How the Universe Synthesized JWST's Beryllium

"If you wish to make a apple pie [next-generation space telescope] from scratch, you must first invent the universe" 
- Carl Sagan

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So we were undoubtedly all thrilled by the long-awaited James Webb Space Telescope's flawless Christmas morning launch. As a hardcore space fan, when I heard that liftoff from French Guiana would occur around 6am my time, I dutifully set my alarm for the early morning countdown. Unfortunately, the timing was such that I'd gotten my COVID-19 vaccine booster the day before; I hadn't thought much of it since my first two shots produced minimal side-effects, but man alive did I feel horrendous by the time that 6am alarm went off! As JWST took flight, the nausea and full-body soreness combined with my sleep deprivation had me almost ready to hurl. But in the end, both boosters - Ariane 5 and Moderna - were more than worth it! (heh heh see what I did there??) 

The Ariane 5 carrying JWST, moments after liftoff

My unfortunate timing notwithstanding, I've actually had this article idea in mind for months now, ever since NASA Earth Observatory published this great article about where the beryllium for those 18 super-shiny gold hexagonal mirrors on JWST came from. As it turns out, the vast majority of the world's beryllium comes from Spor Mountain in Utah, locked away in minerals that formed 25 million years ago from the lava of a volcanic eruption. NASA has long used beryllium from Spor Mountain for its missions; for JWST, beryllium was perfect because it's 1/3 lighter than aluminum and 6 times stiffer than steel, even under extreme temperatures, so the mirrors can withstand any collisions with micrometeoroids

From mine to mirror, the beryllium in JWST's mirrors has undergone quite a transformation!

But I wanted to go one level deeper: how did all the beryllium at Spor Mountain get there in the first place, or any of the beryllium on Earth for that matter? During my elective astronomy classes in college, I really enjoyed the unit on nucleosynthesis (how all the various elements get produced in the Universe), and I remembered that beryllium is a pretty strange edge case…

If you were to take a look at the periodic table of elements and make a wild guess as to the relative abundance of each element in the Universe, you'd probably end up with a result that has lighter elements being way more abundant than those weird heavy elements like bismuth or molybdenum. And you'd basically be right, especially if you knew that hydrogen and helium were the two elements largely produced by default from the Big Bang, and that stars are almost entirely made up of! Looking at the figure below plotting relative abundance in the Universe as a function of atomic number, it's a pretty clear trend… EXCEPT for three elements: lithium, beryllium, and boron. Why is that??

Let's back it up a bit. I mentioned just now that hydrogen and helium were produced by default from the Big Bang. We also know that stars release their energy through nuclear fusion, so shouldn't that solve our problem? Once the Universe cooled enough for matter to coalesce, why couldn't stars just start fusing hydrogen and helium into lithium, beryllium, and boron? 

The problem is that if you take the available isotopes of hydrogen and helium (H-1, H-2, He-3, He-4) and slam them together, none of the isotopes of lithium, beryllium, or boron that you'd get are stable. For example, if you fuse two He-4 nuclei together, you'd get beryllium-8, an incredibly unstable isotope that decays almost immediately! Instead, the first stable isotope heavier than helium that stars can produce through nuclear fusion is carbon-12, and that only happens through an incredible reaction called the triple-alpha process, where three He-4 nuclei (aka alpha particles) collide at the same time! From there, once carbon is formed, the nuclear fusion in stars can continue producing the heavier and heavier elements we see on the periodic table (as least up through iron; after that, more exotic processes like supernovae and white dwarf/neutron star collisions are needed). But the point is that stellar nuclear fusion skips lithium, beryllium, and boron. What gives?
Diagram of the triple-alpha process. In the moment the Be-8 nucleus is formed, another He-4 nucleus
 better come along and slam into it immediately! Otherwise it'll decay right back to He-4

Clearly there's a different process that produces lithium, beryllium, and boron, and it's called cosmic ray spallation, which is basically a fancy way of saying "nature's atom smashers." Think of it this way; here on Earth we humans have built particle accelerators to smash exotic large atoms into interesting smaller ones. The Universe has its own particle accelerators, things like pulsars, supernovae, and black holes that are constantly firing cosmic rays in all directions. Sometimes, these energetic particles will strike a larger atom and cause it to break apart. And sometimes, the smaller piece that breaks off happens to be a stable isotope of lithium, beryllium, or boron! If this sounds like an unlikely scenario, you're absolutely right, which is exactly why these three elements are relatively uncommon in the cosmos! They're the only elements not made from stars or from the Big Bang itself
Simple diagram of boron spallation forming lithium and helium


So the next time you see a picture of JWST's magnificent hexagonal mirrors, know that the beryllium in those mirrors is a rare delicacy of the Universe, and carry a little bit of pride knowing that we humans have taken these very special atoms and constructed them into something truly worthy of their value



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