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Back to Astrophysics
Supergiant Stars - Asymptotic Giants
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 10 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: 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). Below: taking away the outermost atmosphere and convection, for clarity, we can see the 'tiny' central core (shown in orange) which is actually some 6 times the diameter of the Sun in our model.
Above and below: zooming in on the core, we see that it contains other shells within it. The core is tiny compared to the massively extended envelope of the supergiant. Illustrations often exaggerate its size for clarity, but here we are trying to illustrate the actual scale.
Zooming in even closer, right, we see that this inner shell contains still further shells, rather like an onion! These shells are labelled in the diagram below:
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 sulfur 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.
The Thermal Pulse Cycle (TPC)
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
(B1).
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).
Superwind
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 center 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 center. Only the most massive supergiants will end-up going supernova.
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, though it is expected to burn some helium in its core.
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 meter) 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.