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A Brief Description of Neutron Stars

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Hello there you quantum lumps of carbon floating in space-time!

Starring in Neutron Dev’s first science article: Neutron stars!

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Helix Nebula or The Eye of God.

Neutron stars are the collapsed cores of large stars that have undergone a supernova explosion, ultimately having been compressed by gravity into the smallest, yet densest kind of stars.

In principle, when a star of a certain mass, usually bigger than our sun, runs out of hydrogen fuel it explodes into a supernova, leaving behind, or better said, expelling the chemical elements it consisted of, thus having a role in enriching the interstellar space with heavier atomic mass elements.

There are many types of supernovae and, depending on various factors, supernovae can lead to the formation of neutron stars, or other types of stars that don’t have enough mass to become neutron stars, such as white dwarfs. Anyhow, if neutron stars are not able to sustain themselves against further collapse, they will eventually become black holes. This phenomenon is known as neutron degeneracy pressure.

As one star collapses, the energy of the electrons increases to the point where they can combine with protons and produce neutrons  (via inverse beta decay), but just imagine the process happening simultaneously for an awful lot of particles. The result is composed of nuclear matter, or as some scientists name it, degenerate matter, the term having been described ever since 1926.

Degenerate matter in physics is a cluster of non-interacting particles that have a certain characteristic determined by quantum mechanical effects. Basically, it’s pretty close to what an ideal gas means in classical mechanics. This state is defined by incredible high density in compact stars, such as neutron stars. There is also an analog for electron-degenerate matter and so on.

Coming back to neutron stars.

As the name suggests, neutron stars are densely packed with neutrons, particles that are also found in the nuclei of atoms and that are not electrically charged.  In fact, there is so much matter clumped in such a small space, a shot glass filled with neutron star matter would weigh as much as Mount Everest! They are also really hot and possess a strong magnetic field.

Their diameter is something close to 20 kilometers and the mass is around 1.4 bigger than that of our Sun. You could basically jog around it if you were a trained athlete and, of course, had a special suit that’s totally impervious to massive amounts of radiation.

Or you could just compare the size of a Neutron star to the city of Manhattan.

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Credit: NASA/Goddard Space Flight Center

There are supposedly around 100 million neutron stars in the Milky Way, most of the old and cold. The closest is located not too far, in the constellation Cetus, about 424 light-years away.

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Artist’s impression of magnetar SGR 0418+5729. Credit: ESA/ATG medialab

Stars, just like planets and well, everything in space, are spinning. Neutron stars don’t make an exception, as they spin at approximately 24% of the speed of light or 70.000 km per second. While doing this, they can emit electromagnetic radiation that makes them detectable as pulsars, which confirmed the existence of neutron stars, when first observed. Despite the distance from Earth, we can see that this radiation occurs at very precise intervals and is in fact so precise, that it has sparked a debate regarding which one is better at timekeeping, pulsars, or the atomic clocks made by humans.

The phenomenon that occasionally increases the spin of the star is called a starquake and it is a result of a glitch, thus the form of the neutron star becomes more spherical. An anti-glitch is the reverse process, in which there is a small decrease in rotation, first observed in magnetars. Magnetars are a type of neutron star that has a more powerful magnetic field.

Recent studies suggested that there is something slowing the spin, namely virtual particles.  The mechanism roughly includes the principles of friction that also affect movement on our planet. As some believe, there’s a lot of empty space out there, but it turns out the Universe is not that empty, as particles can pop in and out of existence, exhibiting the characteristics of an ordinary particle only for a limited time. This is known as the quantum vacuum, and as the theory states, even if you could take out all its elements, the vacuum still wouldn’t be empty, precisely due to these virtual particles.

And still, this doesn’t break any rules of the quantum laws, because the time of existence of these particles is so small, they just don’t have time to violate these laws.

Any process involving virtual particles has a lot to do with the famous Feynman diagrams, that describe the behavior of subatomic particles. Despite their ghostly appearance, they impact spacetime and various objects in the universe directly. The Hawking radiation is a good example in this matter. It causes black holes to evaporate, due to these particles that pull mass out of the black hole.

Sometimes, even in space, things don’t appear as ordinary as they may seem, despite the fact of them obeying certain laws, so I’ll end this with a neat quotation from the one and only Isaac Asimov:

The most exciting phrase to hear in science, the one that heralds new discoveries, is not ‘Eureka!’ but ‘That’s funny…

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