
22Ne( ,
n)25Mg or from 13C( , n)16O, leading to s-processing,
and the products are subsequently brought up to the surface in what is known as
the third dredge-up process. This process leads to observable abundance
anomalies in the spectra of AGB stars, carbon and S stars; see
Figs. 1.7,
1.8). The products
are then ejected into the ISM by mass loss in the form of stellar
winds and planetary nebulae (PN), leaving a white dwarf as the final
remnant. If the white
dwarf is a member of a close binary system, it can occasionally
be ``rejuvenated''
by accreting material from its companion. giving rise to
cataclysmic variables, novae and supernovae of Type Ia (cf. Chapter 5).
|