ν and You

When a really heavy star (eight or more times the mass of the Sun) runs out of gas, literally, its core collapses and a so-called Type II supernova is born. Within a second, its Earth-size core crumples into a ball — a neutron star or black hole — whose density is akin to that of an atomic nucleus (a million billion kilograms per cubic centimeter). As gravity forces electrons and protons to coalesce and form neutrons, a "gazillion" ghostlike neutrinos are instantly set free to roam the universe, perhaps for eternity. (Neutrinos are chargeless, possibly massless elementary particles.)

Just after the neutrinos begin to zing merrily through space at (or very near) the speed of light, the star's core stops collapsing. Then it rebounds, causing a shock wave to travel out toward the star's surface, which doesn't have a clue about the oncoming disaster (S&T: August 1995, page 30). If the overweight star is a red supergiant (like Betelgeuse) with a hydrogen-rich envelope, nearly a day will elapse between the collapse-induced neutrino emission and the beginning of the supernova light show.

Except for SN 1987A in the Large Magellanic Cloud, no star has been observed before it blew up. As bad luck would have it, SN 1987A's progenitor (called Sanduleak –69°202) was an oddball for a Type II supernova; it was a blue (not red) supergiant and relatively lightweight (six solar masses instead of eight or more). We will probably not see another one like it "for centuries," says Stanford Woosley (University of California, Santa Cruz).

Yet SN 1987A will be remembered for producing the first supernova-spawned neutrino burst detected on Earth, though the event was recognized only after the supernova was seen shining in the sky. Now, more than a decade later, we are armed with hindsight as well as with better and more abundant neutrino detectors. Some even have cute names like Super-K, SNO, MACRO, and AMANDA.