A pulsar on the move: astronomers resolve a long-standing mystery—only to reveal a new puzzle

Natural History, June, 2002 by Charles Liu

About once a century, somewhere in our galaxy, a star destroys itself in an explosion called a supernova. Within ten seconds, this titanic blast releases more energy than our Sun will produce in its entire 10-billion-year lifetime. Then, during the next 100,000 or more years, a glowing, gaseous remnant of the explosion expands outward from the supernova site at hundreds of miles per second, scattering the heavy elements that seed the birth of new stars and planets. The core of the exploded star, compacted by its own gravity, collapses into a superdense ball called a neutron star, ten miles across and 100 trillion times denser than lead. If the neutron star spins with a favorable orientation, it will send regular pulses of electromagnetic radiation toward Earth. We call this kind of star a pulsar.

You might expect to find a neutron star at the center of every supernova remnant. One of these, the Crab Nebula, has a pulsar at its center (spinning thirty-three times a second) and is often cited as a typical remnant, yet the vast majority either have a neutron star off-center or just don't seem to have one at all.

Astronomers are never short on ideas about how the universe works, so we of course have a plausible explanation for this apparent contradiction. Supercomputer simulations of supernovas almost never produce a perfectly symmetrical explosion; factors such as stellar spin or off-center blast waves can impart a fraction more energy on one side than on the other. The imbalance propels the neutron star away from the center of the maelstrom, pushing it outward at tremendous speeds. Within a few thousand years, it is far from the middle of the spidery supernova remnant.

Solid observational evidence now supports this theory. A team of astronomers, led by Joshua Migliazzo at the Massachusetts Institute of Technology and Bryan Gaensler at Harvard University, has just published measurements of the motion of a pulsar dubbed B1951 32, lying near one edge of a supernova remnant with an equally cryptic name, CTB 80. Using data gathered over twelve years with the Very Large Array radio telescope in New Mexico, Migliazzo and colleagues showed that the pulsar is moving across the sky, away from the center of the remnant, at a blistering pace of 0.025 arc seconds per year. Standing on Earth while watching this pulsar move across the sky is similar to standing in New York City while watching a snail in San Francisco crawl one-sixteenth of an inch per day. However, at the remnant's distance from Earth--about 7,000 light-years, or some 40,000 trillion miles--0.025 arc seconds per year translates to a minimum velocity of 550,000 miles per hour. At that speed, an astronaut could get to the Moon and back in less than an hour.

Unsatisfied with merely making a first-of-its-kind measurement, the researchers went one step further. Tracing the pulsar's trajectory backward to the center of the supernova remnant, they computed how much time its journey has taken. The dying star that birthed B1951 32 almost certainly exploded at that central location, so the question they've really asked is, How old is the pulsar? Their answer: 64,000 years, plus or minus eighteen millennia.

In human terms, this pulsar is an ancient artifact, born long before our ancestors began painting animals on cave walls. Astronomically speaking, though, it's young--too young, in fact, according to the most commonly accepted method of measuring the age of a pulsar. Under the conventional formula, this pulsar's age should be 107,000 years.

Migliazzo and his colleagues offer a simple explanation for the discrepancy. The standard formula for calculating age assumes that, as a result of supernova collapse, pulsars are born whirling at extremely high speeds--a hundred or more revolutions per second--and then slow down over time. The researchers state that if B1951 32 began spinning at thirty-seven revolutions per second, then both the standard and the new Calculations would match at 64,000 years.

That sounds like an ideal solution --except that such a conclusion unseats a long-held tenet of pulsar physics: that all neutron stars begin their lives at breakneck rates of spin. Is our theoretical understanding of pulsar formation seriously flawed? Pulsar B1951 32 may now have provided the strongest challenge yet to the conventional wisdom.

The richness of Migliazzo and Gaensler's discovery shows how much we can learn from the study of traveling pulsars. Sadly, only a fraction of the thousand or so known pulsars can be analyzed this way. Many creep along so slowly that measuring their motion with current technology is a hopeless task. Others simply don't have a clear association with a supernova remnant, so we can only guess where they came from. But it's nice to know that, at least in this case, theoretical and circumstantial evidence have come together with reliable observational data to settle a long-standing puzzle--and stir up fresh debate about the death of stars.

Charles Liu is an astrophysicist with the Hayden Planetarium. He is also affiliated with Barnard College as a research scientist in the Department of Physics and Astronomy.