(8 of 12)
But not for long. The instant the remaining silicon in the core is fused into iron, the thermonuclear reactions stop. Without enough radiation pressure to sustain it, the now all-iron core, hidden under the star's outer layers, begins its final, catastrophic collapse. In the incredibly short time of just 1 second, according to University of Arizona Astrophysicist Adam Burrows, the core is compressed to more than the density of an atomic nucleus. "It's as if the earth had suddenly collapsed to the size of New York City," says Burrows. "At this point the rest of the star is oblivious. It doesn't know the core has collapsed and that it's doomed."
Now it is not just the atoms that are touching, as in a white dwarf, but their nuclei. Under the immense pressure, the electrons, no longer able to repel one another, are squeezed into the nuclei, which ordinarily contain just protons and neutrons. In about a thousandth of a second, the negatively charged electrons combine with positively charged protons to form additional neutrons; the process also produces the ethereal neutrinos, which effortlessly zip through the star's outer layers and into space. Under these circumstances, there is a limit to how much the neutrons can be compressed. As gravity tightens its grip further, the neutrons, in what Hans Bethe, Cornell University's Nobel laureate physicist, has called the "moment of maximum scrunch," recoil ferociously.
The resulting shock waves spread outward through the core, enter the star's still unsuspecting outer layers, and hours later reach the surface, spewing the star's laboriously made elements into space in a mammoth explosion. All that is left behind is the neutron core, the strange entity that astronomers call a neutron star.
There is another possible scenario: if a star is a minimum of 30 to 40 times as massive as the sun, its gravitational collapse could be so violent that it may never become a supernova at all. Instead of bouncing back at the instant of maximum scrunch, the core continues its collapse indefinitely, forming a bizarre object of infinitesimal size and nearly infinite density, with a gravitational field so intense that light itself cannot escape -- a black hole. In effect, the entire, tremendous mass of the star has gone down a cosmic drain.
These are the theoretical scenarios. And at first 1987A seemed to be following the rules: it jumped from near invisibility to respectable brightness literally overnight, and while its wave-front speed was high, its spectrum revealed the unmistakable hydrogen-bearing signature of a Type II. But when the International Ultraviolet Explorer satellite reported a rapid drop in ultraviolet light, scientists began to wonder. Says Robert Kirshner, of the Harvard-Smithsonian Center for Astrophysics: "The spectrum we're seeing in the ultraviolet resembles the spectrum of a Type I. That's a puzzle." Admits Texas' Wheeler: "There are some funny features in this supernova."