|Giant and Supergiant Stars
|Above: the blue giant star in the centre is a main sequence O class star (the largest star shown in the Main
Sequence Star picture) with a diameter about 18 times that of the Sun. The relatively small red star at the top
left is a red giant star, with a diameter about ten times that of the Sun. A red giant is an old star that has left
the main sequence as it has depleted its core of hydrogen fuel and is continuing to burn hydrogen in a shell
outside the core. This causes the core to contract and heat up and eventually begin burning helium fuel (which
requires a higher temperature than hydrogen burning) which makes the end of the red giant phase. As the core
contracts, the outer layers expand outwards as the star becomes some ten times larger. Once the red giant has
depleted its core of helium fuel, it may swell up even more, to 100 times its size, as it becomes a red supergiant
star. [Graphic produced using Pov-Ray.]
Above the blue giant is a blue-white supergiant, like the star Rigel in Orion (Beta-Orionis or Beta-Ori) with a
diameter about 70 times that of the Sun. Beneath the blue giant is a red supergiant, like Betelgeuse, also in
Orion, with a diameter some 630 times that of the Sun. A supergiant is an old star that is nearing the end of its
life and has swollen to an immense size as its core is depleted of hydrogen and helium fuels and is in an
advanced stage of burning, producing heavier elements. Supergiants continue to burn hydrogen and helium in
two shells outside the core, this generates an immense radiation pressure that pushes the outer layers
outward, indeed such stars exhibit superwinds as the pressure of the light that they emit blasts material away
from their loose outer layers. As these outer layers move away from the central star they cool and dust may
form within them and the star becomes a shell star, encased in a dusty shell. Lower mass stars become red
supergiants, whilst heavier O and B class stars become extremely hot and luminous blue-white supergiants.
Supergiants tend to be thermally unstable and so pulse in and out once every 100 to 10 000 years or so.
These pulses are caused by temperature instabilities that result from having two burning shells within the same
star, and are called thermal pulses.
The Sun, which is a main sequence yellow dwarf star, was not visible as anything more than a point of light on
this scale and so was omitted.
Sometimes the term 'blue giant' is only applied to hot blue O or B class stars that are middle-aged and in
transition to the blue-white supergiant stage, but these transitional stages last only for a relatively short time
(red giants last longer and so are more common).
There is no single reason why contraction of the core in a red giant should cause the outer envelope to
expand. However, computer models accurately predict this once all the physics is included. It can be thought of
as being a consequence of conservation of angular momentum and conservation of gravitational potential
energy, in addition to the increased thermal pressure as the contracting core heats up as it converts
gravitational potential energy into thermal energy.
Red supergiants are also called asymptotic giant branch (AGB) stars and when they exhibit thermal instabilities,
as thermally pulsing asymptotic branch stars or TP-AGB stars. In these massive giants, radiation becomes an
inefficient mechanism of heat transfer and so the stars become much more convective as convective
turbulence churns their outer layers.
Look at the Hertzsprung-Russell (H-R) diagram (below):
The red giant stars (RG) occupy a region called the red giant branch, which is above the main sequence
(MS). Although their surfaces are relatively cool, their huge size gives them an increased luminosity, so they
are higher in the diagram. Higher still are the supergiant stars (SG) which belong to the asymptotic giant
branch (AGB) which is higher still, due to the immense size of these supergiants. Many red giants clump to
the right of the red giant branch, the so-called red giant clump stars. Stars with lower metallicity, however,
like population II stars are further to the left, extending into the horizontal branch of the red giant branch.
Stars like the Sun, a population I star, will arrive at the red giant clump, where they linger for some time as
they are realtively stable (giving rise to the apparent clumping of stars in this region of the diagram).
Eventually, the horizontal branch crosses a region of the H-R diagram called the instability strip (stars with
open circles are in the instability strip). Those stars at the top of the instability strip are the giant stars and
form RR Lyrae variables, which pulse as they fluctuate due to their instability.
Evolution of a Medium-sized Star of 5 Solar Masses
The evolution of a star of 5 solar masses (5 times the Sun's mass) on the H-R diagram, from the MS to the
giant branches is shown below:
This diagram shows the predicted path or track of the star on the H-R diagram. The table below summarises
the main features and events shown in the above diagram:
When a protostar becomes a main sequence (MS) star it joins the MS along the so-called ZAMS or zero-age
MS. During its life on the MS, the star moves very slowly in the MS on the H-R diagram, moving slowly upwards
as it burns hydrogen in its core. As hydrogen in the core begins to deplete, it is converted into heavier
elements, chiefly helium. This increase in mean molecular mass reduces the thermal pressure in the core. It is
this pressure which resists gravity, and so as the pressure reduces, the core contracts until the pressure is
increased and stability is maintained. As the core contracts, so it converts some of its gravitational potential
energy into heat, heating up as it does so. This gradually increasing temperature of the core causes the outer
layers to expand and the star slowly expands and increases in luminosity and so moves upwards in the H-R
diagram (between points 1 and 2).
Eventually, most of the hydrogen in the core is depleted and the core begins to contract appreciable,
massively expanding the outer layers of the star and increasing its luminosity. The star moves above the MS
and becomes a subgiant (stage 2-3). This increases the temperature of the core and outer regions, until a
shell of hydrogen outside the core becomes hot enough and ignites, burning to form more helium. The star
now has a predominantly inactive helium core surrounded by a hydrogen-burning shell (3-4). Eventually, a
thick hydrogen-burning shell forms (4-5) but this narrows as the hydrogen in the shell is gradually depleted
(5-6) and the helium core contracts still further, resulting in a huge increase in the star's radius. The outer
layers cool until the plasma starts to convert into a gas of H atoms in the outermost layers. Neutral atoms can
absorb radiation in ionisation back to plasma, and so these outer layers become opaque. This opacity means
that heat can not easily escape from the star by radiation through the outer layers and so convection kicks in
to transport the heat. The star's atmosphere becomes increasingly turbulent to deeper depths, until heavier
elements near the core (produced by H and He burning and other reactions) are reached and some of these
are dredged up into the star's atmosphere (the first dredge-up). The surface abundances of C, N and O
increase in these stars (as evidenced by their spectra) (stage 6-7). The star has become a red giant.
All this time, the helium core has been increasing and contracting, heating up as it does so, since the core is
not generating thermal pressure through nuclear reactions. Eventually, however, temperatures in the helium
core become hot enough to ignite helium and the helium burns by nuclear fusion to form heavier elements like
C, O and N. This suddenly increases the thermal pressure in the core, which expands. As the core expands,
the star's outer layers contract and the outer layers heat up, ionising the hydrogen back to plasma, and these
layers become transparent to light radiation again. Surface convection ceases and the star stops dredging-up
its core materials (8-9). The star has contracted and heated-up its surface layers and so becomes bluer and
moves to the left in the H-R diagram, forming the first-leg of the so-called blue loop.
The star readjusts to a new equilibrium (9-10) and then settles down to a relatively stable phase of core helium
burning, during which time the sequence of events parallels those earlier on when the star was burning
hydrogen in its core on the MS - carbon slowly accumulates in the core, which does not burn at these
temperatures, and so the core slowly contracts, causing the outer layers to expand and cool, the star reddens
again, completing its blue loop (10-11). Eventually, helium-burning in the core ceases and the core contracts
massively and the star enlarges even more than it did when it become a red giant and it becomes a red
supergiant (asymptotic giant-branch star or AGB star) during which time the star's outer layers become
opaque again and convection sets-in, resulting in a second dredge-up. This contraction heats-up the core,
igniting an outer shell of helium which continues burning, so now the star depends on helium-shell burning.
The shell thickens, forming a fairly stable early AGB star (E-AGB star), stage 12-13. Gradually, the helium
depletes and the layer thins. Now the star becomes unstable and enters a complicated cycle of thermal pulses,
in which burning alternates between shells of hydrogen and helium, around the carbon core. The star is now a
thermally-pulsing AGB star (TP-AGB) (14-15).
Evolution of Low-mass Dwarf Stars
The evolution of a star the same size as the Sun (a yellow G-class dwarf) is shown below:
The pattern is similar to that of the 5 solar mass star, but with some differences. Core hydrogen exhausts,
and the star swells into a red giant, undergoing the first dredge-up as it does so. For stars of between 1
and 2.5 solar masses, helium does not ignite smoothly. By this time, the temperatures in the core fell to
such an extent that the core contracted massively, until it become degenerate matter supported by
degenerate electron pressure, in a manner similar to that in a white dwarf. Degenerate matter is
thermally uncoupled (it is all 'cold' and isothermic) and has an exceptionally high thermal conductivity.
Contraction and heating of outer non-degenerate layers around the core occurs until the helium-ignition
temperature is reached. This heat is conveyed to the core almost instantaneously, due to its high thermal
conductivity, and the whole core may ignite at once! An explosive reaction ensues in a matter of seconds!
The star readjusts dramatically to this rude awakening by this helium flash. The star survives this
explosive reaction without being disrupted, however, and settles down to a period of stable helium-burning.
Eventually, helium in the core is depleted and the star resorts to helium shell-burning, becoming an E-AGB
star, but without a second dredge-up. It becomes a TP-AGB star. The blue loop is progressively smaller for
smaller stars and may not occur at all for the smallest.
AGB stars develop intense superwinds which blasts away their outer layers, often in two immense bipolar
lobes. The core eventually runs out of fuel and contracts to form a degenerate white dwarf remnant, after
going through a planetary nebula phase. For a star the size of the Sun, the white dwarf will be the
carbon-core, a so-called carbon white dwarf. For stars of less than about 1 solar mass (for stars less
massive than the Sun) the core temperature will not become high enough to ignite helium and no AGB star
is formed. Instead, the star passes from the red giant phase into a contracted helium white dwarf.
The Evolution of More Massive Stars
More massive stars may go through repeated cycles of core-ignition, following from the first shell-burning
AGB stage, if the star is massive enough then its core will reach sufficient temperatures to ignite carbon
(quietly in the more massive stars, or via a carbon flash in the less massive ones). Each fuel-phase (H, He,
C, Ne, O and Si-burning) gets progressively shorter and occurs at progressively higher ignition
temperatures, with only the most massive giants burning Si and ending with a core of iron (iron-56) which
cannot burn any further. Such stars lose massive amounts of material in their superwind, but with enough
core mass remaining they will undergo supernova explosions and leave either a neutron star / pulsar or
stellar black hole remnant.