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8. SUMMARY

There are three kinds of stellar explosions: novae, thermonuclear supernovae, and core collapse supernovae. During a nova explosion, a star's luminosity suddenly increases to about 100,000 times the Sun, then fades after a few weeks or months. During a supernova explosion, a star's luminosity suddenly increases to about a billion times the Sun, then fades after several months. Novae and supernovae are sometimes very bright, and have been observed since prehistoric times. About nine supernovae have been seen during the past 2,000 years. We can see their remnants today. Roughly ten times as many have actually occurred in the Milky Way, but most have been invisible to the naked eye because they lie behind interstellar dust clouds.

Both novae and thermonuclear supernovae occur as a result of mass transfer from a companion star to a white dwarf in a binary system. Novae are explosions of a layer of hydrogen that has been deposited on the surface of the white dwarf. The white dwarf star itself does not explode. The mass transfer may resume after the explosion and the nova may repeat after many centuries. In a thermonuclear supernova, the surface explosion is powerful enough to ignite a thermonuclear explosion of the carbon/oxygen core of the white dwarf, and the entire star blows apart. Astronomers are not sure of the circumstances that make the entire star explode rather than just the surface.

Through many experiments with "atom smasher" machines on Earth, physicists can determine the rates of nuclear fusion reactions at temperatures approaching a billion degrees. By solving equations incorporating these rates on large computers, they have shown that supernova explosions will produce heavy elements in the relative proportions we see in the universe and on Earth, confirming the notion that supernova explosions are responsible for the presence of these elements in the cosmos.

A core collapse supernova is the final fate of a star more massive than 8 times the Sun. Through successive stages of nuclear burning, such a star will evolve to form an iron core that is more massive than 1.4 Suns (the Chandrasekhar limit). Then the core collapses. The collapse suddenly halts when its diameter is about 20 km and a neutron star is formed. Most of the collapse energy is released as a burst of neutrinos, but about 1% of this energy is deposited in the infalling envelope of the star, reversing its direction in a tremendous explosion.

Supernova 1987A (February 23, 1987), is the brightest supernova to be seen since Kepler's supernova (1604 AD). It occurred in the Large Magellanic Cloud, a nearby galaxy (160,000 light years away). Astronomers saw the burst of neutrinos in underground detectors, just as expected. They also saw gamma rays from newly formed radioactive elements. The rings seen around SN1987A by the Hubble Space Telescope were a big surprise and their origin is still a mystery. We suspect that the progenitor star of SN1987A was a binary star system that merged some 20,000 years before it exploded, ejecting the rings during the merger. The blast wave from the supernova explosion is just now beginning to hit the ring, causing a bright spot to appear. During the next ten years, the ring should become several hundred times brighter than it is today, giving us an opportunity to understand the mechanism by which the rings were ejected.

Supernova remnants are giant shells expanding into interstellar space, caused by supernova explosions. They remain visible for thousands of years, especially at radio and X-ray wavelengths, and astronomers have discovered hundreds of them in the Milky Way. Clusters of massive stars will produce several supernovae over a period of a few million years, and their combined action can make an even bigger "supershell" of expanding gas that can actually trigger the formation of new generations of stars.

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