<p>The lines that appear in a stars spectrum mainly reflects its temperature not its composition, <a href="http://www.star.ucl.ac.uk/~pac/spectral_classification.html" rel="nofollow">see here</a></p>

<p>O-type stars start out with the same sort of <a href="https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements#Abundance_of_elements_in_the_Universe" rel="nofollow">composition as other stars</a>, that is they are mainly H and He (approximately 75% and 25% by mass) with traces heavier elements.</p>

CC BY-SA 3.0
2015-11-05T09:58:35.260Z
2015-11-05T09:58:35.260Z
Conrad Turner
<p>Hot stars like <a href="http://en.wikipedia.org/wiki/O-type_star" rel="nofollow">O-type stars</a> show no hydrogen in their <a href="http://en.wikipedia.org/wiki/Stellar_classification" rel="nofollow">spectra</a>. Does this mean they are made entirely of helium?</p>

<p>Any explanation would be really helpful.</p>

CC BY-SA 3.0
2015-11-05T16:08:53.443Z
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)
Eubie Drew
<star><spectra><hydrogen><helium>
Are hot stars like O-type stars entirely composed of helium?
nversusp
# Are hot stars like O-type stars entirely composed of helium?

(Asked by: username_0 on 2015-11-05)

Hot stars like [O-type stars](http://en.wikipedia.org/wiki/O-type_star) show no hydrogen in their [spectra](http://en.wikipedia.org/wiki/Stellar_classification). Does this mean they are made entirely of helium?

Any explanation would be really helpful.

**Tags:** star, spectra, hydrogen, helium
> **Comments:**
> **username_1**: 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)  

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## 1 answers

### Answer by username_2 on 2015-11-05. Score: 4 | Accepted answer

The lines that appear in a stars spectrum mainly reflects its temperature not its composition, [see here](http://www.star.ucl.ac.uk/~pac/spectral_classification.html)

O-type stars start out with the same sort of [composition as other stars](https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements#Abundance_of_elements_in_the_Universe), that is they are mainly H and He (approximately 75% and 25% by mass) with traces heavier elements.


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