Neutron stars are astonishingly unique. Formed from the remnants of supermassive stars after a supernova, they can rotate at speeds exceeding 43,000 revolutions per minute (24% the speed of light), boast temperatures 10 times hotter than our Sun, and possess magnetic fields that are 10^15 times stronger than Earth's. With a density 10^14 times greater than water, just a teaspoon of neutron star material weighs a staggering billion tons. A neutron star the size of Los Angeles would match the mass of our Sun. These extreme conditions give rise to unusual states of matter that scientists whimsically refer to as "nuclear pasta."

Stars generate energy by fusing hydrogen atoms, the simplest element, into progressively heavier elements. Essentially, hydrogen fusion leads to helium, which then fuses into carbon, and so forth along the periodic table. This fusion process results in a small mass loss, which is emitted as energy, as described by Einstein's equation E=mc². In this formula, a minor amount of mass (m) is converted into vast amounts of energy (E) when multiplied by the square of the speed of light (c²), approximately 9 x 10^16 m²/s². All forms of energy emitted by a star, whether visible or not, originate from fusion occurring in its core.

However, this fusion process halts at iron. As lighter elements are transformed into heavier ones, a star nears the end of its life cycle. Heavier elements require increasingly higher temperatures to fuse. Smaller stars can generate sufficient core temperatures to fuse hydrogen, ultimately settling into white dwarfs once their hydrogen is depleted. Medium-sized stars, such as our Sun, can fuse heavier elements but will subsequently expand and cool into red giants when their fuel runs out. In contrast, massive stars that can fuse iron meet a far more dramatic fate. Due to iron's unique atomic structure, it demands more energy to fuse than it releases, leading to a net energy loss in the star’s core, which results in a rapid and catastrophic collapse.

A star maintains equilibrium between the inward force of gravity and the outward force of energy produced through fusion. When a supermassive star starts fusing iron, its energy production diminishes sharply, allowing gravity to take over. This initiates a collapse.

Nonetheless, this collapse can be countered by the Pauli Exclusion Principle, introduced by Wolfgang Pauli in 1925. This principle states that no two electrons in an atom can share the same quantum numbers, meaning that as atoms are compressed within a collapsing star, some electrons must transition to higher energy states to coexist, generating what is termed "electron degeneracy pressure." In stars that lack sufficient mass to exert a strong gravitational pull, this pressure is adequate to halt collapse.

However, in supermassive stars, gravity's force is strong enough to overpower electron degeneracy pressure. Electrons gain enough energy to rearrange, forcing matter into an incredibly dense core. Electrons, being negatively charged, combine with protons (positively charged) to form neutrons (neutral). Neutrons also adhere to the Pauli Exclusion Principle and contribute to "neutron degeneracy pressure," which requires substantial energy for rearrangement and may be sufficient to prevent collapse. For neutron stars, this is where the collapse stabilizes. Should a star possess enough mass to surpass neutron degeneracy pressure, the core will continue to collapse, forming a black hole in mere moments, with its mass crossing the event horizon.

In neutron stars, the outer layers collapse inward, collide with the dense neutron core, and rebound, creating a shockwave that ignites the outer layers and results in a supernova, rivaling the brightness of entire galaxies. The core is left solitary, giving rise to a neutron star.

Once the outer layers have been expelled, the neutron star becomes a tightly packed sphere of neutrons, with minimal protons and electrons remaining. The two primary forces at play in this environment are the strong nuclear force, which binds protons to neutrons and quarks to quarks but operates only over very short distances, and the Coulomb force, responsible for the repulsion or attraction between particles based on their electrical charges. Under less extreme conditions, the Coulomb force can keep nuclei apart due to their positive charges, thereby preventing the strong nuclear force from functioning. However, within a neutron star, these two forces reach a balance, resulting in unique matter configurations.

On the neutron star's surface, typical nuclei can exist, as the gravitational pressure isn't strong enough to override the Coulomb force. Elements like iron can be found in their usual state. Yet, as one descends below the surface, pressure increases significantly. Researchers have 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).” Still, the overwhelming gravitational pressure can overcome the Coulomb force, allowing the strong nuclear force to dominate, trapping particles in a neutron star between these powerful forces.

Just beneath the surface, nuclei are compressed into large, closely packed groups that take on semi-spherical shapes resembling the Italian dish "gnocchi." Studies indicate that these initial formations are resilient enough to navigate through the neutron star's layers with minimal disruption. These shapes are recognized as the first manifestations of "pasta" that can endure to relatively deep layers.

Deeper down, the "gnocchi" morph into a neutron-rich environment. Here, the forces intensify, and the "gnocchi" are elongated into rod-like shapes akin to "spaghetti." If these shapes descend further, the pressure increases enough to bond the "spaghetti" into sheets that first resemble "waffles" and then flatten into "lasagna noodles." Continuing deeper, the pressure can fuse the ends of the "lasagna noodles," creating hollow formations reminiscent of "bucatini." Beyond this point, the structural integrity of the pasta fails, likely disassembling particles into their fundamental quarks.

Researchers have categorized these four stages as: (P1) nearly spherical nuclei represent the minimum energy configuration, while pasta appears as local minima; (P2) pasta phases become the minimum energy configuration, yet spherical nuclei persist in some local minima; (P3) all local minima correspond to pasta configurations, with protons localized in at least one dimension; and (P4) all local minima correspond to pasta configurations, where the appearance of the BCP phase indicates protons are delocalized across all dimensions.

While nuclear pasta has yet to be directly observed, simulations suggest it possesses a strength 10 billion times that of steel, making it the strongest known material in the universe. Unfortunately, its potential applications remain unattainable.

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Originally published at http://thehappyneuron.com on October 9, 2021.