Pulsars

Pulsars emit electromagnetic radiation that pulses on and off with periods ranging from approximately 0.001 seconds (one millisecond) to 4 seconds. Pulsars are thought to be rapidly rotating neutron stars, which are the remnants of massive stars that gravitationally collapse to a size of only about 10 kilometers after their progenitor star erupts as a supernova.

Overview

Pulsars were accidentally discovered in 1967 by Jocelyn Bell (later Jocelyn Bell-Burnell), while working with a group of British radio astronomers led by Antony Hewish. Other members of the group were John D. H. Pilkington, Paul Frederick Scott, and Robin Ashley Collins. The Hewish group was looking for interplanetary Scintillation at radio frequencies. Scintillation is the twinkling of radio sources caused by interplanetary ionized gas.

On August 6, 1967, this team noticed a mysterious signal that looked like random interference, such as that from a passing automobile. The signal reappeared at the same location in the sky, however, and therefore had to be of celestial origin. Using a high-speed recorder, researchers then observed this signal (from what was later named CP 1919) on November 28, 1967. The high-speed recorder showed extremely regular pulses (brief intervals of greater than normal brightness). The timing was repeatable to one part in about one million. That extreme regularity caused the Hewish group to consider briefly the possibility that they were observing a evidence of extraterrestrial intelligence. This conjecture was dubbed the “Little Green Men” (LGM) theory. The subsequent discovery of three additional similar signals forced the Hewish group to consider the natural origins of their mysterious signal since similar multiple sources would not likely all be from intelligent extraterrestrial civilizations.

The Hewish group named this newly discovered class of astronomical objects pulsars, because they seemed to be stars pulsing on and off very rapidly. The first Pulsar discovered by the Hewish group was designated as CP 1919. CP 1919 and other pulsars are named by their position in the sky. In early pulsar nomenclature, CP stands for “Cambridge Pulsar,” the University of Cambridge the location of the discovery group. The 1919 refers to the object’s right ascension, which is one of the position coordinates in the sky and analogous to longitude. In modern nomenclature, pulsars are designated by PSR, short for “pulsar,” and a number indicating the pulsar’s location in the sky. Particularly famous or important pulsars are also given names. For example, PSR 0531 is also known as the Crab Nebula pulsar because it is centrally located within the Crab Nebula.

Like many discoveries in science, the discovery of pulsars was serendipitous. The Hewish group was not looking for pulsars and indeed did not even suspect that pulsars existed. Their equipment, however, was designed to measure the angular structure of radio sources using interplanetary scintillation. They were thus able to detect rapid variations of pulsars. As with all unexpected discoveries, there was a temptation to disregard data not conforming to prior expectations and suspect such data to be the result of either interference or an idiosyncrasy of the equipment. Other groups had in fact observed pulsars at both radio and optical wavelengths prior to the Hewish group’s discovery, but they did not recognize the significance of their observations until the Hewish group announced their findings. Credit for following up on the unexpected data, searching for their extraterrestrial origin, and thereby discovering pulsars justly goes to Bell-Burnell.

Pulsars were originally discovered at radio wavelengths, but they have since been observed over the entire electromagnetic spectrum. W. John Cocke, Michael J. Disney, and Donald J. Taylor, working at the University of Arizona, first discovered a pulsar pulsing at optical wavelengths on January 16, 1969. This pulsar, the famous Crab Nebula pulsar (PSR 0531), has also since been found to pulse over the entire electromagnetic spectrum, from X-radiation to radio waves. Pulses from the Vela pulsar have also been recorded over a wide wavelength range. Pulsars seem to pulse at all wavelengths; this fact has been directly observed, however, for only a small fraction of pulsars. This discovery is significant, because pulsars must produce much more energy to pulse at optical or X-ray wavelengths than at radio wavelengths only.

Since the initial discovery, astronomers have continuously searched for new pulsars and have extensively studied the properties of known pulsars. By 2023, over 3,000 such objects have been discovered. Estimates based on current astrophysical understanding suggest that as many as half a million pulsars should exist in the Milky Way, but most are not readily detectable. No pulsars have yet been detected outside our galaxy, but pulsars cannot be objects confined to existence only in the Milky Way of course. The capability to detect pulsars is limited by their relatively low luminosities.

Pulsar periods, the time required for a complete on-off-on pulse cycle, range from about a millisecond (0.001 seconds) to almost 4 seconds. These periods are very regular and precise, to about one part in 100 million. In addition, periods for the faster pulsars, notably the Crab Nebula pulsar, are known to be increasing—that is, their pulse rates are slowing down. Pulsars also have occasional “glitches,” or abrupt changes in their periods, which are thought to be caused by “starquakes” on the pulsar.

Each pulsar has its own unique pulse shape. Some pulsars have two distinct pulses, a major pulse and an interpulse. The interpulse is usually smaller than the main pulse, but can be as large. Other pulsars have only one pulse. An individual pulsar’s pulse shape over a single cycle can vary, but the average pulse shape over a large number of cycles remains constant. No two pulsars are known to have the same average pulse shape.

Pulsars are also known to be associated with Supernova remnants. A supernova remnant is formed from the material blown away from a massive stellar explosion. For example, the Crab Nebula is the remnant of a supernova that was observed in 1054. The Crab Nebula pulsar is associated with this remnant and is thought to be the collapsed core of the star that went supernova.

A theoretical model explaining pulsars must account for all observed properties. The Hewish group originally thought that pulsars changed in brightness by increasing and decreasing in size, or pulsating. Hence, they were named pulsars. However, this pulsation was not able to account for all the later-observed properties of these objects. The Hewish group did correctly conclude that pulsars were most likely to be either white dwarfs or neutron stars.

Thomas Gold suggested in 1968 that pulsars are actually rotating Neutron stars rather than pulsating stars. According to his “lighthouse model,” the pulsar sends out relatively tight beams of radiation in two directions. As the pulsar rotates, the beam alternately points toward and away from Earth; thus the pulsar appears to flick on and off rapidly. An individual pulsar’s pulse shape is determined by how squarely these beams hit Earth. If both beams hit Earth equally, two equal pulses are recorded. If one beam misses, only one pulse is detected. If one beam hits Earth dead center and the other barely hits, a strong main pulse and a smaller interpulse result. If both beams miss, the pulsar cannot be detected from Earth. Thus, it is likely that there are pulsars that cannot be identified from Earth. Whether a pulsar and its pulse shape are detectable depends on the chance alignment of these beams relative to Earth.

Pulsars are rotating neutron stars rather than white dwarfs because pulsars must be very compact, and the rotation periods, as given by the pulse times, are too fast for objects as large as white dwarfs. A typical White dwarf star will have about the radius of Earth, which is too large to explain the rotation periods of the faster pulsars. A neutron star, on the other hand, with a typical radius of about 10 kilometers, depending on its mass, is small enough for even the most rapid pulsar.

Why is the pulsar/neutron star spinning so fast? Neutron stars are formed when a massive star explodes in a supernova. After the supernova, the core of the star collapses to form a neutron star. If the original star was rotating, the collapsed core turned neutron star would continue to rotate much faster—many times a second. For an illustration of the mechanics of this phenomenon, consider the figure skater who wants to spin: She must pull her arms close to her body in order to spin faster. To slow down, she stretches out her arms. Similarly, as the stellar core collapses into a 10-kilometer neutron star, it begins spinning very rapidly. The spinning property known as angular momentum will be conserved, or remain constant, if there is no net external torque. Because the amount of angular momentum depends on the rotational radius as well as the rotation rate (the mass rotating is also important), as the radius decreases the rotational rate increases. The neutron star must spin very rapidly because it is so small compared to the original star.

As the neutron star collapses, it also compresses the star’s magnetic field. Hence, there is a very strong Magnetic field at the surface. Electrons moving at near the speed of light in a strong magnetic field cause what is known as synchrotron radiation to be emitted. The exact mechanism is still poorly understood, but the magnetic field of the pulsar causes two beams of synchrotron radiation to be emitted from the pulsar’s magnetic poles. If the pulsar’s spin and magnetic axes are lined up in such a way that one or both of the beams are pointed toward Earth, a pulsar is observable. The orientation of these two axes will determine the exact pulse shape that is observed. Ultimately, the energy observed from pulsars and the surrounding Supernova remnant comes from the rotation of the pulsar. If the model is correct, then pulsars must slow down. The observed increase in pulsar periods strongly supports the lighthouse model.

The model of pulsars as rapidly rotating neutron stars, then, explains the observed properties of pulsars. The fact that neutron stars are thought to be formed in a supernova explosion and that several pulsars are associated with the remnants of a supernova explosion strengthens this connection between pulsars and neutron stars.

Knowledge Gained

During the 1930s the possibility of neutron stars was first hypothesized by physicistJ. Robert Oppenheimer, but prior to 1967 they had not been observed. With the discovery and later explanation of pulsars, neutron stars were shown to exist. Their discovery provided an important corroboration of accepted theories of stellar evolution.

However, neutron stars did not have exactly the properties that had been predicted. Few if any astronomers had suspected that neutron stars would wink on and off in less than a second. Franco Pacini perhaps had come the closest to predicting, prior to the discovery of pulsars, this true nature of neutron stars. In 1967, Pacini correctly predicted that neutron stars should be rotating rapidly and suggested that this rotation in a condensed magnetic field provided the energy for the surrounding supernova remnant. In trying to explain the then-unknown energy source within the Crab Nebula and other supernova remnants, Pacini came remarkably close to predicting the existence of pulsars, but he did not predict the beams of radiation that constitute pulses from the pulsar.

The lighthouse model, explaining pulsars as rotating neutron stars, seems obvious now but had not been considered prior to the discovery of pulsars. Even after their discovery, most researchers thought that pulsars were white dwarfs rather than neutron stars, until pulsars too fast to be white dwarfs were discovered. Originally, most astronomers thought that pulsars were more likely to be white dwarfs because the latter had been observed for quite some time. For an object the proposed size of neutron stars to be able to rotate as fast as the observed pulsars yet not fly apart, the density of that object must be much greater than a value typical of the density of white dwarf material. This physical constraint, coupled with the discovery of the faster pulsars, convinced astronomers that pulsars must be neutron stars rather than white dwarfs.

After observing the properties of pulsars, most astronomers agreed that pulsars were indeed neutron stars and realized that pulsars provided an excellent laboratory for the study of a number of phenomena. Neutron stars are a form of highly condensed matter: degenerate neutrons. Degenerate neutrons are compressed to roughly the density of the atomic nucleus, so that if the neutrons were compressed further they would lose their structure as neutrons. These conditions cannot be duplicated in a laboratory on Earth, so to understand how matter behaves under these conditions scientists must study pulsars. A pulsar shines by synchrotron radiation, which is produced by the interaction of high-speed electrons moving in an intense magnetic field. The scale and intensity of a pulsar’s magnetic field cannot be reproduced on Earth, so by studying pulsars researchers can gain information about regions with strong magnetic fields.

A pulsar also provides the energy source that illuminates the supernova remnant surrounding the pulsar. An example is the Crab nebula. Prior to the discovery of the Crab nebula pulsar, astronomers did not understand the energy source for this nebula. It is now known that the pulsar provides the energy to make the nebula shine. The same mechanism can cause other supernova remnants to shine similarly.

Context

Prior to the discovery of pulsars, scientists had suggested that neutron stars might be formed from the stellar core left after a supernova. Once pulsars had been discovered to be associated with supernova remnants, this suggestion had a firm observational foundation. Since the discovery of pulsars, astronomers better understand the formation of neutron stars and the role of a supernova in it. The first supernova observable to the naked eye in nearly four hundred years was observed in the late 1980s in the Large Magellanic Cloud (Supernova 1987A). Scientists were hoping that a pulsar would eventually form, affording them an opportunity to observe directly the formation of a neutron star.

Placed in a broad context, the most significant aspect of the study of pulsars is their role in increasing astronomers’ understanding of stellar evolution. In particular, pulsars have helped clarify the role of supernovae in the process of star death. It is thought that neutron stars and black holes are the final corpses of massive stars and are formed in supernova explosions. The study of pulsars increases scientists’ understanding of supernovae and ultimately of Earth’s origin.

A supernova plays a crucial role in the manufacture of the raw materials needed for life. According to the Big Bang model, in the aftermath of the universe’s formation it contained primarily hydrogen and helium with trace quantities of Lithium and beryllium and no other elements. How, then, are all the other naturally occurring elements formed? Stars manufacture other elements by nuclear fusion reactions that produce stellar energy. For example, stars similar to the Sun can produce elements as heavy as carbon in their cores before they collapse into white dwarfs. Stars that are much more massive than the Sun can produce elements as heavy as iron in their cores, and elements slightly heavier than iron nearer the surface. The heaviest elements probably cannot, however, be manufactured in stellar cores, because the processes by which elements heavier than iron are manufactured require rather than release energy. A supernova releases tremendous quantities of energy, so it can supply all the energy needed to manufacture the heavier elements.

Supernovae also provide the recycling mechanism for the elements manufactured in stellar cores. Such an explosion blows material rich in heavy elements back into space, where it can be recycled in the next round of star and planet formation. Without the supernova, this material would be trapped in stellar cores. With the exception of hydrogen and helium, all the material on Earth and in human bodies was manufactured in the core of a star or in a supernova and blown into space by a supernova, prior to the formation of the solar system.

In June 2023, astronomers from the University of Warwick in England were able to spot and image a white dwarf pulsar, only the second time this has been accomplished. The activity was accomplished at an observatory in La Silla, Chile. The pulsar, named J1912-4410, is located in the constellation Scorpius.

By studying pulsars, scientists gain a greater understanding of the supernova process and the role played by supernovae in manufacturing the raw materials necessary for human existence. Without clues provided by pulsars, scientists could not have unraveled the mystery of stellar death and its essential role in the development of life on Earth.

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