The Italian Smorgasbord Happening Inside Neutron Stars
The extraordinarily dense insides of neutron stars serve up large portions of the strongest known substances: quantum spaghetti, lasagna, bucatini, gnocchi, and waffles for dessert.
Neutron stars are deliciously weird. The post-supernova collapsed core of supermassive stars, they rotate upwards of 43,000 times per minute (24% the speed of light), are 10 times hotter than the Sun, can have magnetic fields 1015 times stronger than Earth’s, and are 1014 times denser than water. In fact, they’re so dense that one teaspoon weighs a billion tons. A neutron star the size of Los Angeles would weigh as much as our Sun. Beneath the surface, these conditions produce some mind-boggling states of matter that scientists have affectionately named nuclear pasta.
The Iron Wall
Stars create energy by forcing hydrogen atoms, the lightest element, together into heavier and heavier elements. Put simply, hydrogen fusion creates helium, helium fusion creates carbon, and so on up the periodic table. Each time this happens, the product weighs slightly less than its parts, with the missing mass released as energy, as explained by Einstein’s E=mc2. In this equation, a small amount of mass (m) becomes enormous amounts of energy (E) when it’s multiplied by the speed of light squared (c2), which is around 9 x 1016 m2/s2. All of a star’s energy, both visible and nonvisible, is produced through fusion in its core.
However, the party ends at iron. As the lighter elements are fused into heavier elements, a star approaches the end of its life. The heavier an element is, the higher temperature it takes to fuse together. Small stars can generate enough temperature in their cores to fuse hydrogen, and when this is used up they will live out their retirement years as a white dwarf. Medium stars like our Sun have enough temperature to fuse heavier elements, but once these are used up, they will expand and cool into a red giant. Stars large enough to fuse iron die a much more violent death. Due to its unique atomic configuration, iron takes more energy to fuse than it produces, meaning the star’s core has a net energy loss, resulting in a quick, catastrophic demise.
Uncle Pauli, the Degenerates, and a Big Bang
Think of a star as being in balance between the inward pull of gravity and the outward push of the energy it produces. When a supermassive star begins to fuse iron, its energy production drops quickly, allowing gravity to win. At this point, the star begins to collapse in on itself.
This collapse, though, can be stopped by the Pauli Exclusion Principle, first proposed by Wolfgang Pauli in 1925. Pauli explained that no two electrons in an atom can have the same quantum numbers: energy, angular momentum, magnetic moment, and spin (n, ℓ, mℓ, and ms). This means that as atoms are forced together in a collapsing star, for electrons to coexist, some of them must move to higher energy states, resulting in what is known as electron degeneracy pressure. If a star doesn’t have the mass to create a powerful enough gravitational inward pull, there won’t be enough energy to rearrange the electrons, and the collapse is halted. For small and medium stars, electron degeneracy pressure is enough to combat gravity.
But in supermassive stars, the force of gravity is powerful enough to overcome electron degeneracy pressure. Electrons have enough energy to rearrange themselves, and matter is squeezed together in the hot, tightly packed core. Electrons (negative charge) are forced into protons (positive charge), creating neutrons (neutral charge). Neutrons also follow the Pauli Exclusion Principle and exert neutron degeneracy pressure, in that rearranging them takes a lot of energy, which might be enough to stop the star’s collapse. For neutron stars, this is where the collapse stops. If a star has enough mass to overcome neutron degeneracy pressure, then the core continues to collapse to form a black hole in a fraction of second and its mass disappears behind the event horizon.
For neutron stars, the outer layers of the star continue to rush in, collide with dense neutron core, and rebound. This forms a shockwave that ignites the outer layers, causing a supernova that can rival the luminosity of entire galaxies. The core is now alone and a neutron star is born.
A Nuclear Feast
Now that the neutron star has shed its outer layers, all that remains is a tightly packed ball of neutrons, with scarce amounts of protons and electrons. Besides the immense gravitational pressure and magnetic fields, the two main forces at play are the strong nuclear force and the Coulomb force. The strong nuclear force binds protons to neutrons and quarks to quarks, but only works over extremely short distance. The Coulomb force is responsible for the natural repulsion or attraction of particles due to their electrical charges. Under less extreme conditions, the Coulomb force is able to keep nuclei away from each other due to their positive charges, not allowing the strong nuclear force to work. However, in a neutron star, these two forces are more or less equal to each other, resulting in some unique combinations of matter.
On the surface of the neutron star, normal nuclei are free to exist, as the gravitational pressure is not strong enough to overcome the Coulomb force and allow the strong nuclear force to take over. Here, it’s possible to find elements like iron existing as it should. Moving below the surface, though, the pressure increases substantially. The power of the Coulomb force climbs exponentially. Some researchers found that “the global balance of the forces allows a huge charge (~ 10(20) Coulomb) to be present in a neutron star producing a very high electric field (~ 10(21) V/m).” However, the immense gravitational pressure is still able to overcome the Coulomb force, allowing the strong nuclear force to work, leaving particles in a neutron star caught between two extremely powerful forces.
Just under the surface, nuclei are forced together into large, unnaturally close groups that bend into semispherical shapes that resemble the Italian pasta gnocchi. Researchers found that these first shapes are strong enough to descend through the layers of the neutron star relatively unscathed. “These shapes are the first pasta to appear as one descends through the crust, can persist to relatively deep layers, making them the most robust.”
Further down, the gnocchi are plunged into a neutron soup. Here, the forces have continued to grow, and the gnocchi are squeezed into long rods that resemble spaghetti. If these are pulled further down, the pressure is enough to fuse the spaghetti together into sheets that first resemble waffles and then flatten into lasagna noodles. Going further down, the pressure increases enough to fuse the ends of the lasagna noodles, leaving a hole in the middle that resembles bucatini. Going down any further, the pasta is not able to exist, as particles likely get broken down into their constituent quarks.
Researchers explained these 4 stages as “(P1) roughly spherical nuclei are the minimum energy configuration, but pasta appears as local minima; (P2) pasta phases become the minimum energy configuration, but spherical nuclei still occupy some local minima; (P3) all local minima correspond to pasta configurations, and protons are localized in at least one dimension, and (P4) all local minima correspond to pasta configurations, and the appearance of the BCP phase indicates protons are delocalized in all dimensions.”
Although nuclear pasta has never been observed, researchers used computer simulations and determined that it’s 10 billion times stronger than steel, making it the strongest material in the universe. Unfortunately, we’ll never be able to make use of it.
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