neutral hydrogen in the galaxy

Neutral hydrogen in the Galaxy

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 Aλ 1/λ gives

Using this, any star can be de-reddened back to the stellarlocus, 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/104)0.25 exp(-39000/Te) = 2.5x10-7 T*

The Cooling Curve

Volume emissivity ε = Λ(T) nH2

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) nH2

Course Summary

1. Observational Astronomy

- Quantifying light (flux density, intensity)

- Magnitude system (m = m0 - 2.5 log10f)

- Measuring distances (parallax)

- Luminosities, absolute magnitudes

- Stars as black bodies (L=4πR2Teff4)

- 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. LM≈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