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Galaxy NGC4414 from HST Astronomy 162:
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

Key Ideas

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?

Main-Sequence (H-burning) Lifetime:

Core Hydrogen Exhaustion

Red Giant Star Interior


Red Giant Branch on H-R Diagram

Climbing the Red Giant Branch

It takes a star about 1 Gyr to climb the Red Giant Branch At the Tip of the Red Giant Branch:

Helium 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:

Triple-Alpha Process

Once Carbon is formed, a secondary reaction forms Oxygen from the fusion of Carbon & Helium:

C12-Alpha-Gamma Reaction

When this occurs, the star once again has a nuclear power source in its core and leaves the Giant Branch.



The new energy source helps the star begin to regain Hydrostatic and Thermal Equilibrium. As it does so, it moves onto the Horizontal Branch.

He Flash to Horizontal Branch on the H-R Diagram

Horizontal Branch Phase

Horizontal Branch Star Interior

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

Asymptotic Giant Branch Phase

After 100 Myr, the core runs out of Helium for Triple-Alpha fusion.


Asymptotic Giant Branch Star Interior


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:

Asymptotic Giant Branch on the H-R diagram

The star becomes an Asymptotic Giant Branch Star

The Instabilities of Old Age

He burning is very temperature sensitive: Triple-alpha fusion rate ~ T40!


Star experiences huge Thermal Pulses that destabilize the outer envelope.

Core-Envelope Separation

Rapid Process: takes ~105 years

Outer envelope gets slowly ejected (fast wind)

C-O core continues to contract:

Core and Envelope separate physically.

Planetary Nebula Phase

Expanding envelope forms a nebula around the contracting C-O core:

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.

Final Stages: Envelope Ejection to White Dwarf

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 over...

Degenerate Electron Gas:

Collapse halts when R ~ 0.01 Rsun (~ Rearth)

Degenerate core becomes a White Dwarf

We will learn more about White Dwarfs in Unit 3.

Return to [ Unit 2 Index | Astronomy 162 Main Page ]
Updated: 2006 January 21
Copyright Richard W. Pogge, All Rights Reserved.