We think that mass transfer onto a white dwarf can also account for type I supernovae. Sometimes enough mass falls onto the white dwarf to make its mass greater than the Chandrasekhar limit, Mch. In this case, electron degeneracy no longer supports the star, and it collapses. The energy from the collapse drives nuclear reactions, which eventually build up 56Ni (with an even number of 4He nuclei). The 56Ni beta decays to form 56Co, which, in turn, beta decays to 56Fe. Type I supernovae have light curves with double exponential behavior. The time scales of the two exponentials turn out to be characteristic of these two beta decays. We think that this process accounts for most of the iron in the universe, since the iron created in more massive stars is destroyed in the type II supernova, as discussed in Chapter 11. Light curves for type I supernove are shown in Fig. 12.6. There is little variation in the peak brightness of type I supernovae. We will see in Chapter 18 that this makes them very good "standard candles" in distant galaxies.

The nuclear energy released in these reactions is greater than the binding energy of the white dwarf, and the star is destroyed, leaving no remnant. This explanation accounts for the light curves of type I supernovae, their spectra and luminosities, and their occurrence in what are thought to be old systems (as we will discuss in Chapter 13).

12.3 I Neutron stars in close binary systems

In the preceding section we saw how mass transfer in semidetached systems can alter the evolution of a star. In this section we consider a neutron star and a normal star in orbit around their common center of mass. As the normal star evolves towards a red giant, it fills its Roche lobe and matter starts to fall onto the neutron star.

At first it was thought that this situation could not develop. It was not clear how such a system could form. The problem is that, for a neutron star to be present, there must have been a supernova explosion in the past. The first star to go supernova in a binary is the more massive star. The supernova explosion drives away most of the mass of the more massive star, meaning that more than 50% of the original mass of the system was blown away. If this happens far too quickly for the system to adjust the orbit radius/period, it becomes unbound. To see this for circular orbits, we assume that the system starts with stars of mass m1 and m2 and a separation R, with speeds appropriate for a circular orbit, given by equation (5.15). Assume that star 1 explodes and is left with a mass m, but that R and the orbital speeds don't change. The new

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