Supergiant stars (also technically known as asymptotic giant branch stars or AGB stars or ASG stars)
are especially large and old giant stars that are nearing the end of their life. Betelgeuse is a classic example, a
bright red star in orion, visible to the naked eye, and which has a diameter 630 times that of the Sun and 14
times as massive. The example pictured above is a supergiant 500 times the diameter of the Sun and about 12
times as massive. Massive, luminous O and B class stars are sometimes classed as blue-white supergiants,
such as Rigel, a B class star with a radius 70 times that of the Sun, 17-times the Sun's mass and 66 000 times
the Sun's luminosity. However, others consider any star with a radius less than about 100 solar radii to be giant
stars, rather than supergiants, and the blue-white giants are massive and more-or-less main sequence stars
and sometimes called hypergiants. Some sources estimate the radius of supergiant stars to be several tens of
light years! Certainly, these stars have a very intense superwind that blows off material that may travel to
these distances in the lifetimes of these supergiants, but such material would be very cool and dim and
probably would not constitute the visible surface, or photosphere, of the star, though constitutes a dense shell,
so I prefer to stick to more conservative size estimates.
When a star like the Sun nears the end of its life, it has burnt the hydrogen in its core, converting it into helium,
and continues to burn hydrogen in a shell around the core. At this stage the star increases to about 1o times
its radius and its surface cools - it becomes a red giant. The shell continues to burn helium, which sinks onto
the core, so the core gets larger and larger. Since no heat (by hydrogen burning) is any longer being
generated in the core, the core has no energy to resist gravity and contracts, and the heavier it becomes the
more it contracts, heating up as it does so (by turning gravitational potential energy into heat) until it reaches a
critical temperature of about 100 million degrees K, at which point the core sparks into life (quietly in
intermediate mass stars of between 2-10 solar masses, violently in low mass stars of between 0.7 and 2 solar
masses, causing a so-called helium flash in these low mass stars, such as the Sun) as it can now burn helium
by nuclear fusion, turning the helium in the core into carbon and oxygen. Note, that to burn a heavier element
by nuclear fusion generally requires a higher temperature. Thus, a star has to be hotter to burn helium than to
burn hydrogen, and a still higher temperature to burn carbon and oxygen. Each time a fuel is burnt by a star, it
is converted into heavier elements by fusion (joining) of the atomic nuclei.
Eventually the giant star turns its helium core into the heavier elements of carbon and oxygen (primarily),
building up a carbon-oxygen (C-O) core. Since it requires a higher temperature to burn carbon and oxygen,
initially this core is too cold to burn and contracts under gravity. As the core contracts, helium burning
continues in a shell around the core, depositing more C and O onto the core (the hydrogen burning shell
further out which burns hydrogen to helium has temporarily extinguished) so the envelope (outer layers of the
star) expands and cools even further. The star expands to about 100 times its original radius (or about 10 times
larger still than the red giant stage) and becomes a supergiant (or AGB star).
AGB supergiants exhibit pronounced pulsations as the star cyclically expands and contracts (these pulsations
are due to thermal instabilities and form the thermal pulse cycle, each thermal pulse is also called a shell
flash) due to the hydrogen and helium burning shells alternately switching on and off (for complex reasons that
will not be discussed here). These stars are called thermally pulsing AGB stars (TP-AGB stars).
The helium burning shell of supergiants reaches very high temperatures and produces neutrons that are
captured by heavy elements in the shell, producing elements heavier than iron, by the s-process. Many of the
elements heavier than iron, such as strontium and zirconium, are manufactured inside supergiant stars!
If the C-O core gets hot enough (which it will do for stars with about 8 solar masses or more) reaching the
critical temperature of 500 million degrees K (at a density of about 3 billion kilograms per cubic metre) the C-O
core begins to burn carbon, converting it into the heavier elements neon, sodium and magnesium. If this neon
rich core exceeds one billion degrees K, then neon is converted into the heavier element magnesium. At two
billion degrees K, after neon burning, the oxygen is burnt into silicon. If, after silicon burning, the core
temperature exceeds 3 billion degrees K then silicon is burnt into heavier atoms, such as sulphur, argon,
calcium and nickel, all the way up to iron, which is stable and cannot be burnt by nuclear fusion. Thus, there is
a succession of burning stages, each beginning one after the other, deeper within the core as the temperature
rises, until the central core consists mostly of iron.
What we end up with is a core that resembles an onion, with different layers, each burning different fuels and
each composed of different elements, with the innermost layers being hotter and containing the heavier
elements. This structure is shown in the diagrams below:
Above left, a look at the structure of a supergiant. Outside the hot central region, the envelope of the star is
convective and probably turbulent. (Convection is the mixing of fluid as hot fluids rise and cooler fluids sink.
The Sun has a much shallower convective layer just beneath its visible surface). Right: taking away the
outermost atmosphere and convection, for clarity, we can see the 'tiny' central core (shown in orange) which is
actually 6 times the diameter of the Sun.
Above: left - zooming in on the core, we see that it contains other shells within it. Zooming in even closer, right,
we see that this inner shell contains still further shells, rather like an onion! These shells arelabelled in the
Above: supergiant core structure. Each shell (or onion 'skin') is a region of burning enclosing a zone of different
elemental composition. 1) A shell burning oxygen to silicon surrounding the innermost zone containing silicon
and sulphur nuclei with the silicon burning to yield iron and nickel. 2) A shell of neon burning to oxygen and
magnesium enclosing a zone containing oxygen, magnesium and silicon. 3) A shell of carbon burning to neon
and magnesium enclosing a zone containing oxygen, neon and magnesium. 4) a shell of helium burning to
carbon and oxygen enclosing a zone containing carbon and oxygen. 5) A shell burning hydrogen to helium,
enclosing a zone containing helium produced by the burning shell as it moves outwards. Outside this is the
convective envelope, in red, which consists of mostly hydrogen and some helium in the proportions which the
star initially contained when it was born. If you cannot see the video on the right then click here to download it.
The lifetime of supergiants
Remember that only the heaviest supergiants complete all the stages of burning up to iron. A relatively small
star like the Sun will end its lifetime with a C-O core. The table below lists the length of time that a star spends
burning each fuel, for a star of 25 solar masses (heavier stars burn their fuel faster):
Hydrogen burning 7 million years
Helium burning 500 thousand years
Carbon burning 600 years
Neon burning 1 year
Oxygen burning 6 months
Silicon burning 1 day
(In contrast a star like the Sun, with one solar mass, burns only hydrogen for some nine billion years and never
gets to the carbon burning stage).
As you can see, the star gets into ever greater difficulty, as each stage extends the star's life by decreasing
amounts of time. When the final stage terminates, with an iron core, the star has run out of life, it has no
more fuel that it can burn in its core, though it has fuel in its outer layers, this will not help it, as the core has no
energy source to resist gravity. The core, which is now extremely dense (weighing 30 million kilograms per
cubic metre) has no energy source to resist its own immensely strong gravitational field, and it implodes. The
outer layers rebound, exploding off in a vast supernova explosion that temporarily outshines a galaxy of several
billion stars! The core will survive, if at all, as a neutron star or black hole, hurtling through space.
Above: a thermal pulse cycle (TPC). Nuclear burning occurs in two shells within an ASG star. The core consists
of heavier medium-weight elements like carbon (C) and oxygen (O) which will only ignite towards the end of the
star's life as a supergiant. Outside of this is a shell of burning helium (He), in which C and O nuclei are formed
by nuclear fusion of helium nuclei. Outside of this is a shell of burning hydrogen (H) where helium nuclei are
being synthesised by nuclear fusion of hydrogen nuclei. The presence of two burning shells creates periodic
instabilities called thermal pulses. The outermost layer consists of inactive (non-burning) hydrogen in which
turbulent convection transports the heat generated by the burning-shells to the surface of the star.
During the longest phase of the cycle (A1 and B1), hydrogen is burnt to helium in the outer H-burning shell
whilst the inner He-burning shell is inactive. This results in a build-up of helium beneath the H-burning layer.
With no thermal generation occurring in the growing helium layer, it contracts under its own mass, heating up
as it does so (converting gravitational potential energy into thermal energy) until it reaches the helium-ignition
temperature and then the helium shell switches on and burns helium into C and O (A2). Thin shells of burning
gas/plasma are subject to Schwartzschild-Harm instability - the rate of heat generation exceeds the rate of heat
loss and so the shell expands, but heats-up further as it does so (in thick layers of gas, expansion results in
cooling) resulting in a runaway nuclear reaction called a helium-flash or shell-flash (A2) which causes a
thermal pulse. The intense energy released causes the outer layers of hydrogen to expand and cool, rapidly
reducing the rate of hydrogen burning in the H-shell which becomes inactive (A3). The helium is rapidly
consumed (converted into C and O, A4) and the burning front of helium, which extends from the core outwards,
catches up with the inactive hydrogen shell and its temperature reignites the hydrogen shell to repeat the cycle
Hydrogen burns at a lower temperature than helium, so does not ignite the helium synthesised (this only
happens when the helium builds up and contracts). Hydrogen also burns with much higher stability, so the star
settles down again to a period of stable burning, until the next pulse. With each cycle, the C/O core increases
in mass. The period (duration) of each cycle is about 100 to 1000 years.
Notice that in stage A3 the extent of the turbulent convective layer deepens dramatically. This causes a
dredge-up of core materials into the star's atmosphere, which become visible as metal lines in the star's
spectrum. (In astrophysics, a 'metal' is any element heavier than helium).
The luminosity of supergiant stars is determined by the core mass, and not the total mass as it is in main
sequence (MS) stars. At the end of their helium-core burning stage, these stars left the giant branch (where
they were red giant stars) and moved onto the asymptotic giant branch (AGB) of the Hertzsprung-Russell
diagram. As the cores of the stars continue to grow, then they move up the AGB as their luminosity rises and
their surface temperature falls. The very high luminosities of these stars (due to their immense size, despite
their relatively cool and red outer layers) creates a high photon pressure - the pressure due to photons
generated by the burning shells colliding with the overlying atmosphere. In addition, the outer layers of the
atmosphere are so far from the centre of the star, that they are tenuous and more weakly bound by the star's
gravity. The Eddington limit is exceeded - the limit at which the outward pushing radiation pressure equals
the inward pull of gravity on the star's atmosphere. As a result, the intense photon or radiation pressure
blows off much of the star's outer atmosphere at high speeds. About 0.0001 solar masses of material are
blasted away each year at speeds of around 500 km/s.
The expanding shell of material blown off the supergiant by its superwind, forms a dense shell of cooling gas,
which becomes cool enough for molecules and dust particles to form, producing a dusty shell around the star.
Eventually, the entire atmosphere is blasted away, and a planetary nebula is formed with a white dwarf star at
its centre. Only the most massive supergiants will end-up going supernova.
The Thermal Pulse Cycle (TPC)