1. Stellar Structure: TCD 2006: 4.1 4 star formation

2. Stellar Structure: TCD 2006: 4.2 massive interstellar clouds The galactic interstellar medium (ISM) consists of a roughly uniform gas with n~10 5 atoms/m 3. At current epoch, star formation takes place in massive interstellar clouds. Clouds have dimensions: R~10 pc, n~5 10 9 atoms/m 3, T~10 K. Galaxy is pervaded by a magnetic field with field lines approximately parallel to the galactic plane. Field strongly tied to the ionized plasma in the ISM. Small local perturbations in the field lead to local potential wells. ISM condenses in these wells, increasing their strength as it pulls the magnetic field with it  Rayleigh-Taylor instability. Provides initial mechanism for the formation of dark clouds of interstellar matter. What causes these clouds to collapse to the point of forming stars?

3. Stellar Structure: TCD 2006: 4.3 Jeans’ criterion for gravitational collapse

4. Stellar Structure: TCD 2006: 4.4 contraction, fragmentation and formation

5. Stellar Structure: TCD 2006: 4.5 contraction, fragmentation and formation (2)

6. Stellar Structure: TCD 2006: 4.6 approach to hydrostatic equilibrium After ionisation is complete (all H  H + ), temperature and pressure rise, contraction slows down  hydrostatic equilibrium. can be estimated using the Virial theorem. Thermal kinetic energy is E kin ~(M/  m H )(3kT/2) = M/m H. 3kT. Gravitational energy at end of collapse is E grav ~-GM 2 /R 2 ~-M/m H (  D /2+  I ). But 2E kin +E grav =0, so protostar approaches equilibrium at given by k ~(  D +2  I )/12~2.6eV4.5 Hence T~30,000 K, independent of M !

7. Stellar Structure: TCD 2006: 4.7 thermal contraction Subsequent contraction governed by opacity. Opacity controls the loss of radiation from the surface, and hence the release of gravitational energy on a thermal timescale (t kin­ ~10 7 –10 8 years). Virial theorem can again be used because the star remains close to hydrostatic equilibrium.

8. Stellar Structure: TCD 2006: 4.8 minimum mass for nuclear reactions

9. Stellar Structure: TCD 2006: 4.9 stellar luminosity We know Sun has been in hydrostatic equilibrium for 4.5 billion years, implies Virial theorem useful. For example: ~10 14 Pa (10 4 atm), ~10 3 kg m -3 (cf. water) ~6.10 6 K. We also know L=4  R 2  T eff 4 (Stefan’s law: Eq. 1.4) But T eff ~6000 K ~ /1000. Why are T I and T eff different ? If star in equilibrium at temperature T I, it radiates as a blackbody with L = 4  R 2  T I 4 ~ 10 12 L ,4.8 and photons would have mean energies kT~0.5 kEV (X-ray). Fortunately, X-rays are trapped (absorbed and re-emitted) by ions, a temperature gradient is established and radiant energy diffuses towards the surface (Section 2).

10. Stellar Structure: TCD 2006: 4.10 stellar luminosity

11. Stellar Structure: TCD 2006: 4.11 4 star formation - review Stars form from interstellar clouds, providing Jeans criterion for gravitational collapse is satisfied. Virial theorem used to determine radius after contraction to point where opacity increases, timescale from earlier, and average temperature. Minimum electron separation used to estimate minimum mass for nuclear ignition. Photon mean free path used to estimate stellar luminosity.