But very red! he wrote in an email. I still remember that exclamation point in the Large Magellanic Cloud.

According to astronomers, there are three possible fates for a star that has run out of fuel and died. It can end up as a hot dense cinder called a white dwarf, as an even hotter and denser neutron star or as a black hole, depending on its initial mass and other details of its composition.

The star that exploded was subsequently identified as a giant blue star known as Sanduleak -69 202, which promptly vanished from the sky. In its prime it was about 19 times as massive as the sun, which puts it in the range that astronomers think should produce a neutron star.

Reinforcing that conviction was the subsequent discovery that two or three hours before the supernova was discovered, a pulse of two dozen lightweight subatomic particles called neutrinos had splashed into particle detectors on Earth. Messengers from the inside of the inferno, they had outraced the visible light in escaping the collapsing star.

Neutrinos are indeed key to the supernova and neutron star process, Dr. Burrows said.

As a massive star like this one undergoes its thermonuclear immolation, he noted, it develops onionskin layers of helium, oxygen, carbon and other newly minted elements. At the center is a growing core of iron, the most stable element. When it reaches a limit called the Chandrasekhar limit, at which atomic forces can no longer support its weight, it implodes and then rebounds, leaving behind a hot, dense neutron star.

A shock wave ripples out through the onion layers. Accompanying it, and powering it by absorptive heating, are copious quantities of neutrinos, created from the energy of the collapse. Indeed, as much as 99 percent of the energy of a supernova goes into these particles and out into the cosmos.

Neutrinos are famous for their spooky ability to pass through solid lead like moonlight through glass, but even neutrinos have trouble escaping the core of a dense proto-neutron star. It is the energy supplied by neutrinos, astronomers think, that provides the oomph to blow the star apart. If the neutrinos cannot emerge fast enough to heat an explosion, the supernova is likely to fizzle and the newly-birthed neutron star will collapse into a black hole, Dr. Burrows said.

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A Star Went Supernova in 1987. Where Is It Now? - The New York Times