Dr. Melissa Graham: The Different Types Of Supernovae
Just as there are different types of stars, there are different types of supernovae. They are classified empirically based on the elements identified in their spectrum. The core collapse supernovae described above are called Type II if they display hydrogen, Type Ib if they show helium, and Type Ic if neither hydrogen nor helium are present (these are arbitrary choices of representative letters). Although these categories were initially defined based on observational evidence, astronomers now understand the physical differences of the progenitor stars and their explosions that give rise to these classifications.
As described above, a massive star becomes like an onion with the heaviest element, iron, fused in the center, and concentric shells of lighter elements out to helium and hydrogen. Since Type Ib do not show hydrogen but do show helium, this indicates that at the time of core collapse, the star did not have a hydrogen shell. Similarly, Type Ic have neither a hydrogen nor a helium shell, and their spectra show heavy elements such as iron from the core. How could this be? In massive stars that burn hotly and brightly, radiation pressures are large enough to blow the outer layers off the star. In more massive stars, more mass is lost from the outer shells - thus it is expected that stars of 8 to 20 solar masses become Type II, and more massive stars become Type Ib and Ic. This hypothesis has been confirmed for some of the nearest such supernovae, when the massive star visible in pre-explosion images has disappeared.
There is one more empirical classification of supernovae called Type Ia. As with the Type Ic, the Type Ia do not show hydrogen or helium, but they do have remarkably strong silicon absorption lines, and also show iron. All Type Ia are very bright, and have similar intrinsic luminosities - this means they all release the same amount of energy, and a lot of it. These characteristics indicate they are not caused by a star's core collapse, but are thermonuclear explosions of 1.4 solar mass carbon-oxygen white dwarf (COWD) stars. A star which is initially 2-8 solar masses is not hot enough to fuse elements heavier than carbon and oxygen. At this stage the star cools, shrinks, loses most of its mass during a planetary nebula phase, and becomes a COWD star. These stars are very dense - the mass of the sun but the size of Earth - and only stable when less than 1.4 solar masses. However, if a COWD has a binary companion it may accrete matter, and grow. At the critical mass, a thermonuclear runaway reaction fuses most of the material to radioactive nickel in a matter of seconds, which then decays to iron. The remaining material is burned into lighter elements like silicon. Although COWD stars are too faint for direct confirmation as the progenitor, they are the only known physical scenario which simultaneously explains the brightness, similarity, and spectra of Type Ia supernovae.
