Astronomy

What is a Pulsar



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A pulsar is an abbreviation for a pulsating radio star. Pulsars are believed to be rapidly rotating neutron stars, which have very strong magnetic fields. As these stars rotate, they emit beams of radio waves from both magnetic poles. If these beams cut across the Earth, then they can be picked up with radio telescopes.


Formation of a Pulsar


All stars will at some point run out of nuclear material to burn, resulting in an enormous explosion called a supernova. At this point, most of the matter in the star is blown away, but the inner core remains, much condensed, as either a white dwarf, a neutron star or even a black hole. The only determining factor as to which will be left, is the size and mass of the original star. In the case of a neutron star, the mass will need to be greater than 11 but less than about 60 times the mass of our sun (the mass of our sun is a convenient unit in astrophysics). For such a star, its core will be mostly iron, surrounded by layers of lighter elements. It is the production of iron (by nuclear reactions) that keeps the star stable in it's final hours - and it is only hours. Once the production of iron stops, the core must contract under gravity.


Once the contraction starts, the core will heat up, reaching a temperature of about 100 billion degrees. The iron nuclei will then disintegrate into electrons, protons and neutrons (the building blocks of all atoms). Despite the incredible temperatures and pressures, the collapse actually speeds up, until the energy acting on the protons causes them to disintegrate. It just so happens (simplifying a lot of complex physics) that a proton and an electron will combine to produce a neutron and a release of energy. At a core temperature of 10 thousand billion degrees, and a density of about 10 million billion kg per metre cubed (think of the Earth squeezed into 2 Olympic size stadiums!), the contraction stops. Now the core is made up of nothing but neutrons, and these are strong enough to withstand the pressure. As all this is happening, shockwaves are passed out through the outer gas layers of the star, throwing them outwards in the explosion that is observed.


The inner core is now all that's left of the star, and has a radius of about 10km. But that is not all. In the final stages, the core may get smaller, but its mass remains the same. Now think of a skater on ice, spinning round. If she holds her arms out straight, and then brings them in tight to her body, she will spin much quicker. This is called conservation of angular momentum. The same thing happens to the core, speeding up the rate at which it turns, until it may be spinning at hundreds of times a second.


There is also a concentration of magnetic fields, trapped within the core, as it collapses. The mechanism for this is not known, but it is believed to be these immense magnetic fields, that create the radio waves at the poles. These are broadcast out like the light from a lighthouse. If you are in the beam's path, you can detect it.


Detecting a Pulsar


Radio astronomy is a relatively young science. Its birth came out the Second World War development of radar. By the 1960's, most university physics departments were conducting looking into this field. One such university was Cambridge in the UK. Tony Hewish had received a grant to map quasars with a telescope that covered 4.5 acres (or 57 tennis courts). One of his research students was Jocelyn Bell, whose job it was to look for the quasar signals. However, after a few weeks, she noticed peculiar pulsing signal that repeated over several days' observation. Further investigation found that it was a new type of star, which was termed a pulsar. It should be noted that Tony Hewish was awarded the Nobel prize in 1974, yet Jocelyn was not included, which has caused some controversy since.


There have been over 1000 pulsars discovered in all parts of the sky, with times ranging from mili-seconds to seconds. Forty years of observations have also shown that these objects are remarkably good timepieces, their rates of spin almost constant. Pulsars have also been discovered at the centre of nebular confirming the theory of their formation.



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