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 ash Because 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 form The 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 together almost 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 means they 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 core Because 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
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°
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 while The 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
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 space By 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 M sun They are composed mostly of carbon with some oxygen