Lecture 16: Evolution of Low Mass Stars

Key Ideas:

Low Mass = M < 4 M_sun

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: H fusion in the core

What happens to the He created by H fusion?

• Core is too cool to ignite He fusion
• Slowly build up an inert He core

Lifetime:

• ~10 Gyr for a 1 M_sun star (e.g., Sun)
• ~10 Tyr for a 0.1 M_sun star (red dwarf)

Hydrogen Exhaustion

Inside: He core collapses & starts to heat up.

• H burning zone shoved into a shell.
• Collapsing core heats the H shell above it, driving the fusion faster.
• More fusion = more heating, so that Pressure > Gravity

Outside:

• Envelope expands and cools
• Star gets brighter and redder & climbs up the Giant Branch.

Climbing the Red Giant Branch

Takes ~1 Gyr to climb 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

• T_core reaches 100 Million K
• Ignite He burning in the core in a flash.

Helium Flash

New fusion source: Triple-alpha Process

Fusion of three ⁴He nuclei into one ¹²C (carbon) nucleus:

A secondary reaction forms Oxygen:

When this occurs, the star leaves the Giant Branch.

Inside:

• Starts generating primary energy from He burning in the core.
• Gets additional energy from an H burning shell surrounding the core.

Outside:

• Gets hotter and bluer.
• Star shrinks in radius, getting fainter.

Moves onto the Horizontal Branch

Horizontal Branch Phase

Structure:

• He-burning core
• H-burning shell

The Triple-alpha Process is inefficient, and can only last for ~100 Myr.

Build up a C-O core, but it is still too cool to ignite Carbon fusion

Asymptotic Giant Branch Phase

After 100 Myr, core runs out of He.

Inside:

• C-O core collapses and heats up
• He burning shell
• H burning shell

Outside: Star swells and cools

Climbs the Giant Branch again, but at a higher effective photosphere Temperature than the Giant Branch, so it ascends at bluer color slightly to the left 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!

Consequences:

• Small changes in T lead to
• Large changes in fusion energy output

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:

• with weight of envelope taken off, heats up less
• never reaches Carbon fusion ignition temperature of 600 Million K

Core and Envelope go their separate ways.

Planetary Nebula Phase

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

• Ionized and heated by the hot central core.
• Expands away to nothing in ~10⁴ years.

Becomes a Planetary Nebula

Hot C-O core is exposed, moves to the left on the H-R Diagram

Hubble Space Telescope Images of Planetary Nebulae:

The ``Hourglass Nebula'' (catalog name MyCn 18). Red is primarily ionized nitrogen gas, blue-green is ionized oxygen and hydrogen gas. Colors are computer enhanced. The central core is barely visible in the center. In this object the envelope blew off along the poles of the star, giving it the hour-glass like appearance. Click on the image to see a full-size view, or go to the description at STScI Public Information web site.
Credit: R. Sahai & J. Trauger, [WFPC2 Science Team], & NASA
Source: STScI/AURA)

More Images of Planetary Nebulae on the Web

Core Collapse to White Dwarf

Contracting C-O core becomes so dense that a new gas law takes over.

Degenerate Electron Gas:

• Pressure becomes independent of Temperature
• P grows rapidly & soon counteracts Gravity

Collapse halts when R ~ 0.01 R_sun (~ R_earth)

Becomes a White Dwarf Star