Stellar Evolution
after the Main Sequence
Low Mass Stars
The Path to the Main Sequence
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Life on the Main Sequence
• Once a low mass star, such as the Sun,
settles down on the Main Sequence, it is in
balance
• In the core, Hydrogen is being converted to
Helium, the resulting energy, in the form of
heat and radiation, works its way to the
surface
• While the force of gravity has not been
stopped, the gravitational collapse has been
halted
• The star will remain in this state for
several billion years or more, depending on
its mass
Nuclear Fusion
• In order to supply today's Solar
Luminosity, the Sun must convert 600
million tons of Hydrogen to Helium every
second.
• Simultaneously about 4 million tons of
matter is being converted to energy.
• Energy in the form of radiation makes
its way out of the sun.
• But what happens to the Helium?
Forming a He Core
• Helium is 4 times heavier than hydrogen, so
the inert helium will begin to slowly collect
at the center of the star
• At first there is no problem, but as the
amount of the He becomes substantial, an
inert core forms
Helium Core
• The Helium "ash" continues to grow.
• The Hydrogen is burning in a shell
surrounding the He
• Once there is enough He, gravity starts to
compress the helium core
– It begins to get hot from the compression, the
heat causes the H  He reaction to increase
causing more He, increasing the core mass
which increases the gravitational force
– Things start getting out of hand
Degeneracy
• The He has had a lot of time to pack together; The electrons have
formed what is called a degenerate electron gas
• At usual stellar densities, the electrons in a gas act as though they
were ordinary molecules and obey the usual gas laws
• As the electrons are squeezed into tighter and tighter spaces,
they begin to encroach upon each others 'territory' - They are
not free to move as particles in an ideal gas, but are constrained to
move only when other electrons move. It is as if the entire mass
of electrons are geared together.
This Star has a Problem
Let's summarize the situation,
The star is converting H to He furiously in a shell
about a contracting He core. The core is
resisting the pressure because of the electron
degeneracy pressure, but that doesn't stop the
heat from increasing causing the H to go to He
more and more rapidly.
Eventually the excess heat, radiation and
pressure overwhelm the force of gravity on the
hydrogen.
Balance is lost and the star begins to expand.
(Of course, the He core is still trying to
compress)
Red Giant
As the star expands, the gas cools.
This has the following effect,
1.
The color of the star changes gradually to red
showing the cooling gas temperature
2. The surface area becomes larger and larger,
causing the brightness to increase despite the
cooling temperature.
3. The He core is still getting hotter causing the
'burning' shell to produce more and more
4. This star begins to travel to the Red Giant
region of the HR Diagram
Leaving the Main Sequence
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Helium Flash
• At this point the core is still contracting against the electron
pressure – It's like a pressure cooker with the lid on tight.
• Finally the temperature exceeds 100 million degrees Kelvin
• At this temperature, the He nuclei have enough energy to
begin to react
• The Triple-Alpha process begins to convert helium into
carbon:
4
2 He
8
4 Be
4
+ He
2
4
+ He
2
8
4 Be
+ 
12
6 C
+ 
Triple-Alpha Process
Helium Flash
• One property of the degenerate electron gas is that it conducts
temperature very well, so as soon as the energy is released in one
part of the core, it is transmitted throughout the core in seconds,
producing a rapid heating of all of the He there.
• The He burning accelerates like an explosion – the He Flash.
• The new energy expands the core rapidly which in turn cools things
abruptly reversing the growth of the red giant. Without the
overwhelming heat and pressure, the outer atmosphere begins to
contract again; the triple-alpha process ceases once the
temperature has dropped less than 100 million degrees.
• The giant reduces its size (at the cost of heating up and shifting
color toward the blue once again)
• This moves the star down and to the left once again
After the He Flash
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And next…
• The pressure has been released, the star has
reduced it size (and consequently gotten a
hotter surface changing color again)
• Now it begins all over,
Hydrogen is begin converted to Helium in a shell
about the remaining Helium. The star once again
collects He in its core, and everything happens all
over again --- back to the Red Giant stage as the
He compresses heating the Hydrogen shell
Only this time there is a difference…
Red Giant again
• This time there isn't enough time to
form the electron gas – these changes
have occurred over tens or hundreds
of millions of years, not billions
• This time when the core temperature
reaches 100 million degrees and the He
 C, there won't be a He Flash instead
the star will be converting H to He and
He to C simultaneously.
Late stage evolution
Thin, cool atmosphere
Hydrogen burning shell
Helium
What's THIS??
It's a core forming – Carbon 'ash'
The final stages
Our star is now creating a carbon core, as that
becomes substantial, gravity begins
compressing it, making it denser and hotter
(Sound familiar?)
The heat from the compressing carbon gas
causes the helium shell to burn furiously;
that in turn increases the rate of burn for
the hydrogen shell. Making the star larger
and hotter (moving it left on the HR)
The pressure finally overcomes gravity in the
helium and hydrogen and the outer layers
begin to expand and lift off into space
Solar Life-Cycle
The Red Giant Sun
1 A.U.
Earth
Sun
Sun: Main Sequence
1 A.U.
Sun: Red Supergiant
The Fate of the Earth
Three possibilities:
• The earth enters the supergiant sun.
–
–
The Earth will vaporize.
The Earth will melt into a cinder, but remain
• The earth remains just outside the
supergiant sun.
–
The Earth will melt into a cinder, but remain
But what happens next to the Sun?
Planetary Nebulae
As the outer layers lift off
they form one of the most
beautiful sights in space.
Emitting mostly in blue and
red the gas above the core
moves into space. The ring
is an illusion as the gas is
spherical about the core.
At one time we thought
that this was a gentle,
graceful process. The
Hubble telescope has
changed our mind
M57 – The Ring Nebula
Planetary Nebulae
Ejected
atmosphere
Exposed
core
Planetary Nebulae
In the Cat's Eye Nebula, we
can see the complex jets and
interactions of the
expanding gas
Hubble's eyesight has shown
us that the stars do not
"gently go into the night"
NGC 2440
Planetary Nebulae
In the 'Twin Jet Nebula', the gas is a bipolar flow moving
at 200 miles/second. The left-over core of this star
has a surface temperature of 200,000 ºK
White Dwarf
What is left is the carbon core
of the original star.
It is very small, very hot
About the size of the Earth,
and many times hotter than the
Sun.
This is a White Dwarf. It is held apart by the degenerate
electron pressure. It will slowly cool over billions of years
to become a burned out carbon core – a black dwarf.
This will be the fate of our Sun.
White Dwarf
• Remember what's left at this point is a '
carbon core' – It's outer atmosphere has
lifted away leaving a very dense, very hot
core.
• The core's intense gravitational field is
balanced by the pressure of the
degenerate electron gas
White Dwarf
• Most of the mass of a
solar sized star is
concentrated in a core
about the size of the
Earth
– This means it is very dense:
A sugar cube’s worth of
material at the Earth’s
surface could weigh up to
200 tons
There is an inverse relationship between the mass and the
radius --- the more massive, the smaller the white dwarf
0.4 kg
0.8 kg
Chocolate cakes grow larger when their mass increases
0.4 M
0.8 M
White dwarfs grow smaller as their mass increases.
(More gravity, but same pressure)
White Dwarf
• The hot core now slowly cools without losing
pressure support
• The cooling process takes billions (perhaps
trillions) of years
• When it becomes cool enough, it can
crystallize
• At some point, when it is cool enough, we
declare it to be a black dwarf
Solar (low mass star) Evolution
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White Dwarf
It becomes natural to ask,
"What if the star has more mass than the
electron gas can balance?"
In order to become a white dwarf, a star
cannot have more mass than
Chandrasekhar's Limit,
Mstar < 1.4 Msun
Exceeding Chandrasekhar's Limit
If a star starts out with more than 1.4 M
it cannot become a white dwarf so its
evolutionary path must be different
(which we will discuss in the next lecture)
Is there any other way to exceed
Chandrasekhar's Limit?
Multiple Star Systems
• Let's digress for a moment and consider
multiple star systems.
• Until now, we have been considering only
'solitares' – stars isolated in space. But this is
actually the rarity. Most stars are found with
one or more companions.
• Their spacing is anywhere from about 2000 AU
down to 'almost touching'
• For simplicity, let's consider only binary star
systems
Algol – The Demon Star
Algol, Beta Persei, was seen to be the blinking eye in Medusa's
head. It fades and brightens in just under 3 days.
A very frightening sight
Algol is an 'eclipsing binary'
Two stars in close orbit oriented so that one passes in front of the
other as seen from Earth
Binary Star Systems
There are other types of variable stars (we will discuss some
later). For now let's take a closer look at that image of the
Algol system
The dotted line about Algol A represents its Roche Limit;
Notice that Algol B is deformed. Material from Algol B is
being pulled into Algol A.
Binary Systems
• Suppose one of the companion stars is a white dwarf
• As its partner reaches the red giant stage, its
atmosphere may impact on the white dwarf's Roche
Limit.
• Material from the companion will be accreted onto
the surface of the white dwarf
Dana Berry
Nova
• If the layer of slowly-accreting hydrogen is
heated to the appropriate temperature, it may
explode – vastly, but temporarily, increasing
the lumenosity of the system.
• This is a Nova, or "New Star"
• This, generally, does not do lasting damage to
the star and, in fact, may be re-occurring –
after burning off the accumulated Hydrogen,
the capturing process begins again
Supernova
What if the hydrogen layer is deposited more
quickly and so that it doesn’t have the time to
heat up enough to 'flash' into a nova, but
instead just adds mass to the white dwarf?
Once Chandrasekhar's Limit is exceeded, the
star reacts by undergoing a cataclysmic
explosion.
Supernova
This explosion totally
destroys the star.
It is a Type Ia
Supernova
It has an Absolute Magnitude
of -19.3 or about 5 billion
times brighter than the Sun
SN Ia LightCurves
Notice that the light
curves from the
various supernova are
nearly all the same.
This implies that the
mechanism is very
similar (and we will
be able to use them
as distance
indicators
From P. Hultzsch, et al
Very distant supernovae
Since they are so bright, they are used to measure
the expansion of the Universe