Pulsars

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Pulsars

The discovery of pulsars in 1967 was a complete surprise. Antony Hewish and his student Jocelyn Bell (later Bell Burnell) were operating a large radio antenna in Cambridge, England, when they detected a celestial source of radio waves that pulsed every 1.3373 seconds. Never before had a star or galaxy , or any other astronomical phenomenon, been observed to tick like a clock.

Hewish and Bell considered a number of exotic explanations for the speed and regularity of the pulsing radio source, including the possibility that it was a beacon from an extraterrestrial civilization. Within a few years, the correct explanation emerged, which is no less exotic. A pulsar is a city-sized spinning ball of ultradense material that emits beams of radiation, which flash Earth-like lighthouse beams, as it spins.

How Pulsars Are Created

Pulsars are produced when certain types of stars stop producing energy and collapse. The attractive force of gravity is always trying to contract the material of a star into an ever-smaller ball, but a star can maintain its size for billions of years because of the heat and pressure produced by nuclear reactions within it. When a star finally exhausts its supply of nuclear fuel, it collapses. An ordinary star (such as the Sun) will quietly contract into an Earth-sized glowing ember called a white dwarf. A more massive star will explode violently in an event called a supernova. It is within the detritus of such explosions that pulsars are born.

The reason for the explosion is that when the star collapses all the way down to a diameter of about 20 kilometers (12 miles), its atoms are packed so closely that their protons and electrons merge to form neutrons , which repel each other by nuclear forces and oppose further shrinkage. The collapsing material suddenly rebounds, producing a huge expanding fireball. In some cases, the neutron matter is obliterated by the blast, or it keeps shrinking all the way down to a single point, forming a black hole. Sometimes, however, the dense nugget of neutrons survives the explosion, in which case it becomes what is called a neutron star.

Pulsars and Neutron Stars

Neutron stars are unfathomably dense. A marble with the same density as a neutron star would weigh as much as a boulder 400 meters (0.25 mile) across. Because the rotation of the star is amplified during its collapse (much as an ice-skater spins faster by pulling in her arms), neutron stars are born spinning quickly, as fast as 100 revolutions per second. They also have the most intense magnetic fields known in the universe. If Earth had a magnetic field as strong, it would erase credit cards as far away as the Moon. This powerful magnetism causes intense beams of radio waves to be launched from both magnetic poles of at least some neutron stars; the poles swing around as the star rotates and may flash Earth if they happen to be oriented in the right direction.

In 1934, just two years after the discovery of the neutron, astronomers Walter Baade and Fritz Zwicky predicted that neutron stars should exist. Five years later, Robert Oppenheimer and George Volkoff published a detailed theory of neutron stars. But none of these scientists knew whether neutron stars could ever be observed with telescopes. They were expected to be so dim as to be invisible; nobody predicted they would emit focused beams of radiation. Even now that more than 1,500 pulsars have been discovered, nobody understands the details of how the radiation beams are produced.

Whatever the mechanism, a pulsar keeps pulsing for millions of years. Although the pulse rate is remarkably steady, it does slow down by a tiny but measurable amount. For example, the pulsar at the center of the Crab Nebula (the site of a supernova that occurred in 1054 C.E. in the constellation Taurus) blips once every 33 milliseconds, but this pulse period is slowing by 0.013 milliseconds every year. Eons from now, the pulsar will spin too slowly to produce radiation beams bright enough to observe, and will spend the rest of eternity as a quiet neutron star.

X-ray Pulsars

A neutron star can be rescued from this oblivion, and resume its identity as a pulsar, if it happens to have a companion star. Stars are often found in pairs (or even triplets or quadruplets) and when one star explodes in a supernova, the other may survive. Eventually the intense gravity of the pulsar may rip material away from the giant star. As the material swirls down to the pulsar's surface, it heats up to millions of degrees and glows brightly in X rays . The swirling matter may be funneled by the neutron star's magnetic field onto a hot spot on the neutron star's surface; as this spot rotates with the neutron star, astronomers see pulses of X rays, and the neutron star regains the limelight as an "X-ray pulsar."

Millisecond Pulsars

It is also possible that the swirling matter will cause the neutron star to spin faster and faster, like a top being spun up. The rotation period can become as short as a few thousandths of a second, which is enough to reactivate the radio pulses, and the neutron star is reborn as a "millisecond pulsar." The fastest known millisecond pulsar spins 642 times per second, which is impressive for something bigger than London and more massive than the Sun.

Areas of Future Research

Despite all of this knowledge, the life cycles of pulsars are still a subject of research. Many of the unanswered questions are about young pulsars: How often do supernovas produce them? Can they be created in other ways? Are they always born spinning quickly?

In particular, due to recent advances in X-ray astronomy, a new category of young pulsars has been discovered consisting of objects that spin relatively slowly and emit X rays rather than radio waves, even though they do not have a stellar companion. A consensus is developing that these unusual pulsars should be called "magnetars," because they seem to have magnetic fields hundreds of times larger than the already enormous fields of "ordinary" pulsars. If proven to be accurate, this would be yet another surprising development in the history of pulsar science.

see also Astronomy, Kinds of (volume 2); Black Holes (volume 2); Einstein, Albert (volume 2); Gravity (volume 2); Stars (volume 2); Supernova (volume 2).

Joshua N. Winn