What is a pulsar? A pulsar is a neutron star that rotates extremely rapidly and releases pulses of radiation, hence their name.
Neutron stars are formed when stars much larger than our sun (a minimum of eight times larger) run out of fuel to burn. All stars begin by burning hydrogen and turning it into helium. This is nuclear fusion. Atoms are usually kept away from each other by the electromagnetic force. This force can be overcome if there is enough energy, and a star has that energy because of the tremendous pressure created by its gravity. Four hydrogen atoms are forced together and when they combine, they make one helium atom. The atomic mass of four hydrogen atoms is slightly more than one helium atom, and that difference in mass is released as energy. This energy is released from the star. Once all of the hydrogen has gone, the star starts to collapse. Stars are massive and the incredible force of gravity is constantly trying to crush them. While they are burning hydrogen, they have enough energy to counter the force of gravity. Once that energy has gone, gravity starts to win. If a star is big enough, the force of gravity create more pressure and helium atoms are fused together to make increasingly heavier atoms, such as carbon, oxygen, neon, magnesium, silicon, and then iron. Iron is the last possible material because fusing iron into a heavier element takes energy rather than producing it. At this point there is no more energy to combat gravity and the star which was at least eight times larger than our sun collapses to the size of a city. The pressure is so great that electrons and protons are fused together. The star collapses, then explodes outwards in a supernova. The dense core that is left is made mostly of neutrons and is some of the densest material in the universe.
So, what makes a neutron star into a pulsar? The first thing is the speed of rotation. All stars rotate at different speeds, but they do all rotate. The speed of their rotation is given by the speed of the rotation of the cloud of gas they were formed from and by their size. This can then be sped up or slowed down by the stellar wind, a magnetic field, and many other things. Generally, the larger a star is, the slower it rotates. When massive stars burn up all their fuel and become neutron stars, their size shrinks drastically. This means they spin a lot faster. A common analogy is an ice skater rotating with their arms out. When they fold their arms in closer to their body, they start to rotate a lot faster. This is called the conservation of angular momentum, and you can try it by spinning on an office chair with two full bottles of drink in each hand. When you pull your arms in closer, you will spin faster. With a neutron star, the outer layers of the star collapse in and the diameter shrinks drastically. The rotation speed goes from once every few Earth days to 700 times a second.
When a neutron star spins so rapidly, it has a lot of rotational energy that makes a very strong magnetic field. Similar to the way our Earth makes its field, but billions of times stronger. This energy accelerates particles from its surface, and they are channeled along the magnetic field, getting ejected at the poles of the neutron star. They are blasted out into space as a very strong electromagnetic beam that we can observe as light. The beam appears to pulse to us, but it is actually a constant beam. The illusion of pulsing is given because the particles are emitted from the poles, but the neutron stars usually don’t rotate along their poles, just as Earth doesn’t. This means that the solid beam of electromagnetic radiation moves towards us and they away from us very rapidly, making it pulse. As these pulsars spin, they slowly lose energy and probably stop pulsing at some point, although nobody knows how long that might be. Not all neutron stars are pulsars, but they may have been at some point. Or they could be emitting their electromagnetic radiation in a direction that we can’t see. It is also much easier to see a pulsar than a regular neutron star, so we may only be finding neutron stars that are pulsars.
The signal from a pulsar is so regular that when they were first discovered in 1967, Jocelyn Bell, the graduate student looking at the data, thought it was either human activity or a signal from extra-terrestrial life. It is very unusual for a natural signal to be so regular. Now that astronomers know what is causing the signals, they are on the lookout for more pulsars. So far, over 3,000 have been found. And this is what I learned today.
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Sources
https://imagine.gsfc.nasa.gov/science/objects/neutron_stars1.html
https://astronomy.stackexchange.com/questions/11386/why-do-some-neutron-stars-become-pulsars
https://en.wikipedia.org/wiki/Pulsar
https://en.wikipedia.org/wiki/Neutron_star
https://www.space.com/32661-pulsars.html
https://science.nasa.gov/universe/stars
https://www.exploratorium.edu/snacks/momentum-machine
https://www.eso.org/public/poland/news/eso0336/?lang
Image By Optical: NASA/HST/ASU/J. Hester et al. X-Ray: NASA/CXC/ASU/J. Hester et al. – https://hubblesite.org/contents/media/images/2002/24/1248-Image.html, Public Domain, https://commons.wikimedia.org/w/index.php?curid=238064