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supergiant stars, neutron star, Jocelyn Bell, supernova explosion, planetary nebula

Pulsar, sources of powerful, pulsating radio waves in space, believed to be neutron stars—the dense, rapidly spinning remains of burnt-out supergiant stars. More than 500 pulsars have been identified in the earth’s galaxy, the Milky Way.

Most of the identified pulsars in the Milky Way are concentrated in the outer spiral arms of the galaxy’s disk, where supernovas—giant explosions that can be seen over vast distances in space—are also most common. Astronomers believe that supernovas occur when supergiant stars—massive, bright stars at advanced stages of evolution—collapse under their own weight following the exhaustion of their nuclear fuels. According to currently accepted theories, the supergiant’s core collapses to form a neutron star—a small, extremely dense ball of rapidly spinning matter that traps and compresses much of the star’s mass and magnetic field. Astronomers believe that the intense magnetic field of a rapidly spinning neutron star causes the emission of powerful radio waves that are detected on the earth as pulsars.

In general, a star rotates about its own axis in a manner similar to the earth spinning on its axis. When the core of a supergiant star collapses following the exhaustion of its fuels, material that was distant from the center moves in closer, causing the star to spin faster and faster. This effect results from the conservation of angular momentum, and it is similar to a spinning ice-skater pulling in his or her arms in order to spin faster. In the formation of a neutron star, a mass of material equal to 1.4 to 3 times the mass of the earth’s sun compresses under its own gravitational forces into a volume measuring about 20 km (about 12 mi) in diameter, or less. As it condenses, the neutron star picks up rotational speed until what was originally a large, spherical mass rotating with a period of days or weeks becomes a small, superdense flattened ball spinning up to several hundred times per second.

Although astronomers were able to predict the existence of neutron stars in the 1930s, they did not attempt to find one for several decades because the star’s properties do not make it visible over the vast distances of space, even to the best of telescopes. However, in 1967 astronomers realized that as the matter in the spinning core condenses, the star’s magnetic field also condenses until the magnetic field at its poles is 1 trillion times greater than the magnetic field of the earth. They realized that such a rapidly spinning object with such a strong magnetic field should emit an intense beam of radio waves that would sweep out a conical shape with each rotation of the star. An observer positioned within the sweep of the beam should observe a rapidly pulsating beam of radio waves. These hypothetical pulsating radio sources were named pulsars. Within months, in late 1967, a graduate student named Jocelyn Bell and her adviser Antony Hewish, an astronomer at the Mullard Radio Astronomy Observatory in Cambridge, England, discovered a pulsating source of strong radio waves. This first identified pulsar is known as PSR 1919+21.

Astronomers theorized that the formation of a neutron star and, hence, a pulsar should be accompanied by a supernova explosion. It was strongly suspected that the Crab Nebula was the remnant of a supernova explosion that Chinese astronomers observed and recorded in ad 1064. Radio astronomers soon verified that a pulsar resides almost at the exact center of the Crab Nebula, confirming the theoretical connection between supergiant stars, neutron stars, supernovas, and pulsars. Although pulsars are known primarily as radio wave emitters, they can emit electromagnetic radiation in all regions of the spectrum, including visible light. The Crab Nebula pulsar is not only the most well-known radio pulsar, but for many years it was also the only known pulsar that emitted pulsed visible light.

Pulsars emit radiation periodically, that is, at regular intervals, with the time between pulses ranging from about 4 seconds to about 1 millisecond. Astronomers have measured the regularity of pulsar emissions to be at least within 1 part in 10 billion, making pulsars one of the most accurate timekeeping systems known. However, pulsars do not rotate at a constant rate, but gradually slow down as they lose energy by radiation. The intensity of visible light and gamma radiation that they emit also decreases as they lose energy. Astronomers have calculated the ages of many pulsars from their current rates of rotation and slowing. Judging by these rates, most existing pulsars appear to be on the order of 1 million years old or older, which is confirmed by the fact that few show any evidence of a planetary nebula or other sign of a violent explosion. The newly formed Crab Nebula pulsar is a notable exception.


Mammana, Dennis L., B.S., M.S.

Resident Astronomer, Reuben H. Fleet Science Center. Author of "Other Suns. Other Worlds?", "The Night Sky: An Observer's Guide", and "Star Hunters".

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