The Case of the Missing Neutron Stars

Why some stars can end their lives as both a supernova and a black hole.

By Tim Folger|Wednesday, December 01, 1993
RELATED TAGS: STARS
Where have all the compact, superdense remains of stupendous stellar explosions gone? Down cosmic toilets, every one.

For a few nights in February 1987, a supernova flared into view in Southern Hemisphere skies. This supernova, now known as 1987A, was the first visible to the naked eye since 1604. That made astronomers extremely happy. The supernova’s aftermath, on the other hand, has left them extremely puzzled. A stellar explosion like 1987A was supposed to leave behind a neutron star, the small, dense remnant of the exploded star’s core. And the neutron star was supposed to announce its existence through its powerful magnetic field, which would heat the stellar debris to a telltale glow by slinging charged particles into it. But nearly seven years after the explosion, the glow has yet to be seen.

It’s become sort of an embarrassment for astronomers, says Gerald Brown, a physicist at the State University of New York at Stony Brook. The problem is not just the 1987 supernova; over the years astronomers have spotted the dust-cloud remains of some 150 supernova explosions in our galaxy, but they’ve seen neutron stars in only 20 or so of those clouds. Brown and the 87-year-old Nobel laureate Hans Bethe of Cornell now think they can explain why so many neutron stars are missing. They say the supernovas gave rise to black holes instead.

If Brown and Bethe are right, it would be a major departure from the conventional wisdom on how stars evolve. The standard theory (of which Bethe was a chief architect) holds that when a star of between 8 and 30 times the sun’s mass exhausts the nuclear fuel in its core, outgoing radiation no longer props up the star against its own gravity. The core caves in until it becomes so dense that electrons and protons get squeezed together to form neutrons. At that point the core collapse suddenly stops. The outer layers of the star, though, continue to implode, and ultimately they bounce off the hard neutron core. Within seconds the resulting shock wave blows them out into space, creating a visible supernova and leaving the naked neutron star behind.

The death of stars weighing more than 30 suns is quieter in the standard theory. Their cores are so massive that they continue to collapse past the neutron-star phase. Since there is no hard surface for the outer layers of the star to bounce off, there is no explosion. Instead the entire star collapses toward a single, infinitely dense point: a black hole.

The standard theory thus allows a star to produce either a supernova or a black hole but not both. That is where Brown and Bethe beg to differ. According to their new model, a star of intermediate mass, between 18 and 30 times heavier than the sun, explodes as a supernova and then hides its core in a black hole.

The star’s death begins with a core collapse, just as it does in the standard theory. But Brown and Bethe argue that the ultrahigh density inside the imploding core can transform the electrons into negatively charged particles called kaons, which protons don’t absorb. This happens before the electrons get a chance to merge with protons to produce neutrons. So instead of a pure neutron core, the collapsing star’s heart contains a mix of neutrons, protons, and kaons. You get nearly equal amounts of neutrons and protons, says Brown. We call it a nucleon star.

This proton-neutron mix is more readily compressed into a black hole because the strong force that binds nuclear particles is stronger between protons and neutrons than it is between neutrons alone. The black hole doesn’t form immediately, however (as it does in the case of much larger stars that don’t produce supernovas). For a few seconds the proton- neutron core teeters on the brink of further collapse. A flood of massless particles called neutrinos, created when the electrons were crunched into kaons, briefly heats the core, stabilizing it. Then the neutrino flood ebbs, and the core sinks into the oblivion of a black hole, vanishing forever from the visible universe.

But those few seconds of hesitation provide enough time for the infalling outer layers of the star to bounce off the core and generate a supernova that leaves no neutron star. According to Brown and Bethe, that is why so many neutron stars are missing from supernova remnants--and why half a billion small black holes may lurk, massive and unseen, in the uncharted recesses of our galaxy.
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