Stellar Evolution Flashcards
How are main sequence stars formed?
1) Cloud of dust and gas (left from previous supernovae). Clumps of the cloud contract under force of gravity.
2) When clumps are dense enough, the cloud fragments into regions called protostars, continue to contract and heat up.
3) Eventually the temperature reaches a few million degrees, and hydrogen nuclei fuse to form helium.
4) As temperature increases and volume decreases. The gas pressure increases.
5) Also radiation pressure in the star, becomes significant in stars because of enormous amounts of em radiation released by fusion.
6) Combination of gas pressure and radiation pressure counteract gravity, preventing further contraction,
7) Star has now reached main sequence.
How do main sequence stars become red giants?
1) Hydrogen in core runs out, nuclear fusion stops, and outward pressure stops. Core contracts and heats up under weight of star.
2) Outer layers expand and cool, and the star becomes a RED GIANT.
What happens to red giants?
1) Heat from contracting core raises temperature enough for hydrogen to fuse. Called shell hydrogen burning.
2) Core continues to contract until it is hot enough and dense enough for helium to fuse into carbon and oxygen. Called Core helium burning. Releases huge amount of energy which pushes the outer layers of the star outwards.
3) When helium runs out, carbon-oxygen core contracts again and heats a shell around it so that helium can fuse in this region. Called shell helium burning.
What happens to low mass red giants?
1) Carbon-oxygen core isn’t hot enough for any further fusion and so it continues to contract under its own weight. Stops at about earth-size due to electron degeneracy pressure.
2) Only works for stars with core mass under 1.4 times mass of the Sun. Max mass called Chandrasekhar limit.
3) If below Chandrasekhar limit helium shell becomes increasingly unstable as core contracts. Star pulsates and ejects outer layers into space as planetary nebular. Leaving white dwarf.
When do neutron stars occur?
If the mass of the white dwarfs core is 1.4 to 3 times mass of Sun.
How are neutron stars formed?
Electrons combine with protons to form neutrons and neutrinos due to contract of core.
When do black holes form?
When mass of the core is more than 3 times the Suns mass. Neutrons cant withstand gravitational force.
