FIGURE 6.3 Time-lapse, false-color x-ray images of the hot spinning disk of gas surrounding the neutron star with the Crab Nebula taken by the Chandra X-ray Observatory over the course of several months. Image courtesy of NASA/CXC/ASU/J. Hester et al.

Supernovae are classified theoretically by mechanism—core collapse or accretion—and observationally by whether hydrogen is present in the ejecta. Type I supernovae lack hydrogen, while Type II do not. (The categories are further subdivided into Ia, Ib, Ic, II-P, II-L, II-n, and IIb according to the pattern of heavy elements ejected.) The two means of classification do not necessarily coincide, and we lack a detailed theoretical understanding of how to make the correspondence. Nevertheless, Type Ia supernovae are observed to have very similar intrinsic luminosities and have provided convincing evidence that the expansion of the universe is speeding up (see Chapter 5). Discovering whether Type Ia supernovae are truly a homogeneous class and learning what spread is to be expected in their properties are high-priority objectives of supernova research.

Supernovae are clearly the factories in which the elements up to and slightly above the iron group of elements are made. Not only are the detailed abundances of the elements lighter than iron quantitatively under-