HELIUMHELIUM
Spectroscope designed by Norman Lockyer
for daylight observations of the spectra of solar prominences (1868) (from
Schellen, 1872. Picture from the
DAHAP page)
Term Structure
Otto Struve (see
1935 paper)
discovered the anomalous singlet to triplet line ratio of hot stars.
In early type B stars, the ratio between lines of the singlet term system and those
of the triplet states disagree with calculations for thermodynamic equilibrium.
Struve correctly deduced that this was due to Rosseland's theory of cycles operating
in the diluted radiation field within circumstellar shells surrounding hot stars.
The theory predicts an overpopulation of the metastable states due to the low
gas density and the geometric dilution of stellar radiation from the lower layers of
the photosphere propagating trough the thin extended atmosphere that often surrounds
hot stars.This mechanism generates a strange absorption spectrum of shell stars and quasars