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Neutron Star
I INTRODUCTION
Neutron Star, rapidly spinning, extremely dense astronomical object. Neutron stars are composed primarily of neutrons, minute, neutrally charged particles that exist in the nuclei of atoms. A neutron star is created when the core of a supergiant star—a massive star that has evolved so that it burns heavy elements instead of hydrogen—has converted all of the material in its core to iron. At this stage, no further nuclear reactions can take place to liberate energy, and so the core collapses under the mutual gravitational attraction of its own matter (see Gravitational Collapse).
II PREDICTION OF NEUTRON STARS
Neutron stars were first predicted by Indian physicist Subrahmanyan Chandrasekhar and others in the 1930s. These theorists predicted that when a massive supergiant star exhausts the nuclear fuel in its core, the core will collapse and condense under gravitational forces. If the mass of the core exceeds about 1.4 times the full mass of the sun—a value known as the Chandrasekhar limit—the core will collapse with such force that the positively-charged protons and negatively-charged electrons of the core will be crushed together to form electrically neutral neutrons. The resulting theoretical body was called a neutron star.
The rotational speed of a collapsing supergiant core increases for the same reason that the rotational speeds of spinning ice skaters increase when they pull in their arms—the conservation of angular momentum. As material that was more distant from the center of the star moves in closer, its rotational speed must increase to compensate and conserve angular momentum. A star that originally required days or months to revolve once on its axis would suddenly accelerate to spin several hundred revolutions per second. Only the tremendous forces generated by gravitation and nuclear interactions keep it from flying apart.
III DISCOVERY OF NEUTRON STARS
At the time Chandrasekhar predicted the existence of neutron stars, calculations indicated that the stars would be relatively cold and small and therefore too dim to observe through an optical telescope. In 1967, however, astronomers realized that the magnetic field of a neutron star would also be extremely condensed and would rotate at the same rapid rate as the neutron star. The intense magnetic field at the neutron star's surface—perhaps a trillion times more intense than the magnetic field of the earth—would cause electrons moving near its magnetic poles to radiate energy in the form of radio waves, creating a signal that would sweep across space with each revolution of the star. An observer positioned within the sweep of radio-frequency radiation would observe a radio signal that pulsates at the same frequency as the rotation of the star. For this reason, neutron stars were also named pulsars because of the hypothetical radio frequency pulsations they were presumed to emit. In early 1968, only a few months after the existence of pulsars was predicted, two astronomers at the Mullard Radio Astronomy Observatory at the University of Cambridge observed the first such pulsating radio frequency source (see Radio Astronomy). Other pulsars were soon discovered, and other evidence indicating that the observed pulsars are neutron stars rapidly followed.
One of the most compelling pieces of evidence that pulsars are neutron stars was the discovery of a pulsar at the center of the Crab Nebula. Astrophysicists studying neutron stars felt that the collapse of a supergiant star's core would be accompanied by a violent explosion that would blow away the outer envelope of the star. The explosive event was named a supernova. Many suspected that the Crab Nebula was the remains of an explosion that Chinese astronomers had recorded 900 years earlier, since it was in the same region of the sky. Discovery of a pulsar at the exact center of the Crab Nebula confirmed the theoretical connection between supernovas, neutron stars, and pulsars.
The sequence of events following the formation of a neutron star appears to be as follows: The outer envelope of a collapsing star, which comprises 80 percent or more of the star's total mass, also collapses when nuclear fusion in the core ceases, but it does not become part of the neutron star. When the outer envelope falls through the intense gravitational field of the neutron star to the neutron star's surface, the material of the outer envelope gains an enormous amount of energy. When this highly energetic material hits the surface of the neutron star, massive thermonuclear reactions are ignited all over the surface of the neutron star simultaneously, blowing the outer envelope into a vast spherical bubble of gas and debris that surrounds the new neutron star.
The total mass of a star immediately before it becomes a neutron star is probably about five times the final mass of the neutron star. Thus, most of the material of the star is blasted into space in the supernova explosion that follows the formation of the neutron star. Stars also lose mass in many other ways as they progress from one phase to the next. A star that eventually becomes a neutron star must first go through several intermediate phases, from a young star probably all the way to the formation of iron in its core. Thus, the minimum mass of a star whose core will eventually become a neutron star could be as much as 20 times the mass of the sun or more.
IV CHARACTERISTICS OF NEUTRON STARS
The gravitational and nuclear forces that hold a neutron star together combine to create the most dense, exotic material in the visible universe. Calculations predict that a neutron star is perhaps only 10 to 20 km in diameter, yet it retains all of the mass of the star's core—at least 1.4 times the full mass of the sun. The density of such material could be as high as 1015 grams per cubic centimeter (1,000,000,000,000,000 gm/cc). A teaspoonful of this material would weigh ten billion tons on the surface of the earth.
Physicists estimate that a neutron star probably has an atmosphere a few centimeters (about 1 inch) thick. Beneath the atmosphere is a surface crust about 1 km (about 0.6 mi) thick, which is made of iron 10,000 times more dense and stiff than any iron found on the earth. Despite the great stiffness of the surface material, the tremendous gravitational forces of neutron stars limit the height of “mountains” on their surfaces to only few centimeters (about 1 inch) in height.
Beneath the superdense iron crust is a superfluid sea of neutrons—a strange, liquidlike substance that is even more dense than the iron crust, yet has no resistance to movement (see Superfluidity). At the center of a neutron star is a core of exotic nuclear particles found under no other conditions in the known universe. The rapid rotation of neutron stars causes their equators to bulge, and they take on the shape of a flattened ball.
When a neutron star is a member of a close binary star system, the intense gravitational field of the neutron star can distort the outer layers of the companion star and pull material from the companion star onto the neutron star. This material accelerates under the influence of the neutron star's gravity to enormous speeds, then crashes into the surface in thermonuclear explosions that release intense beams of X rays and gamma rays.X-Ray Astronomy; Gamma-Ray Astronomy.
If the mass of a collapsing star's core is less than the Chandrasekhar limit, it cannot generate enough gravitational force to cause the fusion of electrons and protons to form neutrons. The collapse of such a star will instead stop at a less extreme state—the white dwarf stage. If a white dwarf is part of an interacting binary star system, it may eventually accumulate enough mass to exceed the Chandrasekhar limit, at which point it will condense into a neutron star.
White dwarfs and neutron stars share a unique property: As they accumulate matter, they actually grow smaller, not larger. This shrinking occurs because the additional mass increases the gravitational pull of the star's material for itself, which squeezes the matter even tighter. If the mass of the collapsing core is greater than about three times the full mass of the sun, the gravitational force will exceed the strength of the material, and the core will collapse until it disappears from the visible universe altogether. This extreme state of gravitational collapse is known as a black hole. Astronomers speculate that neutron stars in interacting binary star systems can become black holes by accumulating mass in the same way that white dwarf stars in interacting binary star systems become more massive.
Contributed By:
Dennis L. Mammana
Stars: Life and Death
Discovery Enterprises, LLC
