Stars of spectral class O have the strongest hydrogen absorption lines, whereas stars of spectral class M have the weakest hydrogen absorption lines.

Question

Quizplus · Accepted Answer

Stars of spectral class O actually have the weakest hydrogen absorption lines, while stars of spectral class M have the strongest hydrogen absorption lines. This is because spectral classes are ordered from hottest to coolest, with class O being the hottest and class M being the coolest. Hotter stars ionize more hydrogen atoms, leading to weaker absorption lines. Cooler stars have more neutral hydrogen atoms, leading to stronger absorption lines.

Stars of Spectral Class O Have the Strongest Hydrogen Absorption