Neutral hydrogen in the galaxy
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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.

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Hii regions l.jpg
HII regions

Orion nebula Triangulum nebula





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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 stellar

locus, allowing both E(B-V) and spectral type to be determined



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HII regions

Orion nebula Triangulum nebula


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HII region spectra

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




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Nebula temperatures

(T/104)0.25 exp(-39000/Te) = 2.5x10-7 T*


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The Cooling Curve

Volume emissivity ε = Λ(T) nH2




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Discussion Question

What would happen if the piston were moved faster

than the sound speed of the gas?

What properties would you expect to be conserved

for material passing through the shock discontinuity?

(With what complications?)


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Supernovae 1A as standard candles for cosmology

  • Light-curve stretch correlates with luminosity

  • Correcting for this gives distances accurate to ~5%




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The Cooling Curve

Volume emissivity ε = Λ(T) nH2


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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)


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Course Summary

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


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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


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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


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