Beginning of Essay
A is a STAR that explodes. It suddenly increases in brightness by a factor of many billions, and within a few weeks it slowly fades. In terms of the human lifespan, such explosions are rare occurrences. In our Milky Way galaxy, for example, a may be observed eve ....

Middle of Essay
.... million years, and the energy expended in ejecting material is much greater even than this. In many cases, including the Crab nebula supernova, the stellar remnant left behind after the explosion is a NEUTRON STAR--a star only a few kilometers in diameter having an enormously large density and consisting mainly of neutrons--or a PULSAR, a pulsating neutron star. There are two common types of supernovas, called type I and type II. Type I occurs among old stars of small mass, whereas type II occurs among very young stars of large mass. It is not known how a small-mass star can release the very large amounts of energy needed to explain type I super ....