The hypothetical matter that neutron stars' cores may contain, including strange quarks

A look inside neutron stars, which are thought to contain nuclear pasta

Simulations show that beneath the outer crust of these objects, the gravitational force is so extreme that atomic nuclei link together into chains of primarily neutrons and some protons called nuclear spaghetti. Deeper in the star, these links combine to form sheets—nuclear lasagna—a quintillion times stronger than steel, making it the strongest material in the universe.

Once called 'aces,' quarks are fundamental pieces of matter that combine to create over 100 known composite particles, including the proton and neutron.

Within ALICE—A Large Ion Collider Experiment—at the Large Hadron Collider, scientists have created quark-gluon plasma—an exotic form of matter present in the moments after the Big Bang.

Why quantum mechanics keeps gravity from collapsing neutron stars into black holes

According to the Pauli exclusion principle, certain particles, such as neutrons, are forbidden from occupying identical states within a system. Like tennis balls tightly packed together, neutrons exert a resistive pressure against this compression—neutron degeneracy pressure—which can help prevent gravitational collapse. (Some readers may experience a paywall.)

Neutron stars, the remains of some massive stars after they explode in a supernova, are the densest directly observable objects in the universe.

How a quantum quirk allowed neutral atoms to form soon after the Big Bang

At the beginning of the universe, high-energy photons prevented atomic nuclei from combining with electrons to form neutral atoms. Rather than waiting for the universe to expand and cool completely, a rare quantum phenomenon—a two-photon transition—allowed neutral hydrogen atoms to form sooner, expediting the formation of stars, planets, and life.

The strong nuclear force, which keeps atomic nuclei intact, is mediated by gluons—massless particles that bind together the quarks that make up protons and neutrons.

Fritz Zwicky predicted the existence of dark matter and neutron stars

The former resulted from analyzing the motion of galaxies in the Coma Cluster, a subset of the approximately 30,000 galaxies he cataloged during his life. When hypothesizing an origin for observed high-energy particles called cosmic rays, Zwicky and colleague Walter Baade thought they could come from exploding stars, or supernovae—a term he coined—which would leave behind a collapsed star made entirely of neutrons.

While atoms are normally made of protons, neutrons, and electrons, exotic ones have been built with antimatter—alternate versions of normal matter that live for fractions of a second.

The formation of a neutron star through a core collapse lasts one-tenth of a second

Massive stars keep fusing heavier and heavier elements in their cores to produce energy that pushes against the weight of the star itself. However, the fusion of silicon into iron consumes energy and initiates a feedback loop that rapidly compresses the stellar core, creating neutrons and neutrinos from the combination of electrons and protons.

Quarks are elementary particles that come in six varieties and combine together to form larger, composite particles, such as the proton and the neutron.

Smaller than atoms—what are quarks?

One of humanity’s eternal questions surrounds what we are fundamentally made of. Many ancient philosophies believed in a set of classical elements: from water, air, fire and earth of ancient Greeks; to…

How neutron stars serve as laboratories for testing Einstein's theory of gravity

Data from a 16-year study of an orbiting pair of spinning neutron stars showed that measurements of gravitational wave energy and the bending of light around the system were consistent with predictions made by general relativity to an accuracy of at least 99.99% across seven tests. (Some readers may experience a paywall.)

Emergent quantum phenomena can produce nonfundamental quasiparticles

While physicists have discovered new particles by colliding known ones at high speeds, placing known particles in extreme environments or configurations can allow quantum mechanics to produce bizarre behaviors, making them seem like new types of matter.

How gold, platinum, and many rare earth elements form during neutron star mergers

While massive stars naturally fuse elements as heavy as iron, an analysis of gravitational waves from the collision of two neutron stars revealed that more massive elements are created in this environment via rapid neutron capture—the r-process. This phenomenon sees existing iron nuclei absorb plentiful free neutrons, many of which decay into protons, creating new elements.

Why starquakes may explain various phenomena associated with neutron stars

Fast radio bursts—repeated millisecond-long pulses of enormous energy from neutron stars—are emitted with a timing that resembles the earthquakes and aftershocks seen on Earth when geological activity releases energy from within the planet. Although pulsars should gradually spin more slowly as they lose energy from emitting jets of radiation, sudden increases may result from interior matter moving during starquakes.

How neutron stars resemble Earth, including harboring an atmosphere and 'mountains'

These stars maintain an atmosphere of hydrogen, helium, and carbon at approximately 2,000,000 degrees Celsius (3,600,000 degrees Fahrenheit), only a few thousandths of a millimeter thick, which envelops "mountains" about a millimeter high. Its interior consists of an inner and an outer crust surrounding inner and outer cores, all held together by enormous gravitational forces despite spinning multiple times a second.

Antimatter's rarity across the cosmos is one of the biggest problems in modern physics because the high-energy environment of the early universe should have produced equal amounts of matter and antimatter.

The Higgs boson is a quanta, or discrete piece, of the Higgs field, which gives elementary particles, such as quarks and electrons, mass as they travel through it.

What does quark-gluon plasma have in common with tap water?

Quark-gluon plasma is a state of matter that formed shortly, very shortly, after the big bang. At one time filling the universe, scientists describe quark-gluon plasma as being similar to tap water, but only in the manner that such matter flows. Incredibly hot upon its emergence, it swiftly cooled and began forming the early building blocks of the universe.

Antimatter particles possess the same mass and spin as their normal particle counterparts, but possess an opposite charge and transform into pure energy upon colliding with normal matter.

View a step-by-step breakdown of how helium is forged in stars alongside antimatter

During the proton-proton chain, four hydrogen nuclei—protons—are fused together to create an alpha particle—a helium nucleus—and produce the energy that supports a star's own weight. Through radioactive decay, two of these protons become neutrons, with each releasing an antielectron, or positron, and an electron neutrino.

How Robert Oppenheimer determined when neutron stars become black holes

Building on the work of Richard Tolman and George Volkoff, Oppenheimer reasoned that, once massive enough, a hypothetical, star-sized atomic nucleus would be unable to stave off gravitational collapse with neutron degeneracy—quantum pressure—alone. The mathematical model estimated the maximum mass of a non-rotating neutron star, known as the Tolman-Oppenheimer-Volkoff limit.

String theory replaces black hole singularities with fuzzballs of tangled strings

Black holes contain singularities where information about the material that fell in is lost, violating quantum mechanics. Instead, modeled as a bundle of strings, information is stored and slowly released as the black hole evaporates.

Neutron star collisions may be the universe's gold factories

In his last broadcast interview, Stephen Hawking comments on detecting gravitational waves from the collision of neutron stars. The event may serve as a new means of measuring cosmological distances and help identify revisions to general relativity.

Dark stars are hypothetical stars that would have existed in the early universe

Primarily composed of visible matter, this celestial object would also contain a significant amount of dark matter that would annihilate with itself, converting its mass into energy. This energy would prevent the star from collapsing under its own gravity and from triggering nuclear fusion until its dark matter fuel was exhausted.

Attempts to 'fix' black hole physics have led scientists to theorize new types

Along with their spin and charge, mass is used to categorize black holes as supermassive, intermediate, or stellar-mass. However, ongoing research suggests various additional types may exist, including primordials that formed one second into the universe's birth, fuzzballs—tangles of vibrating strings—and gravastars, which have a shell of dense matter around repulsive vacuum energy.

Axions, a hypothetical subatomic particle, are a leading dark matter candidate

Among the possibilities for what makes up dark matter, some physicists champion axions, which could clump into galaxy-scale quantum states. These states would act as massive, unified superparticles that could explain the observational evidence for the universe's missing matter.

The one-electron theory describes the universe as having only one electron

The thought experiment explains why every electron has the same charge and mass as every other electron, and describes every positron—the antimatter version of the electron—as that same electron traveling backwards in time. Among other foundational challenges, the model suggests that the universe contains equal numbers of electrons and positrons, yet normal matter is much more common than antimatter.

If particle accelerators did create black holes by packing tremendous energy into a small volume, then trillions would be created in Earth's atmosphere daily by cosmic rays.

The deceptively simple proton

Take a visual deep-dive into the proton, a near-fundamental—but deceivingly simple—building block of nature. Among other beguiling characteristics, the particle can't be described by its constituent particles (quarks), but rather exists in a...

The universe might have emerged from a quantum fluctuation—not absolute nothing

Traditional physics breaks down at the moment of the Big Bang, but quantum theory suggests that “empty” space is never truly empty. In 1973, physicist Edward Tryon proposed that our universe began from a quantum energy blip in the quantum vacuum.

Quasars are extremely luminous cores of distant galaxies, powered by supermassive black holes that consume as much as 100 Earth masses of gas and dust per minute.

View a ranking of the masses of fundamental particles, from the massless photon to the top quark, and a timeline of their discoveries.

After photons—particles of light—neutrinos are the most abundant particles in the universe.

An explanation of the weak nuclear force, which can induce some forms of radioactive decay

While the strong nuclear force holds the protons and neutrons together in an atomic nucleus, despite the electromagnetic force pushing protons apart, the weak nuclear force can change the identity, or flavor, of the quarks that make up these particles. In doing so, protons can be converted into neutrons, and vice versa, altering the stability of atomic nuclei.

The graviton may be the last missing piece of the Standard Model

While the electromagnetic, strong nuclear, and weak nuclear forces can be explained through interactions with particles, gravity is instead best modeled through spacetime curvature. Discovering a quantum particle of gravity would unite the two pillars of modern physics—quantum mechanics and general relativity—toward developing a theory of everything.

Quantum field theory, which has made predictions accurate to 12 decimal places, explained

Instead of being made of indivisible particles, all visible matter in the universe is built from underlying quantum fields—smooth, invisible entities that behave like waves and extend throughout space and time. The theory suggests these fields are fundamental, and particles are disturbances that move through them.

The Standard Model reveals an underlying organization to the universe

Using the framework of quantum field theory, it structures all known particles into 12 matter particles, alongside five particles that carry three of the four fundamental forces. Although remarkably precise and predictive, it cannot yet explain gravity, dark matter, or dark energy.

Some particles that mediate fundamental forces in nature are their own antiparticles, such as photons—particles of light and mediators of the electromagnetic force.

As of 2026, 75 of the 86 particles discovered by the Large Hadron Collider have been found by the LHCb experiment—a detector that explores the decay of particles made of beauty and charm quarks.

Some theorists have suggested that dark matter consists of supersymmetric particles—massive, hypothetical versions of known particles, such as the sneutrino, which is the supersymmetric partner of the neutrino.

The cosmic dark ages: before stars existed

The theories behind the Big Bang suggest the process of the universe beginning to the creation of stable, neutral atoms took just half a million years. But the transition from such atoms to star formation took surprisingly much longer, upwards of 100 million years. Why? This deep dive explores the science behind the unique physical composition of the so-called cosmic dark ages.

Nuclear fusion, where the nuclei of two atoms are combined, can produce near-limitless clean energy, but requires extreme conditions to kickstart because of the natural repulsion of atomic nuclei.

In 1997, the Compton Gamma Ray Observatory identified 'positron geysers'—fountains of antimatter particles—emerging from near the center of the Milky Way.

Neutron star types, categorized by the presence of strong magnetic fields and radiation beams

Magnetars have magnetic fields about 1,000 times stronger than the average neutron star—roughly a trillion times stronger than Earth's. Pulsars emit beams of radiation from their magnetic poles, which appear as flickering radio wave signals as the object spins.

Build atomic nuclei to explore their instability and resulting radioactive decay

In this simulation, users can add protons and neutrons to create an atomic nucleus, view its half-life and decay options, and identify patterns in how specific combinations of particles create extremely stable nuclei, from which the nuclear shell model of stability emerged.

A beam of neutrinos can be created by firing a beam of high-energy protons at a target to produce unstable particles that decay into neutrinos.

What is a neutron star?

Neutron stars are born in supernova explosions. They are stars with a couple of times our sun’s mass, squeezed into a sphere the size of an earthly city.

The deaths of massive stars and the mergers of neutron stars and black holes can produce gamma-ray bursts—jets of particles moving in opposite directions at near-light speed that comprise the universe's most powerful explosions.