Classification groups items or subjects into categories to help the reader better understand them. For example, a discussion of rock music might classify it in categories such as heavy metal, acid rock, hard rock, speed metal, and thrash. In the following passage, an astronomer classifies two types of a rare celestial phenomenon, the supernova.

Supernovae are divided into two classes according to how their brightness rises and falls: Type I reach their peak brightness in about 20 days, then fade slowly over a year or so; Type II reach their peak in about 30 days, then fade away in about 150 days. Spectral analysis shows that Type I supernovae have much more helium in their envelopes than do Type II, but no definite relationship between the two types has been found. There is some indication that Type II explosions occur in more massive stars, which would mean they are related to stars with iron cores.

— Barry R. Parker, Concepts of the Cosmos

 

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