We have just seen how Type II supernova come about. Astronomers at the time weren't quite sure what to make of it, so they decided to create some labels: they imaginatively named the explosions without hydrogen Type I, and the (more normal) ones with hydrogen Type II. The supernovas we've just discussed certainly expel great quantities of hydrogen. Lots of hydrogen is what you'd expect in a stellar explosion, because lots of hydrogen is what stars have. That second observation is quite peculiar.
The two types were identified early in the 20th century, when astronomers noted a very peculiar thing: supernova explosions either exhibit a lot of hydrogen in their spectra, or none at all.
The Universe is an astonishing place, so it is no wonder that there are two ways to make a supernova.
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