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Stellar Evolution
The Basic Scheme
Stars live for a very long time compared to human lifetimes. Even though stellar life-spans are enormous,
we know how stars are born, live, and die. All stars follow the same basic series of steps in their lives:
Gas Cloud à Proto star à Main Sequence star à Red Giant and/or Supergiant à Horizontal Branch
star (only if it has a low mass) à Variable Star (RR Lyra, Cepheid or WVirgins) à Red Giant and/or
Supergiant à Planetary Nebula (low mass) or Supernova (high mass star) à Stellar Remnant (white
Dwarf, Neutron Star, or Black Hole).
Stars shine – we know that. But they can only shine until all their energy is used up. Here we will
consider what happens to stars as they burn their fuel, first hydrogen, then helium, etc. Stars go through
different evolutionary stages as their energy source changes. This results in changes in the surface
temperature (thus color), the luminosity (the energy output per second), and their sizes. In general stars
produce metals (up to iron, but this depends on their initial mass), and during the final stages of their
evolution they shed a large fraction of their material into space, thus recycling interstellar material leaving
behind white dwarfs, neutron stars and black holes. How long a star lasts in each stage, whether it dies
as planetary nebula or a spectacular supernova depends on the initial mass of the star. Massive stars
evolve quicker than less massive stars. Solar-mass stars will end up as white dwarfs, and very massive
ones as black holes.
Life on the Main Sequence
The longest and most stable phase in the lifetime of a star is its main sequence lifetime. The star is in
thermal and hydrostatic equilibrium. The star is converting hydrogen to helium in the “core” of the
star (in the region where the temperature is above 15 million degrees Kelvin), and it will do so for
roughly 100 thousand (massive stars) to 100 billion years (low mass stars). Changes do occur, but over
long, long time scales, so that main sequence stars are indeed in thermal and hydrostatic equilibrium.
The main sequence lifetime (and in fact, any other epoch in the life of a star) depends on its total mass.
Why?
a) Massive stars have more gravitational potential energy, so they can collapse faster
b) Massive stars, even when they start nuclear fusion, have relatively higher pressures in their centers
(because the larger mass is exerting a relatively higher pressure), thus higher central temperatures.
This results in faster reaction rates (since the protons smash together with relatively higher
velocities). In fact, hydrogen will get burned via the CNO cycle (more below) which produced
energy at a higher rate.
c) Thus the main sequence lifetime is shorter
d) The same applies for any of the other stages, which will also be shorter.
On the Way to a Red Giant
All through the long main sequence stage, the relentless compression of gravity is balanced by the
outward pressure from the nuclear fusion reactions in the core. Eventually the hydrogen in the core is all
converted to helium and the nuclear reactions stop. Gravity takes over and the core shrinks. The layers
outside the core collapse too, the ones closer to the center collapse quicker than the ones near the
surface. As the layers collapses, the gas compresses and heats up. Eventually, the layer just outside the
core called the “shell layer” gets hot and dense enough for fusion to start. The fusion in the layer just
outside the core is called shell burning. This fusion is very rapid because the shell layer is still
compressing and increasing in temperature. The luminosity of the star increases from its main sequence
value. The gas envelope surrounding the core puffs outward under the action of the extra outward
pressure. As the star begins to expand it becomes a subgiant and then a red giant.
The Horizontal Branch Phase
In low mass stars (like the Sun), the onset of helium fusion in the core can be very rapid, producing a
burst of energy called a helium flash. The reason for this is that the He-core is degenerate (i.e., the core
is packed as closely together as is possible). A degenerate gas does not obey the perfect gas law
(PV=NkT) where a pressure increase results in a temperature increase. In degenerate gases the
temperature can increase without the pressure adjusting along with it. So the core will get hotter and
hotter very rapidly and very suddenly start helium fusion to carbon. This in turn produces more energy,
which, in turn, increases the reaction rate. The overall effect is an explosive reaction. You can think of it
as a “core explosion”, which however cannot be seen since the core is surrounded by a huge envelope
which dams the explosion. Eventually the reaction rate settles down and the star has become a
Horizontal Branch star. Fusion in the core (Heà Carbon) releases more energy/second than the core
fusion of the main sequence stage, so the star is bigger, but stable. Hydrostatic equilibrium is restored
now.
Becoming a Cepheid Variable
Stars entering and leaving this stage can create conditions in their interiors that trap their radiated energy
in their outer layers. The outward thermal pressure increases enough to expand the outer layers of the
star. The trapped energy is able to escape when the outer layers are expanded and the thermal pressure
drops. Gravity takes over and the star shrinks, but it shrinks beyond the equilibrium point. The energy
becomes trapped again and the cycle continues. In ordinary stars hydrostatic equilibrium works to
dampen (diminish) the pulsations. But stars entering and leaving this stage can briefly (in terms of star
lifetimes!) create conditions where the pressure and gravity are out of sync and the pulsations continue
for a time. The larger, more luminous stars will pulsate with longer periods than the smaller, fainter stars
because gravity takes longer to pull the more extended outer layers of the larger stars back.
Because of the period-luminosity relationship, cepheid variables can be used to determine distances
(more later).
Asymptotic Giant Branch
If the star is not very massive, like for example the sun, its core temperature never gets hot enough for
Carbon to burn. Nevertheless, the total energy output is increasing, which causes the star to puff up
again, the surface temperature to decrease and to appear redder. The star is again a Red Giant on its
way of turning into a Supergiant.
Red giants and supergiants can have strong “winds” that dispel more mass than all of the stellar winds
that occurred during the long main sequence stage. All through the star's life after it first started nuclear
reactions, it has been losing mass as it converted some mass to energy and other mass was lost in the
winds. This means that even though a red giant is large in terms of linear size, it is less massive than the
main sequence star it came from. A red giant has the extremes in temperature and density: its surface is
cold and very low density, while its core gets very hot and extremely dense. Most of the mass loss of
stars will occur at this stage and in the next one.
Planetary Nebulae
In the next-to-last stage of a star's life, the outer layers are ejected as the core shrinks to its most
compact state. A large amount of mass is lost at this stage as the outer layers are returned to the
interstellar medium. For the common low mass stars (those with masses of 0.08 - 5 times the mass of
the Sun during their main sequence stage), the increased number of photons flowing outward from the
star's hot, compressed core will push on the carbon and silicon grains that have formed in the star's cool
outer layers to eject the outer layers and form a planetary nebula. The ultraviolet light from the hot
exposed core, called a white dwarf, causes the gases to fluoresce. Most noticeable is the red emission
from the excited hydrogen and nitrogen, the green emission from doubly-ionized oxygen, and the blue
emission from excited helium.
Planetary nebula get their name because some looked like round, green planets in early telescopes. We
now know that they are entirely different than the planets and are about one or more light years across
(much larger than our solar system!). Many planetary nebulae will look like rings (for example, the Ring
Nebula in Lyra or the Helix Nebula in Aquarius) because when we look along the edge of the
expanding spherical shell, we look through more material than when we look toward the center of the
shell. The round soap bubbles you made as a child (or still do!) look like rings for the same reason.
High-resolution images of planetary nebulae show complex structures in the expanding nebula (check
the Helix Nebula on the Web). The expanding gas from the planetary nebula ejection runs into gas and
dust dispersed in the red giant winds. As it passes the slower moving red giant wind material, the gas
shapes the denser blobs into comet-like shapes. Although they are called “comet knots”, they are not to
be confused with real comets in our solar system. Each of these blobs is over twice the size of our entire
solar system! Other planetary nebulae have a more asymmetrical appearance. The outflow is bipolar,
resulting from a more complex interaction of the final outer layer ejection and the material from the
stellar winds of the earlier stages. Examples of such nebulae are the Cat Eye Nebula and the Dumbbell
Nebula. Selecting the image below will bring up an enlarged view of the Cat Eye (check the Web).
Also, earlier jets of gas from the evolving star and companion stars may be needed to explain the
complex structure of nebulae like the Hourglass Nebula (web) and why the white dwarf is not at the
center of the green region in the middle. The two rings are centered along the star's poles that are
oriented around 60° to our line of sight. The upper ring is around the pole that is coming towards us and
the lower ring is around the pole that is oriented away from us. There is evidence that the Ring Nebula is
similar to the Hourglass Nebula except that we are viewing it from right along the pole, so just one ring
is seen.
White Dwarfs
Nuclear reactions have now stopped; the outer layers have been dispersed, and the only remaining part
of the star is the small carbon core. This core is rather hot and thus blueish whitish, however since there
is no more nuclear fuel left, its luminosity will decrease. Thus it will fade and get cooler, turning from a
White Dwarf into a Red and finally a Black Dwarf. Its size will be roughly comparable to that of the
Earth.
White Dwarfs are degenerate stars, i.e. their material is packed as closely together as is possible. WD’s
still emit light, but their are slowly running out of energy and cooling (analogous to a heated iron rod that
is left to cool). Their energy source is no longer nuclear fuel, not gravity, but just their own “thermal
energy”.
And if we had not been fried and destroyed during the Giant phase, we would freeze the death now…
The Evolution in the Herzsprung Russel Diagram
As the sun evolves its luminosity increases, its surface temperature drops, and its radius increases. These
are all observable quantities, which can be (conveniently) plotted into the Herzsprung Russel diagram
(HRD). We refer to this evolution as “the evolutionary tracks in the HRD”. To understand this evolution
is more detail, I have marked several stages in the HRD.
Stage 1:
The sun just started nuclear fusion of H à He and arrived on the main sequence (generally this is
referred to as the Zero-Age-Main-Sequence, or just ZAMS)
Stage 1 à 2:
• As a main sequence star it will continue to burn H à He in a region where the temperature is hot
enough (T>15x106 K). This region is called the “core”.
• Energy source: pp-process (H à He; 4 protons turning into one He-ion)
à in the core the fraction of helium increases gradually
(no mixing as is the case for massive stars – see below)
à the core becomes degenerate (explained below)
• Energy transport in core: radiative diffusion
• Energy transport in envelope: radiative diffusion
Stage 2 à 3:
• now have a core of helium
• hydrogen burning to helium continues in a shell surrounding this core
Stage 3 à 4:
• more energy gets produced in the center of the star (see below)
this energy has to be carried from the center to the surface of the star
à energy transport via radiation is no longer efficient enough
à the envelope turns convective
à energy can then be transported more efficiently to the surface
à luminosity increases
Overall: Stages 1à 2 à 3 à 4: as more H à He
à He mass of core increases (to roughly 0.5 the total mass)
à core contracts slowly (central temperature increases, reaction rates increase)
à envelope expands (radius increases)
à surface temperature decreases (color gets redder)
à luminosity increases (when envelope turns convective)
à star moves to the top right hand side of the HRD
(The Sun has now become a red giant and Mercury and Venus will be part of the sun, and
maybe the Earth too. In any case we will be fried, if we have not yet died…)
At Stage 4:
• Sudden onset of Helium to Carbon fusion (Tripple Alpha process, “3α”)
• He-flash = Core explosion (invisible)
• Followed by:
Re-adjustment rather rapid
Helium core burning and Hydrogen shell burning
How did a HE-flash happen?
Core is degenerate
“electrons as close as possible”
Central temperature keeps on rising
Perfect gas law does not apply to degenerate gases
à As temperature increases, the pressure does no longer balance it out
à Temp can increase further
à reaction rates speed up until He fusion starts
à this reaction is even more energetic (energy generation rate is proportional to T40)
à faster reaction à more energy output à higher temp
à “run-away” process
à “core explosion” - “Helium Flash”
Stage 5: Horizontal branch phase
• Comparable to main sequence phase with one main difference:
• Energy Source: HeàC (3α) and HàHe
• Helium core burning
• Hydrogen shell burning
à Energy generation is a lot higher
Stage 6: A brief Variable star phase (only in certain place of HRD)
• RR Lyrae star
• Instability
over-expansion of envelope
à collapse of envelope
à too much collapse
à rebounce
à expansion
à too much expansion
à etc
à radius increases and decreases
à surface temperature increases and decreases
à luminosity increases and decrease
à Period-Luminosity Relationship
Stage 6à7: On the way to a Giant/Supergiant
• Energy Source: some C à O; He à C (3α) and H à He
• Carbon/Oxygen core
• Helium shell burning
• Hydrogen shell burning
• Stages 1à 4 repeat themselves (only faster)
à Carbon mass of core increases
à core contracts slowly (central temperature increases, reaction rates increase)
à envelope expands (radius increases)
à surface temperature decreases (color gets redder)
à luminosity increases
à star moves to the top right hand side of the HRD
à Red Super Giant
(The Sun has now become a red giant and Mercury and Venus will be part of the sun, and
maybe the Earth too. In any case we will be fried, if we have not yet died…)
Stage 7à8
• as the stat grows, mass loss happens
outer shells get dispersed
He burning shell gets exposed to the surface
get tripple alpha reactions on the surface
since these are uneven, have several short flashes
these are rather explosive
à material is expelled in shells
• see a planetary nebula
• but the central star appears to be hotter
à hotter means a bluer color
à star moves towards blue in HRD
Stage 9 and beyond
• planetary nebula disappears
• star does no longer undergo any nuclear fusion
• star contracts
• star cools (the light emitted is thermal radiation)
à smaller, dimmer, redder means that the star moves towards the bottom right
in the HRD
• star now is a white dwarf
• eventually it will become an even dimmer red dwarf
Evolution of Low Mass Star
Evolution of High Mass Star