Stellar Evolution --- is the result of the interplay between
gravity and pressure support of the core of a star mediated by nuclear fusion
reactions. As soon as any nuclear fuel is exhausted in a star's core, gravity
will cause the core to contract and heat up because of the release
of gravitational binding energy. In general, what then happens is that
it becomes hot enough to fuse the heavier elements in turn, which require
higher temperatures because of the greater electrostatic repulsion between ever
larger numbers of protons. In principle, this can go on until the formation of
iron in the core.
Low-mass versus High-mass Evolution --- The limiting factor
in evolution is the mass of the star, which determines how hot the
core can become by gravitational collapse. Low-mass and high-mass stars
therefore leave different remnants.
Low-mass evolution---A Red Giant is a post main-sequence star that has a hot
contracting helium core and hydrogen fusing in a shell around the core. The
envelope of such a star expands in response to the very hot conditions at the
center to produce a luminous red star, a red giant. In a low-mass star, the
helium core becomes so dense that further contraction is resisted by the
pressure that results when electrons get too crowded (more on this wierd state
of matter later). Eventually, the helium core begins fusion to carbon in an
explosion --- the helium flash. A star with helium fusion at the core and
hydrogen fusion in a shell is called a horizontal branch star because of its
position on the H-R diagram. When the helium in the core eventually runs out
the process starts again, with the core contracting until it is hot enough for
helium fusion to start in a shell, with hydrogen fusion in an outer shell.
Again the star swells up, this time to a supergiant star (radius maybe around
3AU, e.g. Betelgeuse). For a star of less than around 8 solar masses, the
core never gets hot enough to "burn" carbon. The outer shells of the star
separate from the inner parts and become a planetary nebula (see below) and
the core goes on to become a white dwarf star (see below).
Planetary Nebulae --- are expanding shells of gas, thought to be the
result of the loss of the outer envelope of a low-mass star during the late
stages of its evolution. They have nothing to do with planets. They glow by
a process of fluorescence, pumped by UV light from the very hot, very luminous
central star. They have a temperature of about 10,000~K, maintained by
heating from the central star. The nebulae are expanding at about 20
kilometers per second and dissipate in about 50,000 years. There are about
1000 known in the Galaxy, but their short lifetimes mean that this is only a
tiny fraction of them. They are therefore very common.
Stellar remnant --- is the generic name for the dense collapsed leftovers of
stellar evolution, whether low mass or high mass.
White Dwarfs --- are thought to be the remnant of low mass evolution.
The carbon core of such an object collapses until stopped by electron
degeneracy pressure, which is essentially when the electrons "touch". This
stiffness of electrons and resistance to further compression is a quantum
mechanical phenomenon, and arises from the identical nature of electrons. A
white dwarf is initially very hot but cools as it ages. Since it can no
longer adjust its core, it cools at constant radius. Because the white dwarf
is degenerate, the pressure which supports it doesn't depend on the
temperature, only on the density, so the larger the mass of the WD, the
smaller it is. A 1 solar mass white dwarf is about the size of the Earth and
one teaspoonful of white dwarf matter would weigh about 5 tons.
Chandrasekhar Limit --- A white dwarf can't have indefinitely high
mass. As the density increases, the degeneracy pressure saturates. Formally,
the radius of the star could become zero. The mass at which this happens is
the Chandrasekhar Limit, about 1.4 solar masses for a white dwarf of normal
composition.
Binding energy --- of protons and neutrons in a nucleus is a measure of how
tightly bound the nucleaus is, and so (since energy is equivalent to mass) how
much less mass a nucleus has than its constituent particles. (Recall that a
helium nucleus has less mass than 4 protons.) Apart from some wiggles up and
down, this binding energy increases from hydrogen to iron, and then decreases
again to the heavier elements. This means that fusion of light nuclei into
heavier ones (generally) produces energy, up to the fusion of iron, but beyond
this it absorbs energy. So, once a star has made iron in its core, further
fusion reactions won't provide the energy to keep the star stable against its
luminosity.
High Mass Stellar Evolution --- proceeds much more rapidly and
dramatically than low-mass evolution. The cycles of core fusion followed by
core collapse do not end with the production of carbon in the core, as with a
low mass star, but can proceed all the way to the production of iron in the
core, with fusion of lighter elements in shells round the core. Such a star
will have a core temperature of several billion Kelvin and is very large (a
supergiant). Once this stage is reached, the star can no longer support
itself by fusion reactions because of binding energy considerations (see
below) and will probably undergo a Type~II supernova (see later).