Type I Supernovae
When supernovae were first classified, it was done by looking at spectra. If the spectrum of a supernova contains hydrogen (at visible wavelengths, this would be the Balmer series), the supernova was classed as a Type II, if there was no hydrogen present, it was known as a Type I. As astronomers do, Type I supernovae were sub-divided into Types Ia, Ib and Ic.
Type I supernovae initially confounded astronomers - their understanding of stars suggested that hydrogen made up around 70 - 80% of a star's mass so it was difficult to see how an exploding star could leave no trace of the Universe's most common element.
Sometimes, some massive stars (we think of 'massive stars' as those that are more massive than 10 times the mass of our Sun) are so extreme that in the later stages of their evolution, they start to lose their outer layers and evolve into stars known as Wolf-Rayet stars or Luminous Blue Variables. The cores of these stars remain intact but this material has been processed by the nuclear reactions inside the star. This means that we might expect this material to include carbon, nitrogen, oxygen and silicon (in decreasing amounts) with little trace of hydrogen. These stars will experience a runaway effect and will finally explode in a supernova. In these cases though, since the star has lost its outer layers, it is quite possible that they reveal very little hydrogen in their spectra meaning they are defined as Type I supernovae. They are often referred to as 'stripped core-collapse supernovae'. The presence or absence of additional spectral lines (of helium) allow these to be further divided in Type Ib and Ic supernovae. Ib supernovae appear to have lost their outer layer hydrogen whereas Type Ic have evolved further losing their helium as well (see Figure 1).
This brings us to the Type Ia supernovae (also known as thermonuclear supernovae; see Figure 2) – these involve a binary star system. Unlike a 'normal' binary star system, here we have to imagine a star in an orbit with a compact object known as a white dwarf.
White dwarfs are very dense stars. Although they have masses comparable to our Sun, they are squeezed into a volume similar to that of the Earth. This means a white dwarf exerts a strong gravitational force which can pull material away from its companion towards its own surface. The companion star is usually a star like our Sun or a huge red giant star. The mass of the white dwarf gradually increases as it draws more and more material from its companion in a process is known as accretion.
Gravitational collapse of the white dwarf is prevented by “electron degeneracy pressure” which is exerted by electrons within the white dwarf; this gives a white dwarf some strange properties and makes them quite different from normal stars. An increase in mass from accretion can however cause the white dwarf to become unstable. If the white dwarf reaches 1.44 solar masses (known as the Chandrasekhar limit), it is unable to accrete any more material - its degeneracy pressure is no longer able to balance gravity and the star explodes.
Material in a white dwarf will contain the elements we believe to be results of core fusion in lower mass stars (e.g. helium, carbon, oxygen, neon) meaning that spectra of these explosions are also devoid of hydrogen. A more recent discovery has also shown evidence for the possibility of Type Ia supernovae resulting from the collision of two white dwarf stars. These events, although relatively rare, would be likely to create gravitational waves.
From Figure 3, we can see that the shapes of the lightcurves differ; for Type Ia supernovae, this fading away is driven in the main by radioactive decay of some elements that are released in the explosion.