Low mass stars (like the Sun) - H-burning in core via PP chain.
High mass stars
- H-burning in core via CNO cycle.
- CNO fusion - requires very high temperatures and
very high densities.
12C + 1H + 1H + 1H + 1H ® 12C + 4He 
Lifetimes for all main-sequence stars is proportional to 1/(mass)2. Here, "lifetime" means "time on the main sequence".
B) STEPS LEADING UP TO THE RED GIANT PHASE - LOW MASS STARS
Eventually, all stars will deplete its supply of hydrogen in the core, where nuclear reactions are taking place. What ensues afterwards is the "red giant" phase:
-
core fusion ceases ® central gas pressure decreases
®Gravity begins "winning"
®core shrinks
®temperature rises
®hydrogen fusion ignites in a shell outside the helium core ® hydrogen-shell-burning stage
The increased gas pressure generated by shell-burning
"puffs up" the outer envelope of star (out to 20 R
!)
- a) COMPETING FORCES INSIDE THE RG CORE
- gravity (inwards)
- electron degeneracy pressure (outwards)
b) ELECTRON DEGENERACY
Electrons avoid the situation of simultaneously having the exact same energy and being in the same volume of space at the same time. But with high densities, avoiding this situation is difficult ®electrons resits being "scrunched" together by exerting a pressure on each other.
c) HELIUM FLASH
An explosive onset of the helium fusion process in the core.
Why an explosion? ® because of the degenerate core:
- i) IN A NONDEGENERATE (NORMAL) GAS:
As temperature rises, pressure gets larger.
®gas expands, and thus cools.
®fusion rate decreases
ii) IN A DEGENERATE GAS:
As temperature rises, pressure stays the same.
®no cooling, shell-burning rate increases.
®temperature increases, shell burning increases
®but as burning increases, the temperature increases.
®...(temperature ramps up higher and higher...)
®EXPLOSION!!!! ("Helium flash")
The helium flash produces enough energy to expand the core and eliminate the electron degeneracy.
The core, now mostly helium, is hot enough at this point to begin helium-fusion, which converts 3 helium nuclei into 1 carbon nuclei.
C) HORIZONTAL BRANCH
This is similar to the main sequence stage, but here, red giant stars burn helium in their cores.
Competing forces in the core will be:
- gravity (inwards)
- gas pressure (outwards)
D) ASYMPTOTIC GIANT BRANCH (RED SUPERGIANT PHASE)
- a) STEPS LEADING UP TO THE AGB
- helium runs out in core, gas pressure decreases.
- gravity begins winning
- core shrinks
- temperature rises
- helium fusion ignites in a "shell" outside the core, hydrogen fusion shell occurs outside the helium fusion shell.
- increased gas pressure "puffs up" the outer atmophere
even more than before (out to 100 R!!)
- gravity (inwards)
- electron-degeneracy (outward)
b) COMPETING FORCES
c) CARBON-FUSION?
At this point, the core is mostly carbon. But the core is not hot enough to allow carbon-fusion in low mass stars.
E) WHERE LOW MASS STARS GO WHEN THEY DIE - WHITE DWARFS
At some point, the core will not be hot enough to allow the fusion process to operate on the element that the core is predominantly made of. Usually, this happens at the carbon-core phase (but could happen at the helium core phase for very low mass stars!). When this situation finally occurs, the core fusion process ceases (of course!)
®gravity starts "winning" again, causing the core to shrink. The core becomes electron degenerate once again, which supports the core against the inward-pulling gravity.
Repeated helium shell flashes provide the energy to eventually push the outer atmosphere from the core.
®planetary nebula phase
The star inside of the planetary nebula is a white dwarf ®it is the size of the Earth, but having approximately a solar mass!
®1 cubic cm of this stellar material would weigh 1 ton on Earth!!
The maximum mass that a white dwarf can possibly be is 1.4 ("Chandrasekhar limit"). Given the description above, what would you predict would happen if the mass exceeded this?
F) THE "TURN-OFF" POINT ON THE MAIN SEQUENCE TELLS THE AGE!
G) THE EFFECTS OF HAVING A NEIGHBOR! - CONTACT BINARIES
- a) ROCHE LOBES
The location at which the gravitational force from one star pulling towards it equals the gravitational force from the other star that is pulling in the opposite direction, towards that star.
b) MASS-TRANSFER
Occurs when one of the stars overflows its Roche lobe, and accretes on the neighbor.
c) NOVA
An explosion that is created when material from a neighbor accretes onto the surface of a white dwarf:
-
Brightness lasts for a few weeks
As material gets dumped on the surface, it suddenly reaches temperatures suitable for hydrogen fusion, which it does so explosively.
light curve reveals it:
