What is the difference between a Type II supernova and a Type IIb supernova?

A supernova is a powerful and luminous stellar explosion that occurs when a star has exhausted its fuel and its core collapses. Type II and Type IIb supernovae are both core-collapse supernovae, but they have some differences in their characteristics.

Type II supernovae are classified as such because they exhibit a strong hydrogen line in their spectra. These supernovae are characterized by the presence of hydrogen in their outer envelopes, which absorbs and scatters radiation from the explosion, causing the spectra to show a strong hydrogen line. Type IIb supernovae, on the other hand, have weaker hydrogen lines in their spectra, which suggests that they have lost some of their outer envelope before exploding. This loss of hydrogen is thought to be due to the star's interaction with a companion star or the surrounding interstellar medium before the supernova event.

Another difference between the two types of supernovae is the presence of helium in their spectra. Type IIb supernovae show a stronger helium line than Type II supernovae, indicating a higher abundance of helium in the outer envelope of the star before the explosion.