Neutral hydrogen in the Galaxy

1
Neutral hydrogen in the Galaxy
2
Neutral hydrogen in the Galaxy
3
HII regions

• Orion nebula
  Triangulum nebula

4
Interstellar extinction law
5
Dust in the Eagle nebula
6
Dust reddening in colour-colour plot
7
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? ? 1/? 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
8
Atmospheric Extinction
9
HII regions

• Orion nebula
  Triangulum nebula

10
HII region spectra

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

11
Temperature diagnostics
12
OIII diagnostic temperatures




13
Nebula temperatures

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

14
The Cooling Curve

• Volume emissivity e ?(T) nH2

15
Density diagnostics
16
Shocks in the interstellar medium
17
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?)

18
Supernovae 1A as standard candles for cosmology

• Light-curve stretch correlates with luminosity
• Correcting for this gives distances accurate to
  5

19
Isothermal Shocks
20
Shocks in the interstellar medium
21
The Cooling Curve

• Volume emissivity e ?(T) nH2

22
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 (L4pR2Teff4)
• - Stellar classification (OBAFGKM)
• - Hertzsprung-Russell (colour-magnitude)
  diagram
• - Astronomical co-ordinates (Right
  ascension, Declination)

23
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?M3).
• - Complication convection
• - Upper and lower limits of the main
  sequence radiation pressure (Eddington
  luminosity), and degeneracy pressure

24
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

25
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