Cosmic rays come from three main sources, depending on their energy: the sun, exploding stars within our galaxy, and powerful objects like black holes in distant galaxies. About 90% of cosmic rays are protons (hydrogen nuclei), 9% are helium nuclei, and the remaining 1% are heavier elements, ranging all the way up to lead. These aren’t “rays” at all but high-speed atomic nuclei stripped of their electrons, along with smaller numbers of electrons and neutrinos.
What makes cosmic rays fascinating is that particles arriving at Earth span an enormous range of energies, and each energy range points back to a different type of source. Sorting out which objects produce which cosmic rays has taken decades, and some of the highest-energy particles still defy explanation.
Low-Energy Cosmic Rays From the Sun
The sun is the closest and most familiar source of cosmic rays, though the particles it produces are relatively low in energy compared to those from deeper in the galaxy. Solar energetic particles come in two flavors, tied to different types of solar eruptions.
Impulsive events are fast and brief, linked to solar flares. When magnetic field lines in the sun’s atmosphere snap and reconnect, they release bursts of electrons and ions in minutes. These events produce unusual ratios of certain helium types and come from regions where temperatures reach about 30 million degrees Celsius. Gradual events last longer and are driven by coronal mass ejections, massive clouds of solar material that plow through space and create shockwaves. These shocks are especially good at accelerating protons to high speeds and produce particle abundances that mirror the composition of the sun’s outer atmosphere, at temperatures around 3 million degrees.
Both types of events share a common underlying engine: magnetic reconnection, where tangled magnetic fields release enormous amounts of stored energy. The key difference is scale. Flares accelerate particles in compact, intense bursts, while coronal mass ejections drive shocks across millions of kilometers, sweeping up and energizing particles over hours or days.
Supernova Remnants: The Galaxy’s Particle Accelerators
Most cosmic rays that reach Earth at moderate energies originate inside our own galaxy, and the leading candidates for producing them are supernova remnants. When a massive star explodes, its outer layers blast outward at thousands of kilometers per second, driving a shockwave into the surrounding gas. That shockwave acts as a natural particle accelerator through a process called diffusive shock acceleration.
The mechanism works like a cosmic ping-pong game. A charged particle crosses the shock front, gets deflected by turbulent magnetic fields on the other side, bounces back across the shock, and gains a small energy boost each time. After thousands of crossings, the particle reaches extraordinary speeds. The theory predicts that particles accelerated this way follow a characteristic energy pattern: a power-law distribution where many particles end up at lower energies and progressively fewer reach the highest energies. For a strong shock (which compresses gas by a factor of four), this distribution has a specific mathematical slope that matches what we actually observe in the cosmic ray spectrum.
Not every particle near a supernova shock gets accelerated. A proton needs to already be moving fast enough, roughly three to five times the typical thermal speed of surrounding particles, to get caught up in the acceleration process. The same threshold applies to heavier nuclei, scaled by their charge and mass. This injection requirement helps explain why certain elements are over- or under-represented in cosmic rays compared to their abundance in ordinary matter.
The Knee, the Ankle, and What They Reveal
If you plot the number of cosmic rays arriving at Earth against their energy, the result is a steeply falling curve, but it’s not perfectly smooth. Two subtle bends in the curve carry major clues about where cosmic rays originate.
The first bend, called the “knee,” appears at energies around 3 × 10¹⁵ electron volts. Below this energy, supernova remnants in our galaxy can comfortably produce the particles we observe. Above it, the spectrum steepens, meaning fewer particles arrive than a simple extrapolation would predict. The composition also shifts: cosmic rays above the knee are progressively heavier nuclei, consistent with the idea that galactic accelerators are reaching their maximum capability. Lighter particles (protons) drop out first because they’re harder to confine in magnetic fields, while heavier nuclei can be pushed to somewhat higher energies before they, too, escape the galaxy.
The second bend, the “ankle,” occurs near 3 × 10¹⁸ electron volts. Here the spectrum flattens slightly, and the composition shifts back toward lighter particles. This is widely interpreted as the point where extragalactic cosmic rays begin to dominate over the fading galactic population. Between the knee and the ankle, additional features appear: a spectral hardening around 2 × 10¹⁶ electron volts and a further steepening near 10¹⁷ electron volts, where the heaviest galactic component finally bends downward.
Extragalactic Sources and Ultra-High-Energy Particles
The most energetic cosmic rays, those above the ankle, almost certainly come from beyond our galaxy. No known object in the Milky Way can accelerate particles to 10²⁰ electron volts, yet such particles have been detected. The leading candidates are active galactic nuclei (especially blazars, galaxies with jets of material pointed almost directly at Earth), gamma-ray bursts, and colliding galaxy clusters.
One constraint on these sources is the GZK cutoff, named after three physicists who predicted it in the 1960s. Protons traveling above a few times 10¹⁹ electron volts collide with photons left over from the Big Bang and lose energy rapidly. This interaction effectively limits how far the most energetic cosmic rays can travel. Above roughly 100 × 10¹⁸ electron volts, the “visible universe” for cosmic rays shrinks from billions of light-years to less than about 65 million light-years. Sources farther than roughly 300 million light-years become invisible at any energy when magnetic field effects are included. This means the ultra-high-energy cosmic rays we detect must come from relatively nearby extragalactic objects.
In 2023, the Telescope Array experiment in Utah detected a particle nicknamed “Amaterasu” (after the Japanese sun goddess) with an energy rivaling the famous “Oh-My-God” particle detected in 1991, the single most energetic cosmic ray ever observed. The puzzle: Amaterasu’s trajectory traces back to the Local Void, a vast, nearly empty region of space on the fringes of the Milky Way where no obvious source exists. One hypothesis points to a blazar called PKS 1717+177, located within a few degrees of the particle’s reconstructed arrival direction, as a possible origin. But the mystery remains open, and particles like Amaterasu are exactly why the field keeps searching.
How Scientists Trace Cosmic Rays to Their Sources
Pinpointing where cosmic rays come from is uniquely difficult because they’re electrically charged. As they travel through space, magnetic fields in the galaxy and between galaxies bend their paths, scrambling their arrival directions like a funhouse mirror. By the time a cosmic ray reaches Earth, it generally no longer points back to where it started.
To work around this, scientists use two complementary strategies. The first is building enormous ground-based detector arrays. When a high-energy cosmic ray hits the upper atmosphere, it triggers a cascade of billions of secondary particles called an extensive air shower. Arrays of detectors spread across hundreds or thousands of square kilometers measure the timing and density of those secondary particles at ground level. The Telescope Array in Utah, for example, uses 507 plastic detectors spaced 1.2 kilometers apart, covering about 700 square kilometers, to catch showers from cosmic rays above 10¹⁹ electron volts. Other observatories detect the faint ultraviolet glow or radio emissions that air showers produce as they develop in the atmosphere. By combining measurements from multiple detectors, researchers reconstruct the energy, arrival direction, and approximate mass of the original particle.
The second strategy uses neutrinos as messengers. The same violent environments that accelerate cosmic rays also produce neutrinos when lower-energy cosmic rays collide with gas or light near the source. Unlike charged cosmic rays, neutrinos travel in straight lines because they have no electric charge and barely interact with matter. The IceCube Neutrino Observatory at the South Pole, the Pierre Auger Observatory in Argentina, and the Telescope Array have run joint analyses looking for directional correlations between high-energy neutrinos and ultra-high-energy cosmic rays. Two out of three of these analyses have found hints of overlap, suggesting that both types of particles may come from the same astrophysical objects. Newer methods account for how galactic magnetic fields would deflect cosmic rays, essentially “unscrambling” their paths to search for common source points.
Why Some Origins Remain Unknown
Despite decades of progress, no single source has been definitively confirmed as the origin of the highest-energy cosmic rays. The magnetic deflection problem makes it impossible to simply “point back” along a particle’s path with certainty. The rarity of ultra-high-energy events compounds the problem: the most energetic cosmic rays arrive at a rate of roughly one particle per square kilometer per century, meaning even the largest detectors collect only a handful per year.
What scientists have established is a broad framework. The sun produces the gentlest cosmic rays. Supernova remnants in the Milky Way account for the bulk of particles up to and somewhat beyond the knee. Extragalactic sources, likely involving supermassive black holes, take over above the ankle. The gaps in this picture, particularly at the highest energies, are where the next generation of observatories and multi-messenger techniques will focus their efforts.