When massive stars die, they may leave behind objects that are more unusual and exotic than previously imagined, says Victoria M. Kaspi, assistant professor of physics in the Center for Space Research. For one thing, the pulsar -- a type of neutron star that has long been thought to be the standard product of such stellar deaths -- may be merely one species in a larger zoo.
"There's increasing observational evidence that young neutron stars are not as simple as everyone thought," Professor Kaspi said during an invited talk titled "The Neutron Star/Supernova Remnant Connection" at the annual meeting of the American Astronomical Society on January 8.
By taking a broad look at diverse types of recent observations, she and other astronomers are starting to unify these observations into one cohesive explanation of what happens when a massive star dies.
Among the alternatives to pulsars, Professor Kaspi said, are unusual celestial X-ray sources such as anomalous X-ray pulsars, the unpredictable soft gamma-ray repeaters, and magnetars -- ultra-magnetized neutron stars hypothesized to explain these phenomena. Also in the family may be quiet, isolated, cooling neutron stars that, for unknown reasons, never "turned on" to radiate like their cousins.
Considered some of the most exotic objects in space, pulsars are believed to be spinning neutron stars that have jets of particles streaming at the speed of light out of their two magnetic poles. These jets produce a powerful beam of light that sweeps around like a beam from a lighthouse as the pulsar rotates. If the pulsar is oriented just the right way, we see it turn on and off as the beam sweeps over the Earth.
Pulsars, only 20 kilometers across and extremely dense, are like rapidly spinning magnets. Their incredibly strong magnetic fields accelerate particles around them to tremendous energies. Pulsars are most easily observed using radio telescopes, because, although no one knows why, their emission is brightest in radio waves.
Professor Kaspi is most interested in young pulsars, mere babies that are tens of thousands -- as opposed to millions -- of years old. She is involved in numerous efforts to search for young pulsars in the Milky Way. This is no simple task when, of the more than 1,000 known pulsars, only a couple of dozen are fewer than 100,000 years old.
A veritable newborn is the famous Crab pulsar. As recorded by ancient Chinese, Korean and Japanese civilizations in 1054 AD, a star's last gasp created an explosion of heat and light so intense that it was clearly visible from Earth in the daytime. Such an explosion is called a supernova.
Today, the remains of that supernova, in the form of a rapidly expanding nebula of gas, has been dubbed "the Crab" because gaseous filaments resemble, to some, a crab's legs. It is among the best-studied objects in the heavens.
A rapidly spinning pulsar, the Crab pulsar, lives at the Crab nebula center. The Crab pulsar rotates some 30 times per second.
Unlike the Crab pulsar, which is clearly linked by birth to the Crab nebula, not all young pulsars can be associated with certainty to a specific nebula, Professor Kaspi said.
Even if a pulsar is in the vicinity of a nebulous supernova remnant, its birthplace may not have been the remnant's center. When pulsars are born, they sometimes shoot off at a tremendously high velocity, so they may be propelled far from the site of their birth.
"Pulsars are among the fastest objects in the galaxy. They move much faster than other stars," Professor Kaspi said. In fact, the highest speed pulsars are moving so fast that they will ultimately escape the confines of the Milky Way itself.
THE BIRTH OF NEUTRON STARS
Stars that are between five and 15 times as massive as our Sun end their lives in cataclysmic explosions called supernovae. Supernovae provide one of the most spectacular explosions in nature, equivalent to a 1,000-megaton bomb.
A supernova occurs when fuel for fusion in the star's core runs out. The star can no longer create outward pressure to combat the inward gravitational pull of its great mass, so the core begins shrinking, growing hotter and denser in the process.
When only iron is left in the core, it has nothing left to fuse and the star begins the final phase of gravitational collapse. The core temperature shoots up to more than 100 billion degrees as the iron atoms are crushed together.
At the same time, the nuclei repel each other so forcefully that they overcome gravity and the core bursts from the heart of the star in an explosive shock wave, propelling matter into space. The material that is exploded away from the star is known as a supernova remnant. (Stars with more than 15 times the Sun's mass have so much inward gravitational pull that the neutrons can't survive the collapse of the core. These end up as black holes.)
All that remains of the original star is a small, super-dense core composed almost entirely of neutrons -- a neutron star. Until recently, discovering radio pulsations was the main way to find newly formed neutron stars.
WHERE ARE THE MISSING PULSARS?
There are more than 200 known remnants of supernova explosions, but very few of these seem to have left behind radio pulsars. Professor Kaspi said that according to current theories, most of them should have.
Astronomers are now finding evidence that young neutron stars need not only be radio pulsars.
For example, some of the missing pulsars might be isolated, cooling neutron stars that are not pulsars because they are not "turned on," Professor Kaspi said. These neutron stars have only recently begun to be noticed and studied because they are invisible at practically all parts of the electromagnetic spectrum except X-rays. This is because their surfaces are extremely hot -- some millions of degrees -- so that they glow in X-rays, much in the way our own Sun's surface temperature (a relatively chilly 6,000 degrees) causes it to glow in visible light.
X-ray astronomy, one of the most active areas of research today, promises to hold the key to identifying otherwise invisible isolated neutron stars and possibly find some of the missing pulsars.
In fact, recent observations, also in the X-ray regime, are showing that the young neutron star population may include objects even more exotic than regular radio pulsars and the quiet, isolated neutron stars.
Another class of what were thought to be unrelated objects are X-ray pulsars that spin relatively slowly, once every five to 10 seconds compared with more than 10 times a second for their speedier cousins. These have been dubbed "anomalous X-ray pulsars" and may beyoung neutron stars as well, possibly accounting for some of the missing pulsars.
The anomalous X-ray pulsars have long been thought to be binary stars -- the X-rays emerging from matter falling onto a neutron star, one member of the binary. But a perplexing problem with this explanation has been the absence of any evidence for the other member of the binary.
Recent X-ray observations appear to have resolved this dilemma by showing that these objects can be found in supernova remnants. This suggests that the anomalous X-ray pulsars may be young, isolated neutron stars.
"The existence of these slowly rotating X-ray pulsars, if they are young neutron stars, is a big surprise, given all that we thought we knew about pulsars," Professor Kaspi said. "Their slow spin rate is a challenge to models of how pulsars spin."
Their strange properties can be explained if these objects have magnetic fields 10 to 100 times higher than those of regular pulsars. "This is how the term 'magnetar' recently came into astronomy lingo," she said. "If they really are magnetars, there are clear predictions for how their spin rates should evolve over time. At MIT, we are currently monitoring them using the Rossi X-ray Timing Explorer Satellite to see if these predictions hold water."
Additional evidence in favor of magnetars is that another seemingly unrelated class of objects, so-called soft gamma-ray repeaters, were already proposed to be magnetars. They have recently been shown to exhibit slow X-ray pulsations like the anomalous X-ray pulsars.
The magnetar and isolated cooling neutron star theories may soon be proved or disproved by the massive amounts of data streaming in from increasingly sophisticated X-ray detectors.
Although the heavens seem to be a puzzle filled with missing pieces that may never be found, Professor Kaspi said that every time a clue appears, "it's great. With the new X-ray satellites being launched, this is a very exciting time for astronomers. If there ever was a time when these basic questions can get answered, it is now."
A version of this article appeared in MIT Tech Talk on January 13, 1999.