- Continuum slope & structure: Temperature
- Ionization edge discontinuities: pressure/gravity
- Line widths: pressure/gravity, rotation,
- Line strengths: T, pressure/gravity, chemical abundances.
- Classic "spectral-type" sequence is a
selected species easy to measure: e.g. Ca/H, Mg/H, Fe/H. He/H (hot
- Light element (e.g. C,N,O) abundances more difficult: C IV (UV)
in hot stars; various atomic lines in cool stars require high spec
resol; molecules in cool stars (CH, CN, NH, etc)
- Ionization decreases with increasing gas pressure: e.g. use Mg I
5175 Å strength as dwarf/giant
discriminant for Galactic structure studies
- NB: Derived abundances are sensitive to proper T,P estimation
- Integrated light of stars allows inferences concerning ages,
abundances of distant stellar systems. "Population synthesis":
- Slope & structure of continuum related to energy distribution of
electrons, importance of Compton scattering, accretion disk structure,
dust emission/absorption, etc.
- Less definitive interpretation than stellar continua because
of multiple components, complex generation mechanisms, absence of
- One test for a nonthermal source: polarization. Another:
compare its mean surface brightness to the Planck function and
derive corresponding T:
where f is the flux, B is
Planck function and is the angular area
(or upper limit) of the source. Is T "unphysically" high?
- (Emission) line strengths yield electron temperature & gas density;
- Line ionization
level constrains far-UV continuum ("hardness" of ionizing radiation).
- Line widths,
positions probe kinematics of turbulent gas near BH, outflows, etc.
- Line strengths yield abundances
- ...although of different species than in stars: e.g. O, N,
He, S, Fe (uncommon)...but not Ca, Mg (unless UV access), etc.
- ID's, Surveys: strong emission line sources (photons
concentrated to narrow bands) easier to detect than pure continuum