star sequence, attain a maximum[151] at
, and have disappeared[152] in normal
stars. The ultimate lines are the
series,[153] in the far ultra-violet, and cannot be traced in the stars.
The helium lines vary much in width and definition and are often winged. Their intensity does not certainly appear to vary with absolute magnitude within a given spectral class, and they cannot therefore be used in the estimation of spectroscopic parallaxes.[154] The question of absolute magnitude effects cannot be usefully pursued in the absence of more reliable parallaxes, for the
