Chapter 16: Evolution of Low-Mass Stars

While on the Main Sequence stars of all mass burn hydrogen into helium

How long a star

lives on the Main Sequence depends

on its mass

As the star burns its

hydrogen, it accumulates a helium

ashBecause energy flow in the central regions of the star is by radiation, the helium ash isn’t being stirred out.

As time goes by, the helium ash gets in the way

To continue to burn hydrogen with all that helium in the way, the star gets a little

hotter, a little bigger and a little brighter .

At the center of

the star, a dead helium core starts

to formThe central helium core is not fusing. It’s just being squeezed by gravity and added to by the hydrogen fusing above it

Once the hydrogen runs out, the helium ash gets

compressed until it becomes degenerate

When something is degenerate all the low energy states are filled.Only the highest energy states are left for new electrons

What does it mean to be Degenerate?

• Electron energy levels crowded togetheralmost continuous

• All low energy levels are full according to

the Pauli Exclusion Principle• Only place for additional electrons to go is

in high energy levels which meansthey must move very fast

• Adding more mass decreases the volume

• Temperature is same everywhere

If you add mass to a degenerate object it shrinks

When the helium core shrinks, it heats up. This causes hydrogen to start fusing in a shell around the

coreBecause the core is shrinking as more mass is added to it, it heats up. As it heats up that causes the shell fusion around it to speed up and the star stars to expand

The expansion to a red

giant is all due to the

battle between gravity

and pressure

Evolution off the Main Sequence is the reverse of

forming a protostar

The most massive stars become red supergiants

Once fusion in the core stops the

core shrinks and heats

up while the outer

surface expands and

cools

The core doesn’t completely collapse due to degeneracy

Helium Fusion starts when the core reaches

100,000,000°

CHe 1243

In stars with low mass the helium ignition is explosive

In the degenerate core the temperature is the same everywhere

Once helium fusion settles down the star

resides on the

“horizontal branch” for a

whileThe energy production will stabilize so the star will shrink in size (some). It will then start a second life burning helium into carbon

For low mass stars: a second red giant stage

when the helium in the core runs out

Internal Structure of AGB star

Thermal Pulses cause whole layers of star to lift

off

Near the end, shell fusion becomes unstable resulting in thermal pulses which push layers of the star into

space

Planetary nebulae recycle most of a stars’ matter back out into

spaceBy this time, convection is starting to reach farther down into the interior of the star and dredge up the products of fusion

The Death of a Low Mass Star<8 solar masses

Planetary nebulae can have very complex forms. The details of how they create those forms is not well understood but probably has something to do with magnetic fields or if the star is in a binary system.

The death of a low mass star on the H-R Diagram

After planetary nebula dissipates only a white dwarf is left

Visible light

X-ray light

White Dwarf Stars are degenerate matter

Chandrasekhar Limit

1.4 Msun

They are composed mostly of carbon with some oxygen

Many stars live and die in binary systems

The most massive will form a white dwarf first

Eventually, the other star evolves off the main sequence

The white dwarf can become very active as it gains mass

White dwarfs in binary systems are called cataclysmic variables because they can vary

cataclysmically

As hydrogen builds up on the white dwarf it can ignite

If the white dwarf mass exceeds the Chandrasekhar limit it explodes in a supernova. If not, it can undergo an ordinary nova outburst