User:SunderB/Draft:Spectroscopy/HII Regions/General Properties

General Properties of HII Regions

 * HII regions are areas of low-density hydrogen gas, ionised by hot massive stars (O or B-type)
 * They are generally a few parsec in size, but can be as small as ~0.1pc or as large as ~100pc!
 * They have electron temperatures of around 10,000 K and densities of 108 to 1010 m-3

Spectral Features

 * Show Balmer lines due to recombination
 * Strong forbidden lines of metals: OII, OIII, SII, NII
 * Bremsstrahlung continuum at radio wavelengths

Optical emission lines are heavily affected by extinction due to dust in the line of sight, but IR lines (such as Paschen and higher-order recombination lines) are not as affected.