Option D2: Life Cycle of Stars Stellar Evolution https://www.youtube.com/watch?v=-QGU0m7T9Q4 Option D2: Life Cycle of Stars

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

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

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

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

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 12-40 M will supernova and collapse into a neutron star Stars bigger than 40 M will supernova and collapse into a black hole

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

Outer layers White dwarf will eventually cool down Ring Nebula

M< 12 M

Death of the Sun

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

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

Stellar Death Oppenheimer-Volkoff limit: If the core mass is less than around 1.5-3 M, the star will collapse into a neutron star (neutron degeneracy enough to stabilize it) If the core mass is greater than 1.5-3 M, the star will collapse into a black hole (neutron degeneracy NOT enough to stabilize it as neutron star)