Lecture 13 Flashcards
(29 cards)
what are the six phases of star formation?
1) dark cloud
2) gravitational collapse
3) protostar
4) T tauri star
5) pre main sequence star
6) young stellar system
what is ZAMS? describe it
- zero age main sequence
- where H burning phase begins
- star chemical composition and structure is always changing b/c it is always converting H to He
describe the band width of the main sequence
it is a thick band, not a thin band
stars start on the line (on ZAMS). As stars are, the line gets thicker and moves above/left of ZAMS. Thus sequence line gets thicker as stars age
what are opposing inward/outward pulls in stellar evolution?
gravity (pushes inward)
and pressure of gas/radiation (pushing outward)
how does stability of the main sequence compare to other stages of a star?
main sequence is the longest period of equilibrium
other stages have lost equilibrium
the star’s fate depends on its _____
mass
what are the fates of…
1) low mass stars
2) intermediate mass stars
3) high mass
1) low mass stars: become white dwarfs
2) intermediate mass stars: become red giants, planetary nebulae and white dwarfs
3) high mass: explosion
what causes the stellar core to shrink?
when H depletes, an the core is mainly He
there is the same amount of gravity pulling inward but less outward pressure available
what are all the parts of the red giant phase?
- He core: inert (no fusion)
- H shell: surrounding the core; has SOME fusion
- yellow envelope: non burning envelope
describe the red giant phase?
has H burning from the second layer (H shell), which is more efficient than just the core burning
has higher luminosity
outer regions are cool as they puff out
how is the subgiant phase created? how does this lead to the triple alpha process?
in the red giant phase, when the H shell burns, it creates more He.
Thus, the He core grows, which contracts inward b/c of gravity
causes temp to rise in the core –> causes He to fuse –> “triple alpha process”
why does the inert He core need to reach 100MK before triple alpha fusion can begin?
need high temp to overcome electrostatic repulsion of alpha particles
describe what happens with He core fusion
what is the origin of it?
starts explosively (“He flash”) –> results in huge E release –> causes mass loss –> inert C core forms –> transitions to double shell burning
origin: electron degeneracy at the core
how long does a He flash last for?
100 Myr
describe the layers in double shell burning
- has an inert C core
- He burning shell surrounds C core
- H burning shell surrounds He burning shell
describe the death of a low mass star
- He is depleted in the core
- C core shrinks (b/c of gravity)
- double shell burning causes outer layers to expand again
- star becomes larger and more luminous than ever
what conditions are needed for C to fuse?
can this be reached by solar type stars?
temp higher than 600 million K
solar type stars never reach it
describe planetary nebulae
what does it leave behind?
- final end to solar type stars
- hot C core is visible as ejected gas (of outer layers) is dispersed into the interstellar medium
- leaves behind a hot inert C core (white dwarf)
what is the correct sequence for the evolution of a low mass star?
main sequence
giant phase
He flash
planetary nebula
describe the evolution of high mass stars compared to lower mass stars
in lower mass: star isnt big enough for the core to contract, or to reach high enough temp –> thus electron degeneracy pressure slows core shrinkage
high mass stars: C can be fused to form O/Ne/Mg core –> forms onion structure of different element layers
describe H burning in low mass vs high mass stars.
low mass: uses pp chain
high mass: have diff rxns available b/c of higher core temp
- it uses the CNO cycle (4H makes 1 He)
- faster rxn rate; thus uses H up faster
what is the helium capture reaction?
reactions involving alpha particles (4He)
in post-main sequence evolution, what happens when temp increases?
heavy elements fuse with each other
in post-MS evolution, what happens when core fusion ceases?
core fusion ceases –> hydrostatic equilibrium is disrupted –> core contracts while outer layers expand –> core temp increases –> next heaviest element starts fusion –> hydrostatic equilibrium is resotred
