Expert: Courtney Seligman - 2/7/2011

Question
The sun fuses hydrogen to make helium on the sun. Does the helium fuse with other helium or hydrogen to make lithium or beryllium? If not, why not?

Answer
Lithium is produced in the Sun (and other stars and, for that matter, in the Big Bang); but it has a low bonding energy, and is easily broken down into smaller nuclei, so on the average it is destroyed as fast as it is formed, and the total amount present at any given time is too small to be of much importance. So you just don't read much about it, because it is a very minor player, so to speak.

Beryllium can be formed by the fusion of helium nuclei, but both beryllium 7 and beryllium 8 are very unstable, and decay right back into the particles that made them. However, at temperatures exceeding a hundred million Kelvins, a helium 4 nucleus may collide with a beryllium 8 nucleus before it has a chance to decay, with the resulting formation of a carbon 12 nucleus. This is the basis of the "triple alpha" process, which temporarily powers the cores of red giants, and creates the carbon so essential to life as we know it.

All other forms of beryllium are produced by "spallation" reactions -- the breakdown of heavier nuclei by collisions with extremely high energy particles, such as cosmmic rays. Most of the beryllium isotopes produced in this way are also unstable, but beryllium 10 has a half-life of a million and a half years, and accumulates in the soil in amounts proportional to the average cosmic radiation during the previous million or so years. Only beryllium 9 is stable, and gradually builds up over time in space, or at the surface of the Earth. However, it cannot form in stars, because by the time they could contain large amounts of heavier nuclei and have temperatures high enough to create beryllium 9 by spallation, the star has already blown itself to bits in a supernova explosion.

So the answer is that the relative instability of lithium isotopes and the almost total instability of lower-mass beryllium isotopes makes the production of lithium and beryllium in stars too insignificant to be concerned with under most circumstances.