Astronomy
Cosmic Ray Spallation and the Nucleosynthesis of Light Elements
Quick fact
Almost all of the universe's beryllium and boron, and about half of its lithium, are created when high-energy cosmic rays slam into the nuclei of carbon, nitrogen, and oxygen atoms in space, shattering them into lighter fragments—a process called cosmic ray spallation.
Why this is interesting
You've heard that stars forge the elements we see around us, but some of the lightest elements in your smartphone's lithium battery were not born in stars at all. Where do they come from?
Read the full explanation
Understanding Cosmic Ray Spallation and the Nucleosynthesis of Light Elements
Stars fuse lighter elements into heavier ones in their cores, but they have a hard limit: elements heavier than iron require more energy to fuse than they release. For very light elements like lithium, beryllium, and boron, stars actually destroy them rather than create them—they are consumed in stellar furnaces. So where do these elements come from? The answer lies in the violent ballet of cosmic rays. Cosmic rays are high-energy particles—mostly protons and alpha particles—that zip through space at near-light speeds. They can originate from supernova explosions, active galactic nuclei, or even the Sun. When these speedy particles crash into the interstellar medium—the sparse gas and dust between stars—they smash against heavier nuclei like carbon (12C) or oxygen (16O). The impact is so energetic that the nucleus literally shatters into two or more fragments, a process called spallation. These fragments include the very elements we're missing: lithium, beryllium, and boron. So while stars are the cosmic blacksmiths for many elements, the light trio of Li, Be, and B are forged in the cosmic wrecking yard of high-speed collisions.
A deeper explanation
Cosmic ray spallation is a type of nucleosynthesis that occurs through the fragmentation of atomic nuclei when they are struck by high-energy cosmic rays. The cosmic rays themselves are primarily protons and alpha particles, accelerated to relativistic energies by shock waves in supernova remnants. When a cosmic ray strikes a 'target' nucleus—typically a carbon (12C), nitrogen (14N), or oxygen (16O) atom in the interstellar medium—the collision has enough energy to break apart the nucleus. The result is a set of lighter nuclei, including lithium-6, beryllium-9, and boron-10, among others. This mechanism is unique because it does not require the extreme temperatures and pressures found in stellar cores. Instead, it relies on the kinetic energy of the cosmic ray. The efficiency of spallation depends on the flux of cosmic rays, the density of the interstellar medium, and the abundance of target nuclei. Over the history of the galaxy, the cumulative effect of spallation has produced most of the beryllium and boron and a significant portion of lithium-6 and lithium-7. This process explains why the abundances of these light elements are much higher than would be predicted from Big Bang nucleosynthesis and stellar sources alone. It also explains the characteristic isotopic ratios observed, such as the lack of beryllium-9 in stars, because beryllium is destroyed in stellar interiors. Thus, cosmic ray spallation is an essential contributor to the chemical composition of the galaxy, linking the life cycles of massive stars, supernova explosions, and the composition of the interstellar medium.