The Theory of Nothing

What the heck is a pulsar?

What the heck is a pulsar?

A pulsar is one of the more interesting astronomical objects, and the wild thing is that it wasn't something that was known until 1967.

A pulsar is a special kind of neutron star. A neutron star is what is left when a massive star goes supernova and blows its outer layers away when it's core collapses because of gravity. What's left is a core of packed together neutrons, making this core material extremely dense and hot. A neutron star is very dense. It has a density of up to three solar masses stuffed into a sphere with a seven miles radius, giving it a density of ten to the seventeenth kilograms per meter cubed. A matchbox full of neutron star would weigh five billion tons. A thimble full would cut down through the Earth like a knife through butter. Keep in mind that neutrons are extremely tiny compared to the size of an atom; so squeezing neutrons together would be very dense. That's heavy!

What makes a pulsar so special is that it emits a periodic pulse of focused electromagnetic radiation that can only be seen if it's pointed directly at the observer, and this pulse of radiation is periodic because the pulsar is rotating. The frequency of the pulses is often in the millisecond range and can be more precise than an atomic clock. In fact, that's how pulsars were discovered. After determining that radio pulses from a specific location in space were not man made, Jocelyn Bell Burnell and Antony Hewish discovered the first pulsar in 1967. At first, they labeled the signals as LGM, which stands for little green men. Eventually, astronomers realized that these pulsar stars were remnants from supernovae. A pulsar was detected in the center of the Crab nebula, which is the result of a supernova. This led to the idea that a neutron star with a strong magnetic field could generate electromagnetic energy out of its magnetic axis, which is not necessarily its spin axis. The name 'pulsar' is a contraction of the term 'pulsating star.'

So, a neutron star becomes a pulsar when it emits pulses of radio, X-ray, UV and even visible light. The reason it rotates so fast is that when the massive star that it originated from collapses, the core retains its angular momentum and the spin velocity increases as the size decreases.

Astronomers use the pulses of pulsars to investigate the interstellar medium. Most people think that there is nothing in space between stars, but that's not true. There are ionized electrons out there in interstellar space and they affect the radiation from a pulsar. It turns out that lower frequency radio waves travel through this electron soup slower than higher frequency waves. This change in timing of the pulse at different frequencies is called dispersion and it causes scintillation or twinkling just like starlight does when it goes through Earth's atmosphere. This dispersion effect allows astronomers to determine the changes in density in the interstellar medium.
Scientists are also using pulsars to find gravitational waves. Changes in the frequency of pulsars could lead to a proof of gravitational waves left over from the Big Bang as well as gigantic supernovae. Essentially, pulsars have become useful as cosmic clocks.

Thanks for reading.