Astronomy 100 -- Death of Low Mass Stars
POST MAIN SEQUENCE EVOLUTION
"The Death Processes of Stars"
How the life of a star ends depends on the mass of the star.
Low-mass stars (less than 8 solar masses) "die" in a gradual
process while high-mass stars (greater than 8 solar masses)
"die" in rather spectacular fashion. But, for all stars,
their MS stage ends when the hydrogen in the core of the star
is exhausted. Gravity takes over which results in contraction
of the core to higher density, temperature and pressure while at
the same time, the outer part of the star expands. Stars evolve
to the red giant and super giant regions.
This process continues until the next
nuclear fusion reaction can take place in the core of the star.. This is the
Triple-Alpha Process which takes
place in all stars.
THE TRIPLE-ALPHA PROCESS
This nuclear reaction fuses 3 HELIUM NUCLEI
into 1 CARBON NUCLEUS and releases energy.
He4 + He4 <==> Be8 + photon
Be8 + He4 ==> C12 + photon
The beryllium (Be) is very unstable and will break apart
very quickly (3 x 10-16 seconds!) unless it is
hit by another helium to form carbon.
For this reason, the densities and temperatures
must be very high (greater than 150 million Kelvin!)
In massive stars, this reaction occurs in a controlled
manner. But, for stars less than 2 solar masses,
the helium core is fused into carbon
in just a few seconds in
what is a called the
Helium Flash!
Sometimes, the CARBON NUCLEUS
can fuse with another HELIUM NUCLEUS which produces
an OXYGEN NUCLEUS.
C12 + He4 ==> O16 + photon
- - - - - - - - - - - - - - - -
While all stars will have the Triple-Alpha Process
take place in their core, stars will now evolve differently
depending on whether their mass is less than or greater than
8 solar masses. To understand how stars less than 8 solar
masses evolve, we will look at how astronomers expect the
Sun to evolve.
THE EVOLUTION OF
A 1 SOLAR MASS STAR
Evolutionary Track of a 1 Solar Mass Star.
An Animation of this Evolutionary Track.
- Main Sequence phase ends due to core hydrogen depletion:
As fusion fuel
(hydrogen) is used up, the core starts contracting
and temperature increases.
- "Hydrogen Shell Burning": Helium core heats
contracting shell of hydrogen around it so that
P-P fusion takes place outside of the core.
- Star Ascends the Red Giant Branch
:
The gravitational energy from the core and the
energy from the H-burning shell cause the atmosphere
of the star to expand and cool.
- Star outer parts of the star continue to expand
and the core contracts until a new fuel
source for fusion is found. Most logical is fusion
of helium by the Triple-Alpha process.
But, this requires extremely high
temperature and pressure.
Temperature = 100 to 200 million K !!!
- The Helium Flash:
The helium core is fused into carbon and
oxygen in a matter of seconds. Gut-wrenching
experience for the star. The interior structure is
quickly changed and the star shrinks in size.
- The "new" interior core has some helium that fuses
into carbon and oxygen. Star is now a
Horizontal Branch star.
This is a fairly short phase as
very little fuel remains for the star to burn.
- The star then ascends the
Asymptotic Giant Branch:
Collapsing core of carbon and oxygen is surrounded by
helium-burning shell, inert helium layer, hydrogen-
burning shell and inert hydrogen layer. Outer layers
of the atmosphere, once again, expand and cool.
- Helium burning shell is unstable and burns erratically,
causing thermal pulses. These pulses drive away the outer
atmosphere of the star forming a PLANETARY NEBULA.
The carbon-oxygen core of the star is exposed.
This stellar "corpse" is a
White Dwarf .
Examples of Planetary Nebulae
PROPERTIES OF WHITE DWARFS
- About the diameter of the Earth.
- Mass is about the same as the Sun.
(High density! 1 teaspoon = 5 tons!!)
- Larger Mass ==> Smaller Radius!
- Upper mass limit is 1.4 solar masses.
- First discovered in binary star systems.
Difficult to find.
Small size ==> not very luminous.
- Slowly cool and fade way.
"Oldest" white dwarfs are between
9 and 10 billion years old.
Accepted age of the Universe is 15 billion years.
Death of High-Mass Stars
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