An Etymological Dictionary of Astronomy and Astrophysics

The total number of → Lyman continuum photons emitted by an → H II region. It is usually derived using → radio continuum observations which are less affected by → interstellar extinction. The measured value is often a lower limit because of photon leakage from the H II region and absorption.
See also → density-bounded H II region.

See also: → H II; → region; → luminosity.