Beginner

Scientists have discovered something amazing about the most powerful particles in the universe. These particles are called cosmic rays. They travel through space and sometimes hit Earth's atmosphere. Scientists have been studying them for a long time, but they were not sure what they are made of.

A team from Penn State University in the United States studied the cosmic rays very carefully. They found that the strongest cosmic rays may be made of very heavy atoms. Iron is a heavy metal that you might know from everyday life. These cosmic rays might be made of atoms even heavier than iron.

Why does this matter? Atoms heavier than iron lose energy more slowly as they travel through space. This means they can travel farther without slowing down. If the particles are very heavy, they must come from very powerful places in the universe, like exploding stars or crashing neutron stars.

Scientists use large detector systems on Earth to study cosmic rays. One famous detector is in Argentina and is called the Pierre Auger Observatory. Another is in the state of Utah in the United States. These machines help scientists learn more about the most energetic things in the universe.

cosmic ray

a very fast-moving particle from space that travels through the universe and sometimes hits Earth

particle

a very tiny piece of matter, much smaller than what you can see with your eyes

atom

the smallest unit of a chemical element, made up of a nucleus and electrons

iron

a common heavy metal element with the symbol Fe, found in Earth's core and in stars

nucleus

the heavy central part of an atom, made of protons and neutrons

neutron star

an extremely dense star that forms when a massive star collapses, packing a huge amount of matter into a tiny space

detector

a scientific instrument designed to sense and measure particles or radiation that cannot be seen by the human eye

energy

the ability to do work or cause change; in physics, particles carry energy as they move

Elementary

A research team led by Penn State University has published new findings about ultrahigh-energy cosmic rays, the most energetic particles ever detected, in the journal Physical Review Letters. The study suggests that these extreme particles may be made of atomic nuclei heavier than iron, which would be a significant revision to what scientists previously assumed about their composition.

Cosmic rays are subatomic particles that travel through the universe at speeds very close to the speed of light. While lower-energy cosmic rays come from familiar sources such as the Sun, the origin of the most energetic ones, which carry energies a million times greater than anything produced by particle accelerators on Earth, has been a major unsolved mystery in astrophysics for decades.

The Penn State team's calculations show that heavier nuclei lose energy more slowly than protons or lighter nuclei as they travel through the vast distances of intergalactic space. This process, called energy loss during propagation, means that very heavy particles can travel greater distances and still arrive at Earth with enough energy to be classified as ultrahigh-energy. If the particles are indeed that heavy, scientists can narrow the list of possible source environments, since only the most extreme astrophysical events can accelerate such heavy nuclei to those energies.

Scientists believe the likely candidate sources include colliding neutron stars and hypernovae, which are the explosions of very massive stars at the end of their lives. A separate study from the Max Planck Institute for Physics, published in February 2026, independently pointed to M82, a star-forming galaxy roughly 12 million light-years from Earth, as a possible production site. The two studies together add weight to the idea that ultrahigh-energy cosmic rays come from violent, rare cosmic events rather than from an evenly spread background of ordinary sources.

ultrahigh-energy cosmic ray

a cosmic ray carrying an energy above 10 to the power of 18 electron volts, far exceeding anything produced in particle accelerators on Earth

subatomic particle

a particle smaller than an atom, such as a proton, neutron, or electron

composition

what something is made of; the types and amounts of its component parts

intergalactic space

the enormous, mostly empty region of space between galaxies

propagation

the way a particle or wave travels through a medium or through space over distance

hypernova

a particularly powerful supernova explosion from a very massive star, producing an enormous burst of energy and heavy elements

light-year

the distance that light travels in one year, approximately 9.5 trillion kilometers, used to measure vast distances in space

astrophysics

Intermediate

A Penn State-led research team has published a study in Physical Review Letters arguing that the highest-energy cosmic rays detected at Earth, those exceeding the so-called GZK threshold of roughly 5 times 10 to the power of 19 electron volts, may consist of atomic nuclei heavier than iron, including elements from cobalt to uranium and beyond. Previous composition analyses based on data from the Pierre Auger Observatory in Argentina had suggested a trend toward intermediate-mass nuclei such as nitrogen or silicon at the highest energies, but the Penn State calculations use updated cross-section data and a revised intergalactic background photon model to extend that trend toward the superheavy regime.

The key physical argument rests on how different nuclei lose energy through what physicists call the GZK mechanism. When an ultrahigh-energy proton or nucleus collides with photons of the cosmic microwave background radiation, it loses energy through pion production, a process that sets a natural maximum range, around 160 million light-years for protons, on where the most energetic cosmic rays can originate. Heavier nuclei undergo a related process called photodisintegration, where CMB photons strip individual nucleons from the nucleus. Critically, the energy loss per nucleon during photodisintegration is lower for heavier nuclei when expressed in terms of total kinetic energy, allowing superheavy primary nuclei to survive intergalactic propagation from sources at distances beyond the classical GZK horizon for protons.

If the superheavy hypothesis is correct, the population of candidate source environments shrinks dramatically. Accelerating a nucleus of cobalt-59 or uranium-238 to 10 to the power of 20 electron volts requires a source region with both an extremely strong magnetic field, above one microgauss, and a high enough turbulence level to confine the particle long enough to gain sufficient energy through repeated diffusive shock acceleration. The astrophysical objects that plausibly satisfy these constraints at extragalactic distances include the merger sites of neutron star binary systems, the jets of gamma-ray bursts, and the actively star-forming cores of starburst galaxies such as M82, which was independently identified by a Max Planck Institute team in February 2026 as a directional correlate for a subset of Auger events.

The observational community faces a measurement challenge: current ground-based arrays such as Auger and Telescope Array infer nuclear mass composition only indirectly, through the depth of shower maximum in the atmosphere, which depends on the hadronic interaction model used to simulate air showers. Systematic uncertainty in proton-air inelastic cross sections at energies above 100 TeV in the center-of-mass frame propagates directly into composition inference, meaning the superheavy interpretation cannot yet be distinguished from an intermediate-mass composition with high-cross-section hadronic physics. The AugerPrime upgrade, deploying surface scintillators and radio antennas above the existing water-Cherenkov detectors, is expected to resolve the composition ambiguity by providing complementary muon-count and electromagnetic-to-muonic ratio measurements at each shower.

Advanced

The Penn State study, published in Physical Review Letters and amplified widely in late May 2026, extends the compositional inference of ultrahigh-energy cosmic rays beyond iron into the superheavy regime by combining updated photonuclear cross-section compilations with a revised extragalactic background light model derived from the latest Fermi-LAT blazar stacking analyses. The core result is that, under the photodisintegration energy-loss parameterization of Allard et al. as updated for heavier species, primary nuclei with mass number A above 56 can survive propagation from sources at redshifts z between 0.003 and 0.05, corresponding to 14 to 230 megaparsecs, while arriving at Earth with energies above 6 times 10 to the 19 electron volts. This places the GZK-suppressed flux at the Auger and Telescope Array energy threshold attributable to sources within the Local Volume, narrowing the pool of plausible accelerators to the most energetic subclass of extragalactic objects.

The theoretical accelerator physics constraining superheavy UHECR production is governed by the Hillas criterion: the maximum energy achievable by diffusive shock acceleration scales as the product of the nucleus charge number Z, the shock region size R in parsecs, and the magnetic field strength B in microgauss. For a cobalt-27 nucleus to reach 10 to the 20 electron volts, the source must satisfy ZBR above approximately 4.3 times 10 to the 6 microgauss-parsecs, a criterion met only by the most compact, magnetically dominant acceleration zones: the termination shocks of gamma-ray burst jets, the millisecond magnetar wind nebulae that emerge from binary neutron star mergers, and the nuclear starburst regions of compact starforming galaxies. The M82 correlation reported by the Max Planck team provides the first observationally motivated directional anchor consistent with the superheavy composition hypothesis, though the statistical significance remains at approximately 3.6 sigma pre-trial-correction at the Auger energy bin above 8 times 10 to the 19 electron volts.

Composition disambiguation at the highest energies requires breaking the degeneracy between shower-maximum depth, which is well-measured by Auger's fluorescence telescopes, and the hadronic interaction model, which introduces systematic uncertainties of order 20 to 30 grams per square centimeter in the Xmax scale from EPOS-LHC versus SIBYLL 2.3d versus QGSJet-II.04. The AugerPrime upgrade addresses this via the surface scintillator detector, which measures the muon content of air showers independently of the fluorescence measurement. Muon production scales approximately as A to the power of 0.9 in the superposition model, meaning superheavy primaries produce substantially more muons than proton showers at equivalent total energy, providing a discriminating observable that is insensitive to the hadronic cross-section systematics that degrade Xmax-only analyses. Cross-calibration with the Telescope Array Low Energy Extension and the planned GRAND radio array will further constrain the composition transition region between 10 to the 18 and 10 to the 19 electron volts where the galactic-to-extragalactic handoff is expected.