The appearance of the spectrum allows astronomers to classify supernova into two main types: Type I and Type II. Basically, supernova arise from two very different classes of stars, massive ones and old, non-massive ones. The Type II supernova very strongly show the presence of the element hydrogen in their spectra. Type I supernova do not show any hydrogen in their spectra. The astronomer Rudolf Minkowski discovered this distinction in 1941, and this classification scheme was used for about five decades. It was thought that Type II supernova are the explosions of massive stars, while Type I supernova arise from old, low-mass stars.
Types of Supernova (source from Wikipedia)
In about 1985, things got a little more complicated. Some Type I supernova discovered and studied in the early 1980s appeared to be peculiar in nature. They did not exhibit a characteristic spectral signature, thought to be due to the presence of silicon, seen in many other Type I supernova spectra. Additionally, a few of these peculiar supernova showed very strongly the presence of helium. Furthermore, these supernova appeared to be occurring among populations of massive stars in galaxies. For these reasons, it was realized that Type I supernova can be further sub-classified into those with the silicon spectral feature, and these were called Type Ia supernova, and those that do not show this feature, this latter group were called Type Ib supernova.
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