Neutron Stars; the younger sibling of Blackholes

Stars

Inside a star, there is a constant battle between the nuclear fusion taking place in its core and the gravity that pushes the molten outer surface inward. The constant outflow of energy from the fusion prevents the star from being crushed under its own weight. The outward push by the energy from nuclear fusion opposes the inward pull of gravity. These two forces exist in equilibrium for billions and billions of years, giving the star its stability.

However, the hydrogen that is being used in the nuclear reaction eventually runs out as it turns into helium. For stars that are smaller than the sun, this would mark their demise but for stars that are larger, once the helium has formed, there is enough mass for it to undergo nuclear fission to form carbon. This process takes place once again for carbon, which forms neon. Neon forms oxygen, which then forms silicon and finally, the reactions end with iron. Iron is known as nuclear ash as fusion beyond this point will take up energy instead of giving it out. The lack of energy from the nuclear fusion tips the scales, gravity taking over and causing the star to collapse on itself. This marks the death of the star.

However, death doesn’t need to signify the end. It can often mark the start of a new beginning. The deaths of these massive stars often lead to the formation of other cosmic entities such as white dwarfs, novae or if they are large enough, the star’s collapse will cause an explosion. This is called a supernova.

Formation

Once there is no more nuclear fusion, gravity takes over, pushing the molten plasma into the star’s core. Gravity pulls the outer layers of the star in at a speed that is a fraction of the speed of light. These outer layers, which consist of molten plasma weigh as much as multiple solar masses (1 solar mass is 2*10^30 kg) causes the iron in the star’s core being crushed. From a sphere that used to have a diameter of about 5000 miles to one less than a dozen miles across (a 99.76% decrease in its diameter), this drastic reduction changes the structures of the atoms inside the core.

Usually, atoms are 99.99% open space, with almost the entirety of their mass being held by the nucleus which constitutes only 1/100,000th of their size. This is because the electrons orbit away from the nucleus. However, with the collapse of the star, these electrons are forced closer and closer to the nucleus to a point at which they combine with the protons to form neutrons.

The collapse of the outward layers of the star causes the newly formed neutron core to heat up to billions of degrees. The absurd temperatures cause the star to explode. The shockwave expects the outer layers of the star into space and can often lead to the formation to new star clusters and interstellar clouds. Now, only the core remains.

This neutron core is as dense as an atomic nuclei, tightly placed with negligible empty space. A neutron star weighs as much as a million Earths but is only about 20 km wide! That means one cubic centimetre of the neutron star weighs almost as much as 100 million tons, that’s about a trillion kilograms!

Neutron stars are one of the densest material discovered till date, only second to the black hole. Something which would have been formed if the parent star was larger than 3 solar masses.

Inside a Neutron Star

Neutron stars are made up of several layers,. The first is the atmosphere, which is only about a metre thick and its made up of hydrogen and helium atoms which have been ionised due to the skyrocketing temperatures.

Then, we have the outer crust, which is actually a solid crystalline material. But how? Formation of crystals require electrons to form bonds right? Well, yes. But this material is a lattice of nuclei, all positively charged which are forced to come closer to each other because of the insane gravity. This causes them to slot into place and form a lattice. Throughout the lattice, there are clouds of electron gas, called degenerate fermi gas that helps keep the crust from collapsing. But as we do down further, the electrons start to be pushed into the nuclei of an atom, forcing them to combine with the protons to form neutrons.

Next is the inner crust. In this we find strange and irregular nuclei, with extremely high neutron to proton rations. These nuclei would explode on earth, ejecting its neutrons. These nuclei are stabilised only in the depths of a neutron star, where the pressure, temperature and electron energy are at their all-time highs. This neutron-high nuclei start to drip, the excess neutrons start to fill the narrow spaces between the neutrons in the process called neutron drip.

At the very bottom of the inner crust, the neutron drip fills the spaces with neutron gas. The degenerated fermi (electron) gas thins as the electrons fuse with protons. Now, there are 5 neutrons for 1 proton and the nuclei, which were in the lattice structure start to touch.

Then we have the nuclear pasta, a mysterious and strange substance formed almost entirely of neutrons. We see the nuclei forming long cylinders, which are called nuclear spaghetti. Even lower, this spaghetti stacks atop and beside each other to form sheets I.e. nuclear lasagna. This nuclear lasagna is almost a quintillion times stronger than steel!!

Types

There are many types of neutron stars, the two main ones are Pulsars and Magnetars.

1. Pulsars

Pulsars are the most commonly known subclass of neutrons stars. They form when the collapse of the star causes the core to spin, gaining more and more speed as the diameter shrinks.

Pulsars have a very powerful magnetic field, the poles funnelling particles. These particles are accelerated and they produce very bright beams of light. As the poles aren’t aligned with the stars axis, the light that is being produced is swept around, rotating with the star. These stars are basically cosmic lighthouses, and occasionally cross Earth’s line of sight.

They’re young and full of energy, spinning up to 700 times per second. This makes them way more observable, they literally have beams of radiation shooting out of their poles. Which is why we have discovered more than 2000 of them. But there are supposed to be billions of neutron stars right? One way to explain the lack of more pulsars is that as they grow older, they lose both speed and their emission becomes weak, making them very hard to detect and observe.

Light emitted by the pulsar can contain mountains of information, helping scientists understand what exactly happens inside of a neutron core since it is the densest material that we have any hope of studying (Black holes are notoriously mysterious). Scientists have discovered that under such high pressures, matter tends to behave differently and forms small nuggets which are called nuclear pasta.

2. Magnetars

From their name it’s obvious that a magnetar has a prominent magnetic field. In fact, magnetars have the most powerful magnetic field discovered till date, probably even the strongest in the universe! Their magnetic field is at least 1 trillion times stronger than the Earths, the strongest can go up to a quadrillion. That is 1015 gauss (Earth’s magnetic field is 1 gauss). If you get even a 1000 kms near it, it will pull apart your electrons, breaking you down at an atomic level.

Scientists still aren’t sure how these stars form; there are two theories. One is that the extreme temperature and spin, combined with the already existing magnetic field before the star died, cause the magnetic field to be amplified by 1000 times or more. This is the dynamo mechanism. Another theory is that these stars used to be a part of a binary pair, i.e., two stars orbiting each other, transferring material back and forth until one of them detonates, transferring its mass to the other.

An interesting fact about them is that they experience star-quakes, similar to earthquakes, as the stars release a blast of radiation. But these star-quakes release more energy than the sun gives off in 100,000 years in less than a tenth of a second. The most powerful starquake was recorded from a magnetar about 50,000 light years away. If this magnetar found its way near our solar system, a star-quake would destroy the ozone layer as well as any organic matter in hundredths of a second.

Sources:

https://science.nasa.gov/astrophysics/focus-areas/how-do-stars-form-and-evolve#

https://chandra.harvard.edu/xray_sources/neutron_stars.html

https://www.space.com/32661-pulsars.html

https://www.nasa.gov/mission_pages/GLAST/science/neutron_stars.html

https://science.howstuffworks.com/star6.htm

https://imagine.gsfc.nasa.gov/science/objects/neutron_stars1.html

https://phys.org/news/2016-08-magnetars.html