ASTR 511 (O'Connell) Lecture Notes


Sun in H-alpha showing active regions
and a flame-like "prominence."


There are many distinct types of astrophysical sources: stars, AGNs, planets, H II regions, synchrotron jets, hot intracluster gas, shocked gas (SNRs), etc

Main issues:

    1. Generation of photons
    2. Transfer of photons to observer = radiative transfer
    3. Deduction of physical properties from emergent spectrum

(1) Photon generation:

(2) Radiative Transfer:

(3) Deduction of Physical Properties


In this section, we discuss two canonical types of astrophysical sources: STARS & AGNs and compare them in the following characteristics:


  • Dense spheres, r ~ 106-13 cm

  • Stars are dynamically stable, apart from convection and surface phenomena, throughout most of their lifetimes

  • Isotropic radiators.

  • Nuclear reactions maintain high-temperature (>107 K) core except when masses are too small or after fuel exhaustion

  • XR and GR radiation transferred slowly through high optical depth envelope. Degraded to UVOIR band.

  • Observed radiation escapes from a very thin (300 km in Sun) layer = "photosphere"

      NB: photospheres (1017 particles/cm3 in the Sun) are DENSE by the standards of AGN emitting regions

  • Subsidiary processes (e.g. driven by magnetic fields, accretion in binaries) can radiate in non-UVOIR bands.

AGNs ("Active Galactic Nuclei"):

  • Supermassive black hole (M ~ 107-9 Msun;   RSchw ~ 3 x 1013 M8 cm) at center of an accretion disk with r 1016 cm.

  • Accretion disk is fed by infalling material; matter is continuously transported through disk

  • Though disks can be semi-stable, radiative properties are continuously variable; stars that approach event horizon can produce brief, luminous "tidal disruption events"

  • Dissipation and magnetic processes near center of disk generate relativistic particles, . These can generate relativistic jets.

  • Observed radiation emerges from a large volume with nonuniform properties

  • Relativistic jets and disk confinement produce anisotropic radiation.

  • Relativistic particles produce broad-band non-thermal synchrotron radiation, directly observed at radio wavelengths.

  • Direct thermal radiation from denser, hot inner disk (UV).

  • Photon boosting of low-energy photons by inverse Compton scattering to UV/XR bands.

      Compton boost:
  • Hard radiation field produces strong ionization of gas in a large, low-density volume around disk, ===> UVOIR, XR emission lines.

  • Strong heating of surrounding dust grains ===> IR continuum & emission lines (3-1000µ). (But grains vaporized near center.)

  • Star-formation regions are often associated with AGN in disk galaxies (chicken or egg?).


  • Primary component is UVOIR radiation from photosphere.

    • Thermal source; emergent spectrum Planck function

    • Small spread of temperature around characteristic effective Te of photosphere, approximately depth where at any wavelength.

    • peak at Å
    • Photospheric temperatures 1000-100000 K.

    • Strong time variation only in minority of cases

  • Strong concentration to UVOIR, with rise to peak, then dropoff.

  • Spectral slope at higher allows estimate of mean Te.

  • Major absorption discontinuities from ionization edges of abundant ions (e.g. H: Lyman edge 912 Å, Balmer edge 3646 Å).

  • In normal stars, there is only low level radiation in other bands from high temperature corona, synchrotron radiation, etc.



  • Complex, narrow, absorption lines

  • Doppler widths small (thermal), ~ few km s-1

  • Line spectrum reflects physical state (composition, temperature, pressure) of photosphere

  • Lines and local continuum usually coupled (imply ~ same T)


  • Emission lines since generally not viewed against continuum source.

  • Wide range of ionizations (e.g. neutral to Fe XIV)

  • Wide range of Doppler widths in different galaxies, to >104 km s-1

  • Lines and local continuum decoupled since originate in different volumes

  • Doppler widths reflect kinematic motions of gas clouds

    • "Broad" lines from 1 pc
    • "Narrow" lines to 100 pc from BH
  • Line spectrum depends on viewing angle (inner regions can be concealed by thick tori, dust clouds if viewed at large angle to axis). Polarization.


  • Continuum slope & structure: Temperature

  • Ionization edge discontinuities: pressure/gravity

  • Line widths: pressure/gravity, rotation, outflows

  • Line strengths: T, pressure/gravity, chemical abundances. E.g.:

    • Classic "spectral-type" sequence is a temperature-sequence

    • Abundances: selected species easy to measure: e.g. Ca/H, Mg/H, Fe/H. He/H (hot stars only)

    • 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 near-TEQ.

    • 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 sources



Previous Lecture Lecture Index Next Lecture

Last modified December 2020 by rwo

Text copyright © 2000-2020 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 511 at the University of Virginia.