Pulsars were discovered quite accidentally in late 1967. Lead by Professor Antony Hewish, Cambridge graduate student Jocelyn Bell was surveying the sky for scintillation phenomena due to interplanetary plasma in a certain radio frequency range.(Chiu, p.965) Among the expected random noises, Bell noted a repeating signal. Thought at first to be sporadic radio interference, it was soon noted that the emissions always occurred at the same position on the celestial sphere; that is, at the same right ascension and declination. Also, the burst of noise was appearing approximately four minutes earlier each evening, similar to the rise of stars.(Mackay, 1996) This indicated that the source was not of terrestrial origin. Ensuing observations showed the signal to be a series of pulses with a repetitive period of ~1.337 seconds and duration of ~0.3 seconds.(Hewish et al, p.709) Nonetheless, it was such an unexpected discovery that it was deemed most likely to be a yet unknown man-made source, such as a satellite. Further study showed, though, that the source lacked any indication of parallax, and was therefore at a distance far outside of our solar system.
The tentative explanation for Bell's finding was the theoretical concept of neutron stars. Neutron stars are the end stage of stellar evolution for massive stars. Gravitational self-contraction acting on the large mass serves to extend the nuclear fusion processes; hydrogen fuses to create helium, helium fuses to create carbon, and so on. All of these reactions give off energy, which serves to keep the star burning. When the nuclear process attempts to fuse iron, however, energy is required, and the endothermic nature of the reaction causes the star to implode, creating a stellar explosion known as a supernova.(Mackay & Hawkes,1996) Remains of the explosion having a mass greater than the Chandrasekhar limit of 1.4 solar masses, but smaller than 3.0 times the mass of the sun are called neutron stars.(Stellar Evol., p.54). As all thermonuclear reactions have ceased, the gravitational force of contraction acting on the star generates a density so great that electrons and protons combine to form neutrons, hence the name of the star. With this compaction and decrease in size, conservation of angular momentum causes the neutron star to spin at an increased rate, and this provided the basis for the theoretical concept of rotating stars.
Pulsars served to prove this theory of star rotation. Perhaps the greatest verification comes from the Crab Nebula. Documented by Chinese astronomers in 1054, a supernova explosion created the luminous remains known as the Crab Nebula.(Stellar Evol., p.54) The source for the luminosity was unknown until the presence of a high density pulsar was discovered near its centre. The rate of rotation of this galactic object is about 33 ms, which is much quicker than the average pulsar period of 1.34 seconds.(Hewish, p.740) To be rotating at such a fast speed, the star had to be extremely compact. However, with such pulse precision and energy output, the star also had to be extremely massive. A high density star would allow for both characteristics, and the detection of the Crab Nebula pulsar thus helped to substantiate the claims for a rotating star.
Another aspect of the neutron star theory is the systematic increase in period. Detailed observation of pulse timing has shown pulsar spins to be slowing down about one part in 10^15 each rotation.(Rankin, p.567) Generally, this can be attributed to conservation of energy. Rotational energy is the main energy source of a neutron star, and as it radiates energy, it does so at the expense of its rotational kinetic energy, thereby causing the gradual `spin-down' of the pulsar. This not only accounts for the radio emissions discovered by Jocelyn Bell, but also for the visible light from the nebula around the Crab pulsar.(Hewish, p.741) The conversion method of the rotational energy of a pulsar into electromagnetic radiation of radio or visible wavelengths is unknown, but the identification of the pulsar as a likely source of nebular luminosity gives additional evidence in favour of the neutron star theory.
Further pulsar research has revealed over 700 rotating neutron stars to this date.(MPIfR, 1997) Of these, Jocelyn Bell discovered the first four in a space of three months.(Weatherall, 1995) At this time, the findings were published in the Feb. 24, 1968 issue of Nature, with Hewish, Bell, and three other associates from Cambridge listed as co-authors. The identification of the existence of the high density, strongly magnetized neutron stars stimulated research into many new areas of Physics, and was undoubtedly a notable discovery. However, in 1974, it was Antony Hewish who received a Nobel Prize in Physics for "the discovery of pulsars".(Martens, 1996) Despite considerable documentation to prove Bell actually discovered the pulsar, the Nobel Foundation ignored her and gave the prestigious award to Hewish.
This is only one example of the contributions women have made to science today that have gone largely unnoticed, and is evidence for a hierarchical bias.(Prewitt, 1995) On the broad level of general areas of study, `hard' sciences such as physics and chemistry are considered to be more prestigious than `softer' sciences like psychology and social science. Even within a science, there exists a ladder-like structure, with more importance placed on the higher levels and positions. Leaders, having superiority, generate the ideas, and then delegate the tasks to be performed. This hierarchical structure, and its associated reward system is often reflected in the authorship of papers. Graduate and postdoctoral students are usually listed as co-authors, but if the work is noteworthy, the scientific community believes that the credit should be given to the person in the superior position, namely the lab chief.(Shepherd, p.126) The contributions of Antony Hewish's team members were acknowledged, yet it was he alone who received the critical acclaim of a Nobel Prize.
It is interesting to note the lack of representation of women in the list of Nobel Prize winners. From 1901 to 1996, only two women have received a Nobel Prize in Physics, both sharing half of a divided prize with one of their peers.(Stanford, 1996) The prestige attached to a Nobel Prize extends far beyond the immediate award; former recipients, such as Einstein, Bohr, and Van der Waals continue to be important figures in science today, due to the magnitude of their discoveries, and the inherent importance they hold for current scientific research. Physics is traditionally a male-dominated science, and it is extremely difficult for women to move ahead in this field, as in others where females are under-represented. Jocelyn Bell's discoveries and research initiated one of the major movements in radio astrophysics, and the lack of acknowledgement, and possible future scientific status, is evidence of the hierarchical and gender bias found in science today.
Since 1967, Jocelyn Bell-Burnell, as she is now known, has been involved in many aspects of astrophysics, including radio, gamma-ray, x-ray, and infrared astronomy. Her work during this period has not gone unnoticed. In 1995, the National Radio Astronomy Observatory and the Trustees of Associated Universities, Inc. presented Bell-Burnell with the Jansky Award in recognition of her outstanding contributions to astronomy.(Finley, 1997) Presently, she is an astronomy professor at The Open University, in the UK.(Burnell, 1997)
The discovery of pulsars altered the direction of modern cosmological research. Supernova explosions of massive stars were long thought to result in compact, high density neutron stars, but until the pulsar radio emissions were acknowledged in 1967 as having a celestial origin, rotating neutron stars were purely theoretical. Extensive study showed that the rotation of the stars was due to the gravitational contraction of the massive remains of the supernova explosion, and the pulsar centred in the Crab Nebula provided further proof of the neutron star theory of pulsars. Increasing rotation periods were cause for concern, but were easily explained as a consequence of radiative bursts of energy released from the rotating star as pulses. Working as a graduate student at Cambridge University, Jocelyn Bell was the first to discover this amazing phenomenon, but in spite of documentation attesting to her principal role, the credit for the work and discovery went to her professor Antony Hewish in the form of a Nobel Prize in Physics in 1974. Our current scientific hierarchy considers the leader of the research team to be the rightful recipient of recognition for outstanding performance. In the case of Jocelyn Bell, this practise proves to be unfair. While she has continued to be successful in the field of astronomy, the lack of official recognition for her primary role in the discovery of pulsars is, indeed, an astronomical injustice.