Skip to main content

D.4 Stellar Processes (HL)

D.4 Stellar processes

Type 1a Supernovae Type 2 Supernovae
Formation Core produced when mass from a companion star accretes onto a white dwarf, forcing it to exceed the chandrasekhor limit (1.4M~sun~)
Produced when a massive red supergiant stars explodes
- Core collapse
- Implosion is opposed by neutron degeneracy pressure that causes an outward shock wave
Do not active hydrogen lines in their spectra (But carbon, oxygen, silicon, iron) Have hydrogen line
Have a luminosity which falls off sharply after explosion Luminosity falls off gently after the explosion

Star formation

  • Interstellar gas cloud (nebula) from earlier supernova
  • gravitational attraction between particles
  • Jeans criterion
    • The magnitude of the gravitational potential energy of the cloud must be greater than the total random kinetic energy of the particles of the cloud.
    • If the mass of cloud is greater than the Jeans' mass M~j~ for a particular redius and temperature, the interstellarr gas collapse/coalesces
  • As gas collapse, temperature increases leading to nuclear fusion
    • Protostar: when the temperature rises sufficiently for visible light to be emitted

Nuclear fusion

  1. Main sequence stars:
    • Proton-proton cycle

      • Net effect: $4^{1}_{1}H \rightarrow$ $^4_2He$ + 2 positron + 2 electron neutrino


  2. For stars more massive than M~sun~
    • CNO cycle 8da1d1ec60bbd09cf6a0a31b50a8a71.jpg

Lifetime of main sequence stars


T与M^-2.5^ 成比例(T是寿命)

$$ \frac{T}{T_{\bigodot}}=(\frac{M}{M_{\bigodot}})^{-2.5} $$


Layered structure of massive start

  • The elements arrage themselves in layers, haviest at the cave, lightest in the envelope.
  • Nuclear fusion produce ever heavier elements depending on the mass of the star

Fusion cannot produce elements heavier than iron

  • Binding energy per nucleon peaks near iron => one of the most stable elements (Ni-62)
  • Formation of heavier elements than iron is not energetically possible.

Neutron capture

To produce the heavier elements than iron.

S-process R-process
Slow neutron capture Rapid neutron capture
Small number of neutrons are produced in fusion
Have enough time to undergo beta decay before further neutron capture Further neutron are captured before the beta decay
Ends with the production of bismuth-209 Can form nuclides heavier than bismuth-209