Horizontal Branch
Production of 12
C
and 16
O until Helium
in the core is
exhausted
Horizontal Branch to Asymptotic Giant
Branch
• Core temperature too low for C or O to ignite
• When Helium is exhausted, core begins to
contract
• Releasing gravitational potential energy
• Increasing the fusion rates in the He and H
fusion shells
• Atmosphere expands and temperature
decreases
• Red Giant reaches the Asymptotic Giant
Branch (AGB) phase
Asymptotic Giant Branch
Asymptotic Giant Branch
• Core is the size of the earth
• Convection in large portion of envelope
Asymptotic Giant Branch
• Convection in large portion of envelope
• Heavier elements formed in the star’s interior
mixed (second dredge-up)
• Strong stellar wind
• Large radiation pressure drives stellar wind
• Particles absorb photons from radiation field
and be accelerated out of the gravitational
potential
• Interstellar medium enriched with mostly
carbon, but also oxygen and nitrogen
• The H-burning shell adds mass to the He-rich
region between the burning shells (the intershell
region) => increases the pressure and
temperature at the bottom of this region
• When the mass of the intershell region reaches a
critical value, helium is ignited in an unstable
manner, called a helium shell flash
• The large energy flux drives convection
in the whole intershell region =>
intershell convection zone
• Mixing
Thermal Pulses
• Large energy release => expansion of the intershell region
• He-burning shell expands and cools down, time scale of
about a year => H-burning shell extinguishes => deeper
penetration of the outer convective envelope
• Convection can even penetrate beyond the now extinct Hburning
shell => material from the intershell region is
mixed into the outer envelope
• Third dredge-up
• He-burning and H-burning shell
• Long phase of stable H-shell burning follows => mass of
the intershell region grows until the next thermal pulse
• The duration of this interpulse period depends on the
core mass, lasting between 50 000 yrs < 1 000 yrs for the
most massive AGB stars
Thermal Pulses
• During helium shell flashes ejection of outer
atmosphere regions
Planetary Nebula
• Planetary Nebula expands at a speed ~20 km/s
• Radius of about 1 pc in 50 000 years
• The driving force is the radiation pressure caused
by intense ultraviolet emission from the central
core, acting upon dust grains in the nebula
• Dust grains condense out from the cooling
nebula gas, because of the existence of heavy
elements, such as carbon
Planetary Nebula
Times Scales
Phase yrs
Main Sequence 9x109
Subgiant 3x109
RGB 1x109
HB 1x108
AGB 5x106
PNe 1x105