National Aeronautics and Space Administration

From the "Binding Energy Per Nucleon" chart (Figure 4) we see that moving from hydrogen (¹H) to helium (⁴He) creates a more stable nucleus, but moving from helium (He⁴) to lithium (⁷Li) does not create a stable nucleus. In fact, the next element with a more stable nucleus than helium is carbon (¹²C). The nuclei between helium and carbon are much less stable, and thus they are rarely produced in stars. So what is the origin of lithium, beryllium, and boron? As it turns out, some of the heavy elements, including lithium, beryllium and boron, are produced from cosmic ray interactions.

Cosmic rays are high-energy particles traveling throughout our galaxy at close to the speed of light. They can consist of everything from tiny electrons to nuclei of any element in the periodic table. Scientists first observed evidence of high speed particles entering our atmosphere in 1929, although their exact nature was unknown. They were initially dubbed "cosmic rays" due to their mysterious origin, since scientists believed these particles were high energy photons from outer space.

Today, scientists know that cosmic rays are atoms and subatomic particles that are accelerated to near the speed of light, most likely by supernova explosions. The particles may have originally been part of an exploding star, or they may have been atoms of material near the star in open space. The shock wave from the supernova explosion picks up these particles and accelerates them to high speeds, sending them zinging across the galaxy. They may travel for thousands or millions of years without hitting anything, since space is relatively empty.

Figure 6: A collision between a cosmic ray and an atom, showing the resulting fragmentation.

When cosmic rays hit atoms, they produce new elements. During its journey across the galaxy, a cosmic ray may hit an atom of hydrogen or helium in interstellar space. Since the cosmic ray is traveling so fast, it will hit with great force, and part of its nucleus can be "chipped off." For example, the nucleus of a carbon atom in the outer layers of a large star may be accelerated to near light speed when the star explodes as a supernova. The carbon nucleus (which we now call a cosmic ray) flies through space at a high speed. Eventually, it collides with a hydrogen atom in open space. The collision fragments the carbon nucleus, which creates two new particles: helium and lithium.

This same process can happen for all elements. Since lithium, beryllium, and boron are small atoms, they are more likely to be formed in cosmic ray collisions.

Lithium provides an interesting special case. Of the two stable isotopes of lithium, ⁷Li is more abundant. We know from the Big Bang theory that some of it was created shortly after the Big Bang. The amount produced is small, however, and does not account for all the lithium we can see today. A larger portion is made in the Asymptotic Giant phase of a small star's lifetime, and some is made is supernova explosions. The other isotope, ⁶Li, however is made only via cosmic ray interactions.