Neutron Stars – The Most Extreme Things that are not Black Holes

Neutron stars are one of the things the most extreme and violent in the universe. Giant atomic nuclei, just a few kilometers across, but as massive as stars. And they owe their existence upon the death of something majestic. Stars exist because of a fragile balance. The mass of millions of billions trillion tonnes of hot plasma is deeply drawn to gravity and compresses the material with so much force that its atomic nuclei merge. The hydrogen then turns into helium, It releases energy fighting gravity and trying to escape. As long as this balance is maintained, the stars are fairly stable. In the long run, the hydrogen will be used up.

Photo by Alex Andrews from Pexels

Medium stars, like our Sun, become giants where they burn their helium in carbon and oxygen before eventually transforming into white dwarfs. But in stars several times more massive than our Sun, an interesting phenomenon occurs when their helium is depleted. For a certain time, the balance between pressure and radiation is broken and gravity wins, compressing the star even more than before. The core burns stronger and faster, as its outer layers swell until reaching more than a hundred times their original size, merging increasingly heavy elements.

 Carbon burns neon over several centuries, the neon in oxygen over a year, oxygen in silicon in a few months, and silicon in iron in one day. And then.. the death. Iron is atomically inert, it has no energy to supply and cannot initiate a fusion. The fusion suddenly stops and the balance disappears. In the absence of the pressure generated by the fusion, the nucleus is crushed by the huge mass of the star above him. The following now is incredible and scary. Particles, like electrons and protons, really don't want to be close to each other.

But the pressure from the collapse of the star is so large that they merge into neutrons, which then tighten as tightly than in atomic nuclei. An iron ball, the size of Earth, is compressed into a ball of pure nuclear material the size of a city. But not only the nucleus; the whole star implodes, gravity pushing the outer layers of the star at 25% of the speed of light. This implosion bounces off the iron core, producing a shock wave which explodes outwards, catapulting the rest of the star into space. This is called a supernova explosion, and that will dazzle entire galaxies. What's left of the star is now a neutron star.

Its mass, about a million times that of Earth, is compressed into an object 25 kilometers long.

It's so dense that the mass of all living humans would only fill one cubic centimeter of its material. This is equivalent to a billion tonnes contained in a sugar cube. In other words, this is equivalent at Mount Everest in a cup of coffee. From the outside, a neutron star is incredibly extreme.
Its gravity is the strongest, outside the black holes, and, if it were denser, it would become one. The light is bent around it, that is to say that we can see the front and partly the rear.

Their external surface reaches 1,000,000 degrees Celsius.

Ok, let's look inside a neutron star. Although these giant atomic nuclei are stars, in many ways, they also look like planets, by their solid crust covering a liquid core. The crust is extremely hard. Its outermost layers are composed the rest of the iron from the supernova, arranged in a crystalline matrix with a bath of electrons traveling between them. Deeper, gravity brings the nuclei closer and closer. We find less and less protons, like most fuse into neutrons. Until reaching the base of the crust. Here the pits are compressed so strongly that they start to touch. Their protons and neutrons rearrange creating long cylinders or sheets, huge pits, made up of millions of protons and neutrons, in the form of spaghetti and lasagna that physicists call "nuclear pulp".

Nuclear pulp is so dense that it may well be the most solid material in our universe, literally unbreakable. These aggregates of pasta inside a neutron star can even form mountains a few centimeters high, but many times the mass of the Himalayan range. Finally, under the pasta, we reach the heart. We are not really sure of the properties matter when compressed so hard. Protons and neutrons could dissolve into an ocean of quarks, creating a kind of gluon-quark plasma. Some of these quarks could transform in "strange quarks". Creating a kind of strange material, with such extreme properties, that we dedicated a post to it. Or maybe they keep their protons and neutrons. No one knows for sure. And that's why we do science.

 It's all very heavy, literally.

 So let's go back to space.When the neutron stars collapse, they start spinning very, very fast like a ballerina tucking in her arms. Neutron stars are celestial ballerinas, rotating several times per second. This creates pulsations because their magnetic field creates a beam of radio waves with each revolution. These radio pulsars are the best known category of neutron stars. About 2,000 are known in the Milky Way. These magnetic fields are the most powerful in the universe, about a billion times more intense than that of Earth at their appearance. They are called magnetars until they calm down a little. But the best neutron stars are friends with other neutron stars.

By emitting energy in the form gravitational waves, affecting space-time, their orbits can deteriorate, causing them to crash into each other in an explosion projecting their materials into space. At this time, the conditions become so extreme that new heavy nuclei are formed. Not by fusion, but by collapse and reassembly of matter rich in neutrons. We only learned recently that this phenomenon must have been at the origin of the formation of heavy elements of the known universe such as gold, uranium, platinum and dozens of others. So our two neutron stars collapse and form a black hole, dying once again. The stars must not only die in order to create these elements.

They must die TWICE!

 Over millions of years, these new atoms spread throughout the galaxy. But some of them end up in a stellar cloud, where gravity brings them together to form stars and planets, thus repeating the cycle. Our solar system is an example, with the remains of ancient neutron stars all around us. The whole of our modern world was built from elements coming from neutron stars, which died a long time ago, thereby sending atoms on a 13 billion year journey to create us and our world. And that is cool ! Until then, you can admire them on paper.

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