Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30
Lecture 16: The Evolution of Low-Mass Stars
Readings: Ch 21, sections 21-1 & 21-2, and
Ch 22, sections 22-1 to 22-4
- Low-Mass Star = M < 4 Msun
- Stages of Evolution of a Low-Mass star:
- Main Sequence star
- Red Giant star
- Horizontal Branch star
- Asymptotic Giant Branch star
- Planetary Nebula phase
- White Dwarf star
Main Sequence Phase
Energy Source: Hydrogen fusion in the core
What happens to the He created by H fusion?
- Core is too cool to ignite He fusion
- Slowly builds up an inert He core
Main-Sequence (H-burning) Lifetime:
- ~10 Gyr for a 1 Msun star (e.g., Sun)
- ~10 Tyr for a 0.1 Msun star (red dwarf)
Core Hydrogen Exhaustion
- He core collapses & starts to heat up.
- H burning zone moves into a thin shell surrounding the core
- Collapsing core heats the H shell above it, driving the fusion
- More fusion = more heating, so that Pressure > Gravity
- Envelope expands and cools
- Star gets brighter and redder & climbs up the
It takes a star about 1 Gyr to climb the Red Giant Branch
At the Tip of the Red Giant Branch:
- He core contracting & heating, but no fusion
- H burning to He in a shell around the core
- Huge, puffy envelope ~ size of orbit of Venus
- Tcore reaches 100 Million K
- Ignite He burning in the core in a flash.
At 100 Million K, a new fusion source ignites: the Triple-alpha Process.
This is the fusion of three 4He nuclei into one 12C
(carbon) nucleus through a multi-step nuclear reaction chain that involves
the momentary formation of 8Be:
Once Carbon is formed, a secondary reaction forms Oxygen from the
fusion of Carbon & Helium:
When this occurs, the star once again has a nuclear power source in its
core and leaves the Giant Branch.
- Starts generating primary energy from He burning in the core.
- Gets additional energy from an H burning shell surrounding the core.
- Gets hotter and bluer.
- Star shrinks in radius, getting fainter.
The new energy source helps the star begin to regain Hydrostatic and
Thermal Equilibrium. As it does so, it moves onto the
- He-burning core
- H-burning shell
The Triple-alpha Process is very inefficient at producing energy, so it
can only last for about 100 Myr.
While it goes on, the star steadily builds up a C-O core, but it is
still too cool to ignite Carbon fusion
After 100 Myr, the core runs out of Helium for Triple-Alpha fusion.
- C-O core collapses and heats up
- He burning shell outside the C-O core
- H burning shell outside the He burning shell
- Star grows rapidly in radius and cools
Climbs the Giant Branch again, but at a higher effective Temperature
than the Giant Branch, so it ascends with a bluer color, putting it
slightly to the left of the original Giant Branch on the H-R Diagram:
The star becomes an Asymptotic Giant Branch Star
He burning is very temperature sensitive: Triple-alpha fusion
rate ~ T40!
- Small changes in T lead to
- Large changes in fusion energy output
Star experiences huge Thermal Pulses that destabilize the
Rapid Process: takes ~105 years
Outer envelope gets slowly ejected (fast wind)
C-O core continues to contract:
- With the weight of envelope taken off, the core heats up less
- It never reaches the Carbon fusion ignition temperature of 600 Million K
Core and Envelope separate physically.
Planetary Nebula Phase
Expanding envelope forms a nebula around the contracting C-O
- Ionized and heated by the hot central core.
- Expands away to nothing in ~104 years.
The star briefly becomes host to a Planetary Nebula
The hot C-O core is exposed, and moves quickly to the left on the H-R Diagram
at nearly constant luminosity and increasing temperature.
Images of Planetary Nebulae
Planetary nebulae are among the most beautiful objects in the sky. Below
are links to PNe pretty-picture sites:
Enough, already, back to the story...
Core Collapse to White Dwarf
The contracting C-O core becomes so dense that a new gas law takes
Degenerate Electron Gas:
- Pressure becomes independent of Temperature
- P grows rapidly & soon counteracts Gravity
Collapse halts when R ~ 0.01 Rsun (~ Rearth)
- Degenerate core becomes a White Dwarf
We will learn more about White Dwarfs in Unit 3.
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Updated: 2006 January 21
Copyright © Richard W. Pogge, All Rights