Evolution of Medium Mass Stars (0.4 MSun < MStar < 8 MSun )

After ~ 10 billion years, a 1 MSUN star will exhaust its core Hydrogen. But H fusion takes place in a shell surrounding an inert Helium core.

Energy generated in the shell goes into the envelope which expands.

The star becomes a subgiant. (Luminosity class IV on the M&K “Yerkes” system)

Core contracts and T, ρ & ε all increase. Eventually the envelope becomes convective. Original Convection Zone Convection

While the core contracts, the envelope expands to become a Red Giant Star’s luminosity increases quickly. It “climbs the Red Giant Branch” (RBG) in the HR Diagram.

Elements can be “dredged up” from the core to the surface of the star.

Core temps. increase until T=100 million K.

Then Helium fusion begins... Evolution of a 1 MSUN Star Triple Alpha Process (He nucleus = “Alpha Particle”)

1. 3.

2.

4He + 4He + 4He ----> 12C + γ 4He now fuses in the core (Triple Alpha) creating a new phase: the “Helium main sequence”. 1H continues to fuse in the shell.

Red giants in this phase experience mass loss. Stars of various masses will have the same Luminosity but different temperatures, creating the “Horizontal Branch” in the HR diagram.

1 MSun

(NOTE: if MSTAR <1.8 MSUN, He fusion begins with a sudden “Helium Core Flash” because the core is supported by “electron degeneracy pressure” ...more on that later)

He fusion continues until He is exhausted in the core.... When 4He is exhausted in the core, it continues to fuse in a thin Helium Shell. There are now two fusion shells!

The star’s Luminosity 1 MSun increases dramatically. (just like when H shell fusion began)

It quickly moves up the “Asymptotic Giant Branch” (AGB) in the HR diagram. At the top of the AGB, fusion occurs in two shells: a thin He-fusion shell below and a thicker H-fusion shell above

R=44R SUN

Newly formed Helium rains down onto the He-fusion shell causing periodic flare-ups.

Star’s structure is unstable to oscillation, and it becomes a long- period variable star (P~100-700 days). 1 MSun

Mira, the first variable star 5 MSUN discovered, is a pulsating AGB star. V=2.0 to 10.1 !!! Mira (Omicron Ceti)’s variability was discovered in 1559 (& perhaps earlier) It varies by ΔV = 6 magnitudes! Sometimes it disappears to the naked eye! Pulsations in giant stars cause outer layers to be lost to space. Mass- loss rates can reach:

dM/dt = 10-4 MSUN/year ~ 30 MEarth /year !

Matter ejected into space becomes a Planetary Nebula.

1 MSun

0.6 MSUN Planetary Nebulae Are the last gasps of dying stars

(and are not related to planets!) Helix Nebula (closeup view) The Ring Nebula: A Planetary Nebula The Cat Eye Nebula Evolution of 1 MSUN Star Evolution of a 5 MSUN Star White Dwarfs

Stars with Mass between 1-8 MSUN lose varying amounts of mass.

Only their cores (composed of Carbon & Oxygen) remain as White Dwarfs.

All WD’s observed have a mass of 1.4 MSUN or less.

WD’s have no fusion. What prevents gravity from crushing them?