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Hot stars like O-type stars show no hydrogen in their spectra. Does this mean they are made entirely of helium?

Any explanation would be really helpful.

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    $\begingroup$ You missed this sentence from the Wikipedia article you cited: Stars of this type are identified by their dominant Helium II absorption lines, strong lines of other ionised elements, and Hydrogen and neutral He lines weaker than spectral type B. (Emphasis added) $\endgroup$
    – Eubie Drew
    Nov 5, 2015 at 16:08

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The lines that appear in a stars spectrum mainly reflects its temperature not its composition, see here

O-type stars start out with the same sort of composition as other stars, that is they are mainly H and He (approximately 75% and 25% by mass) with traces heavier elements.

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