cal calculations but also to the validity of the assumption that the same physical laws found on the earth apply to distant parts of the universe.

White dwarfs are the corpses of stars whose initial mass was less than about eight times the mass of our sun. More massive stars that have exhausted their nuclear fuel face a different end. For such stars, no amount of resistive pressure can stave off the overwhelming crush of gravity. These stars will explode in a brilliant supernova, which for a brief time can shine with the power of 100 billion stars. The core remaining after the explosion will become a neutron star, or, if the core is massive enough, an object called a black hole, whose gravity is so intense that not even a ray of light can escape it. The intense bursts of radiation given off by magnetized, rapidly rotating neutron stars are often detected by radio telescopes as pulsars. Stellar black holes are inferred to exist from the x-ray emission seen from some binary stars.

In early 1987, astronomers were handed a rare opportunity to test their theories of the evolution, collapse, and explosion of stars. A star exploded nearby, without warning, offering an unprecedented view of a supernova. By carefully monitoring the light from Supernova 1987A, as it is called, and by identifying in older photographs the star that blew up, astronomers have learned a great deal about the origin and nature of supernovae. The event confirmed the general theoretical outlines. The infrared and gamma rays from the radioactive decay of cobalt, and the amount of nickel and other elements ejected by the explosion, could all be understood. Also detected from Supernova 1987A were neutrinos, whose properties are not completely known. According to previous theories, neutrinos should be manufactured in great numbers during the formation of a neutron star. The neutrinos detected on the earth from Supernova 1987A not only confirmed the predicted temperatures and densities inside a supernova, but they also allowed physicists to learn more about the neutrino.

Supernovae play a vital role in the life cycle of stars. The debris from stellar explosions spreads out into space and adds new ingredients to the gas between stars from which new stars form. Thus supernovae are beginnings as well as ends. Theoretical calculations suggest that essentially all of the chemical elements except hydrogen and helium, the two lightest elements, were manufactured by nuclear fusion inside stars. The vast majority of the 100 chemical elements, including oxygen and carbon and other elements that earthly life depends on, were synthesized in stars and blown into space. Some of this seeding occurs during the red giant phase, as a star sheds its surface layers, and some of it occurs in the wind of particles that flow from the hot stellar atmosphere. The rest happens in supernova explosions. Later generations of stars, such as our sun, are born from the gas enriched by these new elements. The gas between stars connects the generations, receiving from the old stars and giving to the new.



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