Pulsars

Pulsars, like Quasars are one of those things that don't get much attention these days, but 40 years ago they really were the talk of the town.

A pulsar is a star that has collapsed to create a rapidly spinning neutron star. The name pulsar comes from pulsating star because the neutron star gives out very regular pulses of electro-magnetic radiation very similar to a light house.

They were discovered by a PhD student Jocelyn Bell Burnell on the November 28, 1967. I can only try and imagine just how exciting that must have been. A pulsating signal that was amazingly regular, pulsing every 1.33 seconds and came from the same location in the sky. At the time there was no known astrophysical sources that could generate something like this, I can't help thinking that there must have been many who thought that it just might be some extraterrestrial signal.

Jocelyn Bell Burnell said this

 "we did not really believe that we had picked up signals from another civilization, but obviously the idea had crossed our minds and we had no proof that it was an entirely natural radio emission. It is an interesting problem—if one thinks one may have detected life elsewhere in the universe, how does one announce the results responsibly?"

The gave the signal the name LGM-1 (little green men). It was only when a second source from an entirely different part of the sky was discovered that the idea of LGMs was entirely dismissed.

The idea that pulsars may be neutron stars came about a year later. Fritz Zwicky, he of dark matter, had first proposed the idea of a neutron star back in the 1930s about a year after the discovery of neutrons by Chadwick. These seemed the best fit. This was strengthened by the discovery of a very rapid pulsar, 33 millisecond (a very rapid clicking) shortly afterwards.

The discovery also gave strength to the ideas coming out of general relativity relating to black holes. Black holes are believed to be stars that have under gone a catastrophic collapse resulting in something called a singularity. The gravitational pull of these objects is so great that not even light can escape, hence the name, black hole. If neutron stars can exist, why not black holes?

In 1982 a pulsar with a rotation period of just 1.6 milliseconds, about the same frequency as an E5 on a piano. Some of these millisecond pulsars are considered to be excellent clocks with an accuracy and stability comparable to the best atomic clocks. They are currently being used to to help try and determine if gravitational waves exist. These where predicted by Einstein back in 1916 but have yet to be seen.

A pulsar forms when the center of a massive star is compressed during a supernova. The neutron star keeps the majority of its angular momentum and since it is only a tiny fraction of the size of the original star it has a very high rotation speed. This is in much the same way that a rotating skater with their arms out stretched suddenly starts to spin faster when they draw their arms closer to their body. The exactly same rules apply to the skater and the neutron star, how amazing is that?

The beam of radiation that we detect as the pulse is emitted along the magnetic axis of the pulsar which may not be the same as the rotational axis and it is this misalignment that givesthe pulse effect (think of a light house). The reason for the beam is thought to be due to the rotational energy of the pulsar. A changing magnetic field causes an electric field to be created (as explained by Maxwells equations, we'll cover these else where).

The strong electric field accelerates protons and electrons on the stars surface which in turn creates an electromagnetic beam.

It is thought that as the electromagnetic power is emitted that stars start to slow down and when they reach a certain point the pulsar mechanism stops, this is called the "death line". It is thought that neutron stars are pulsars for only 10-100 million years. I say only, but 10-100 million years is a staggering amount of time. Once again it was one of those things we say without really thinking about it. 100 millions years, that is such a long long time. It is also thought to be the length of time it takes our sun to do one orbit of our galaxy.

100 million years, compared to the length of the universe,  means though is that most of the neutron stars in the universe don't pulsate any more. Which I think is rather sad.


I suppose the best thing about pulsars are that they gave us the neutron star. While black holes are still theoretical, neutron stars seem to definitely exist and is the only object we know off that has such high densities and have allowed us to test general relativity under truly enormous gravitational fields.

It staggers my brain that something larger than the sun can collapse into something so small that can spin so fast giving out so much power. I wonder what they would look like up close?