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Pulsars are neutron stars which spin rapidly and produce huge electromagnetic radiation along a narrow beam. Neutron stars are very dense, and have short, regular spins. This produces a very precise interval between pulses that range from roughly milliseconds to seconds for an individual pulsar. The pulse can only be seen if the Earth is close enough to the direction of the beam.
The pulses match the star's turns. The spinning causes a lighthouse effect, as the radiation is only seen at short intervals. Werner Becker of the Max Planck Institute for Extraterrestrial Physics recently said,
- "The theory of how pulsars emit their radiation is still in its infancy, even after nearly forty years of work. There are many models but no accepted theory".
The first pulsar was discovered in 1967 by Jocelyn Bell Burnell and Antony Hewish of the University of Cambridge. The observed emission had pulses separated by 1.33 seconds, coming from the same place in the sky. The source kept to sidereal time. At first, they did not understand why pulsars have a regular change in the strength of radiation. The word pulsar is short for "pulsating star".
In 1974, Antony Hewish became the first astronomer to be awarded the Nobel Prize in Physics. Controversy occurred because he was awarded the prize while Bell was not. She had made the initial discovery while she was his Ph.D student. Bell claims no bitterness upon this point, supporting the decision of the Nobel prize committee. "Some people call it the No-Bell prize because they feel so strongly that Jocelyn Bell Burnell should have shared in the award".
In 1974, Joseph Hooton Taylor Jr. and Russell Hulse discovered for the first time a pulsar in a binary system. This pulsar orbits another neutron star with an orbital period of just eight hours. Einstein's theory of general relativity predicts that this system should emit strong gravitational radiation, causing the orbit to continually contract as it loses orbital energy. Observations of the pulsar soon confirmed this prediction, providing the first ever evidence of the existence of gravitational waves. As of 2010, observations of this pulsar continue to agree with general relativity. In 1993, the Nobel Prize in Physics was awarded to Taylor and Hulse for the discovery of this pulsar.
Kinds of pulsars
Astronomers know that there are three different kinds of pulsars:
- Rotation-powered pulsars, where the radiation is caused by the loss of rotational energy; radiation is caused by the neutron star slowing down in the speed in which it turns
- Accretion-powered pulsars (which are most but not all X-ray pulsars), where the gravitational potential energy of matter that falls onto the pulsar causes X-rays that can be received from Earth, and
- Magnetars, where an extremely strong magnetic field loses energy, which causes the radiation.
Although all three kinds of objects are neutron stars, the things that they can be seen to do and the physics that causes this are very different. But there are some things that are similar. For example, X-ray pulsars are probably old rotation-powered pulsars that have already lost most of their energy, and can only be seen again after their binary companions expanded and matter from them started falling onto the neutron star. The process of accretion (matter falling onto the neutron star) can in turn give enough angular momentum energy to the neutron star to change it into a rotation-powered millisecond pulsar.
For some millisecond pulsars, the regularity of pulsation is more precise than an atomic clock. This stability allows millisecond pulsars to be used in establishing ephemeris time, or building pulsar clocks.
Timing noise is the name for rotational irregularities observed in all pulsars. This timing noise is observable as random wandering in the pulse frequency or phase. It is unknown whether timing noise is related to pulsar glitches.
The study of pulsars has resulted in many uses in physics and astronomy. Major examples include the proof of gravitational radiation as forecasted by general relativity and the first proof of exoplanets. In the 1980s, astronomers measured pulsar radiation to prove that the North American and European continents are drifting away from one another. This movement is evidence of plate tectonics.
- The magnetar SGR 1806-20 produced the largest burst of energy in the Galaxy ever seen in an experiment on 27 December 2004
- PSR B1931+24 "... looks like a normal pulsar for about a week and then 'switches off' for about one month before producing pulses again. [..] this pulsar slows down more rapidly when the pulsar is on than when it is off. [.. the] way it slows down must have to do with the radio energy and the things that cause it, and the extra slow-down can be explained by a wind of particles leaving the pulsar's magnetic field and slowing down the speed at which it turns. 
- PSR J1748-2446ad, at 716 Hz (times it turns per second), is the fastest spinning pulsar known.
- European Space Agency, press release, Old pulsars still have new tricks to teach us, 26 July 2006
- Hewish A; Bell S.J. et al 1968. Nature, 217, 709. Observation of a rapidly pulsating radio source
- Sidereal time is a "time scale that is based on the Earth's rate of rotation measured relative to the fixed stars". It is how astronomers keep track of a given star in the night sky.
- Burnell, S. Jocelyn Bell 1977. Little green men, white dwarfs, or pulsars? Annals of the New York Academy of Science. 302, 685–689. 
- BBC Radio 4 – The life scientific: Dame Jocelyn Bell Burnell. BBC (2011-10-25). Retrieved on 2012-07-27.
- Weisberg J.M; Nice D.J. & Taylor J.H. (2010). "Timing measurements of the relativistic binary pulsar PSR B1913+ 16". The Astrophysical Journal (IOP Publishing) 722 (2): 1030. http://iopscience.iop.org/0004-637X/722/2/1030/pdf/0004-637X_722_2_1030.pdf.
- "Nobel Prize in Physics 1993". http://nobelprize.org/nobel_prizes/physics/laureates/1993/. Retrieved 2010-01-07.
- Matsakis D.N; Taylor J.H. & Eubanks T M. (1997). "A statistic for describing pulsar and clock stabilities". Astronomy and Astrophysics 326: 924–928. http://aa.springer.de/papers/7326003/2300924.pdf. Retrieved 2010-04-03.
- An ephemeris is a table of values that gives the positions of astronomical objects in the sky at a given time.
- Backer, Don (1984). "The 1.5 millisecond pulsar". Annals of the New York Academy of Sciences 422 (Eleventh Texas Symposium on Relativistic Astrophysics): 180–181. . http://www3.interscience.wiley.com/journal/119527609/abstract. Retrieved 2010-02-14.
- "World's most accurate clock to be built in Gdańsk". Polska Agencja Prasowa. 2010. http://www.naukawpolsce.pap.pl/palio/html.run?_Instance=cms_naukapl.pap.pl&_PageID=1&s=szablon.depesza&dz=szablon.depesza&dep=374908&lang=EN&_CheckSum=620107168. Retrieved 2012-03-20.
- African Skies 4 - Radio Pulsar Glitch Studies
- Lorimer D.R. & M. Kramer 2004. Handbook of pulsar astronomy. Cambridge Observing Handbooks for Research Astronomers.
- A pulsar discovery - the detection of the first optical pulsar from the American Institute of Physics. Includes audio and teachers guides.
- The listing for the first pulsar (PULS CP 1919) in the Simbad database
- The ATNF Pulsar Catalogue
- Scientists can predict pulsar starquakes (SpaceDaily) Jun 07, 2006
- List of pulsars in binary systems
- XMM-Newton makes new discoveries about old pulsars (SpaceDaily) Jul 27, 2006