1. Option D2: Life Cycle of Stars
Stellar Evolution
Option D2: Life Cycle of Stars

2. Stellar Formation Stars form in nebula
Gravity starts to pull the individual particles together
As the particles move together under their mutual gravitational attraction they loose EP and gain EK
When protostar acquires enough mass, the temperature is high enough for fusion to start
Enters main sequence

3. Stellar Formation Nuclear fusion process stops further contraction
Hydrostatic equilibrium
Initial stellar mass determines location on main sequence
greater the initial mass = higher final surface temperature and greater luminosity

4. Stellar Death Hydrogen fusion continues for the entire main sequence
Length of time on main sequence is determined by initial stellar mass
More mass = LESS time
At the end of a star’s lifetime, most of the hydrogen in the core is gone
Fusion reactions will happen less often, interrupting hydrostatic equilibrium causing core to collapse
Collapse increases the temperature until helium fusion starts
Process causes star to increase massively
Expansion means that the outer layers are cooler
Becomes a red giant star

5. Stellar Death
If there is sufficient mass, a red giant can continue to fuse higher and higher elements

6. Stellar Death How a star dies depends on its mass:
Stars up to about 12 M will blow away outer layers and collapse into white dwarf
Stars between M will supernova and collapse into a neutron star
Stars bigger than 40 M will supernova and collapse into a black hole

7. M< 12 M
After Hydrogen is “used up”, equilibrium is disrupted and the star expands to a red giant
Helium fusion starts
Helium is “used up” and it expands again
Depending on the mass, heavier elements will be produced
At the end of the last fusion phase, the core shrinks while still emitting radiation, “blowing away” the outer gas layers
Forms a planetary nebula around star
Remnant star has shrunk to a much smaller size
Electron degeneracy pressure prevents it from shrinking further
Electrons repulse each other
White dwarf is left
Cools over billions of year due to lack of fusion into Black dwarf

8. Outer layers
White dwarf will eventually cool down
Ring Nebula

9. M< 12 M

10. Death of the Sun

11. Stellar Death Chandrasekhar limit:
If the core mass is less than 1.4 M, the star will collapse into a white dwarf (electron degeneracy enough to stabilize it)
If the core mass is greater than 1.4 M, the star will collapse into a neutron star or black hole (electron degeneracy NOT enough to stabilize it as white dwarf)
Subrahmanyan Chandrasekhar

12. M> 12 M
In red SUPERgiant phase, MASSIVE core creates such extreme temperatures that fusion creates elements heavier than carbon
Electron degeneracy pressure cannot provide enough outward pressure to stabilize the star
Electrons combine with protons to form neutrons
Neutrons get smashed together as star collapses
Emitting neutrinos
Outer layers of star collapse extremely fast and bounce off core creating a SUPERNOVA
Neutrons provide neutron degeneracy pressure that might stabilize the contraction

14. Stellar Death Oppenheimer-Volkoff limit:
If the core mass is less than around M, the star will collapse into a neutron star (neutron degeneracy enough to stabilize it)
If the core mass is greater than M, the star will collapse into a black hole (neutron degeneracy NOT enough to stabilize it as neutron star)