Readings: Ch 21, sections 21-1 & 21-2, andCh 22, sections 22-1 to 22-4
Low-Mass Star = M sun Stages of Evolution of a Low-Mass star:Main Sequence starRed Giant starHorizontal Branch starAsymptotic Giant Branch starPlanetary Nebula phaseWhite Dwarf star
Main Sequence Phase
Energy Source: Hydrogen fusion in the coreWhat happens to the He created by H fusion?Core is too cool to ignite He fusionSlowly builds up an inert He coreMain-Sequence (H-burning) Lifetime:~10 Gyr for a 1 Msun star (e.g., Sun)~10 Tyr for a 0.1 Msun star (red dwarf)
Core Hydrogen Exhaustion
Inside: He core collapses & starts to heat up.H burning zone moves into a thin shell surrounding the coreCollapsing core heats the H shell above it, driving the fusionfaster.More fusion = more heating, so that Pressure > Gravity
Outside: Envelope expands and coolsStar gets brighter and redder & climbs up theGiant Branch.
Climbing the Red Giant Branch
It takes a star about 1 Gyr to climb the Red Giant BranchHe core contracting & heating, but no fusionH burning to He in a shell around the coreHuge, puffy envelope ~ size of orbit of VenusAt the Tip of the Red Giant Branch:Tcore reaches 100 Million KIgnite He burning in the core in a 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 involvesthe momentary formation of 8Be:Once Carbon is formed, a secondary reaction forms Oxygen from thefusion of Carbon & Helium:When this occurs, the star once again has a nuclear power source in itscore and 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.The new energy source helps the star begin to regain Hydrostatic andThermal Equilibrium. As it does so, it moves onto the Horizontal Branch.
Horizontal Branch Phase
Structure:He-burning coreH-burning shell
The Triple-alpha Process is very inefficient at producing energy, so itcan only last for about 100 Myr.While it goes on, the star steadily builds up a C-O core, but it isstill too cool to ignite Carbon fusion
Asymptotic Giant Branch Phase
After 100 Myr, the core runs out of Helium for Triple-Alpha fusion.Inside:C-O core collapses and heats upHe burning shell outside the C-O coreH burning shell outside the He burning shell
Outside:Star grows rapidly in radius and coolsClimbs the Giant Branch again, but at a higher effective Temperaturethan the Giant Branch, so it ascends with a bluer color, putting itslightly to the left of the original 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 fusionrate ~ T40!Consequences:Small changes in T lead toLarge changes in fusion energy outputStar experiences huge Thermal Pulses that destabilize theouter envelope.
Rapid Process: takes ~105 yearsOuter envelope gets slowly ejected (fast wind)C-O core continues to contract:With the weight of envelope taken off, the core heats up lessIt never reaches the Carbon fusion ignition temperature of 600 Million KCore and Envelope separate physically.
Planetary Nebula Phase
Expanding envelope forms a nebula around the contracting C-Ocore:Ionized and heated by the hot central core.Expands away to nothing in ~104 years.The star briefly becomes host to a Planetary NebulaThe hot C-O core is exposed, and moves quickly to the left on the H-R Diagramat nearly constant luminosity and increasing temperature.
Images of Planetary NebulaePlanetary nebulae are among the most beautiful objects in the sky. Beloware links to PNe pretty-picture sites:Hubble Space Telescope Gallery of Planetary Nebula ImagesBruce Balick”s Planetary Nebula Gallery at SEDs.George Jacoby”s Planetary Nebula Sampler at NOAO.Enough, already, back to the story…