Core mystery.

In 1987, a supernova visible to the naked eye erupted in the nearby Magellanic Cloud galaxy. Astronomers hadn't witnessed such a brilliant stellar explosion since 1604. Known as supernova 1987A, the explosion remains the brightest known supernova of the last 4 centuries.

In at least one respect, it also remains one of the most puzzling. Several features indicate that 1987A is a core-collapse supernova, in which gravity crunches a massive star's core down to a sphere only 20 kilometers in diameter. In core-collapse supernovas, this core, which is called a neutron star, then rapidly generates a stream of subatomic particles that blows away the star's outer layers.

For about the past 18 years, however, astronomers have searched in vain for 1987A's collapsed core. New images provided by the Hubble Space Telescope only add to the puzzle. The photos, which are the sharpest ever taken of the supernova, show no evidence of a neutron star, astronomers report in an upcoming Astrophysical Journal.

There may be ways out of the conundrum, says study coauthor Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. If a neutron star isn't pulling in material from its surroundings, it can't be seen, he notes. That could be the case for 1987A. At the other extreme, the missing neutron star could have packed on so much material that it collapsed into a black hole. Kirshner notes, however, that astronomers have so far seen no sign that a black hole has formed.--R.C.