Neutral hydrogen in the Galaxy

HII regions Orion nebula Triangulum nebula

Interstellar extinction law

Dust in the Eagle nebula

Dust: reddening in colour-colour plot

Calculating E(B-V) from colour-colour plot Consider observations of a set of stars in the (U-B) vs (B-V) plane. The reddening vector will have a specific direction: which for A λ  1/λ gives Using this, any star can be de-reddened back to the stellar locus, allowing both E(B-V) and spectral type to be determined

Atmospheric Extinction

Discussion Question Given that we see emission lines (and hence on-going recombination) from ionised regions, what does this mean for the growth of the HII region? It will continue to grow for ever, faster than previous calculations, because of the additional radiation It will continue to grow exactly as before It will grow to a peak size and then stop It will grow to a peak size and then shrink again

HII regions Orion nebula Triangulum nebula

HII region spectra Different HII regions can have very different ratios of emission line strengths.

Temperature diagnostics

OIII diagnostic temperatures

Nebula temperatures (T/10 4 ) 0.25 exp(-39000/T e ) = 2.5x10 -7 T *

The Cooling Curve Volume emissivity ε = Λ(T) n H 2

Density diagnostics

Shocks in the interstellar medium

Discussion Question When a shock develops in the interstellar medium, a discontinuity of properties is produced. What properties would you expect to be conserved for material passing through the shock discontinuity? With what complications?

Supernovae 1A as standard candles for cosmology Light-curve stretch correlates with luminosity Correcting for this gives distances accurate to ~5%

Isothermal Shocks

Shocks in the interstellar medium

The Cooling Curve Volume emissivity ε = Λ(T) n H 2

Course Summary 1. Observational Astronomy - Quantifying light (flux density, intensity) - Magnitude system (m = m log 10 f) - Measuring distances (parallax) - Luminosities, absolute magnitudes - Stars as black bodies (L=4πR 2 T eff 4 ) - Stellar classification (OBAFGKM) - Hertzsprung-Russell (colour-magnitude) diagram - Astronomical co-ordinates (Right ascension, Declination)

2. Main sequence stars - Energy generation (nuclear fusion; tunnelling; pp/CNO) - Escape of light from a star (random walk diffusion process) - Equations of stellar structure (mass continuity, hydrostatic equilibrium, energy generation and radiative diffusion) - Simple solutions (dimensionless variables) - Explained observed main sequence properties (e.g. L  M ≈3 ). - Complication: convection - Upper and lower limits of the main sequence: radiation pressure (Eddington luminosity), and degeneracy pressure Course Summary

3. Degenerate stars - Later stages of stellar evolution (red giants etc; briefly) - Electron degeneracy pressure - Accurately with 6D density of states - Roughly, using the uncertainty principal - Fermi momentum - Maximum mass for White Dwarfs (Chandrasekhar limit) - Sizes, densities and ages of White Dwarfs - Neutron stars and black holes Course Summary

4. The interstellar medium - Its effect on starlight (extinction and reddening) - Photo-ionisation by stars, giving HII regions - Radiative recombination, and the Strömgren radius - Temperatures and densities from emission line ratios - Propagation of perturbations: sound waves - Shocks: derived conditions of the step-change - Supernova shocks: feed metals back in to new star formation Course Summary