The University Record, November 19, 1997

Supernova in Cassiopeia celebrates 425th anniversary

This was the first "nearby" supernova in the last 3 centuries, and for the first time astronomers not only observed the light show, but also detected 19 of the elusive neutrinos (the detectors observed electron anti-neutrinos, to be more precise) produced by the collapse of the star's core. The burst of neutrinos preceded the first sighting of the supernova's light by about 3 hours, in agreement with the expectations of current supernova theory. It is estimated that for an instant in 1987 on the earth the neutrino luminosity of SN1987A was as large as the visible-light luminosity of the entire universe. The adjacent figure is a 1994 Hubble Space Telescope image of the region surrounding SN1987A. The supernova is in the center. The two bright stars are just in the field of view and are not associated with the supernova. The bright yellow ring is thought to be gas and dust heated by the supernova (the expanding shell of the explosion itself that will produce the supernova remnant is still too small to be seen in this photograph). The two large rings are not yet completely understood, though they appear to be associated with the supernova.

A supernova, the explosion of an entire star, is one of nature's most powerful phenomena. At its brightest, a supernova can outshine an entire galaxy of stars and can be seen at very great distances across the cosmos.

"Supernovae are also rare," says astronomer Richard Teske. "The last time one was detected in our home galaxy was hundreds of years ago. This month marks the 425th anniversary of a supernova observed in the W-shaped constellation of Cassiopeia. On November evenings in Michigan, the site of this historic cosmic blast lies directly overhead."

The middle hump of Cassiopeia's "W" bulges northward toward the North Star. In November 1572, a supernova appeared adjacent to the fainter star seen just northwest of the middle of the "W."

"For two weeks it was brighter than any other star in the sky, was visible in daylight and may even have cast faint shadows upon early season snows," Teske says. "After the supernova began to fade, it remained visible for another 16 months. Sixteenth-century observers left us a good record of its appearance and brightness changes."

Cassiopeia's supernova was caused by the thermonuclear detonation of an entire compact star. "The pre-explosion star, called a white dwarf, contains as much material as our sun does, but it has shrunk to the size of the Earth. It is made almost entirely of the chemical elements carbon, nitrogen and oxygen," Teske explains. "The explosion takes place when rapid thermonuclear reaction of these elements begins. It then becomes a huge thermonuclear bomb."

An explosion is just the rapid release of great amounts of energy, according to Teske. In dynamite, the chemical reaction that liberates energy is initiated by a detonator at one end of the dynamite stick. The reaction travels through the stick at a speed of many miles per second, ending a tiny fraction of a second later at the other end.

"Within a compact pre-supernova star, the thermonuclear reactions begin somewhere-perhaps at the center, although nobody knows for sure. The reactions propagate at many hundreds of miles per second, generating so much energy that the star flies apart in an expanding fireball. Within a minute the entire store of carbon, nitrogen and oxygen has been converted mostly into radioactive nickel along with other elements. The atomic debris is exceedingly hot and bright at first. It glows less and less as it expands, cools and radioactively decays."

Every supernova explosion of this kind has just about the same maximum brightness, no matter where it takes place in the universe. This property, together with the great brilliance that makes supernovae visible at vast distances, endows them with astronomical importance, since they can be used to measure the remoteness of the farthest galaxies in which they are faintly detected. When the distance and brightness of one is known, a measurement of the brightness of the other reveals its distance, too.

Astronomers are hunting for far-off supernovae and are crafting a "cosmic yardstick" based upon their observations of them, according to Teske. So far the record holder for distance is a supernova detected nine billion light-years away. It exploded when the universe was only one-third of its present age and just half as large as it is now.

Astronomers expect that accurately gauging such extreme distances with supernova observations will eventually reveal the curvature of space itself. The theory of general relativity, as well as other theories, suggests that the space of our three-dimensional universe might be "curved" in a way very different from people's everyday conceptions of it.

"One of the goals of the scientists trying to find remote supernovae and determine their distances is to decide whether the geometric fabric of the universe is curved and by how much," Teske says. "The solution to this quest will also tell us whether or not the universe will continue expanding forever."