Posted by Earlier this week, I wrote about how the heavier elements in the Universe were made. Specifically, that they are made in stars. These stars then explode in a variety of ways, enriching the Universe with these heavy elements, and allowing us to form glorious things, like our planet!
By contrast, the big bang makes light elements, but not heavy ones. Why is this the case, and how do we know? Let’s find out.
Image credit: Stephen Van Vuuren, created from a simulation of 80,000 star images.
Just a few seconds after the big bang, the Universe is filled with protons and neutrons, in roughly the same numbers as we have today. But, none of these protons and neutrons are bound together, they’re all free and lonesome. Why? There’s certainly enough energy for them to interact with one another, and there’s certainly a high enough density for them to interact frequently. But — in addition to protons and neutrons — the Universe is full of photons, or particles of pure energy. These are important because photons outnumber protons-and-neutrons by almost 2,000,000,000 to 1! So even though a proton and neutron can fuse together to make deuterium at any time:
the photons are so energetic and so abundant that they immediately blast the deuterium back into a proton and neutron. This continues until the Universe has cooled enough for deuterium to be stable, which doesn’t happen until the Universe is a little more than three minutes old. Once the Universe cools enough, protons and neutrons can combine to not only form deuterium, but also tritium, Helium-3, and Helium-4. And it does this pretty easily, so that relatively quickly over 99.98% of the Universe is either a Helium-4 nucleus (2 protons and 2 neutrons) or a Hydrogen nucleus (1 proton alone).
But now, we’ve got a problem. We can’t really go up from Helium-4. By this point, a little more than 3 minutes after the Big Bang, the Universe is already 1,000,000,000 times less dense than the interior of the Sun. When densities are low, it means that reactions occur at lower rates. So let’s take our Helium-4 and try adding things to it:
- Helium-4 + Hydrogen = Lithium-5, which is completely unstable and decays back into a proton and a helium nucleus after less than 10^-21 seconds.
- Helium-4 + deuterium = Lithium-6, which is stable, and of which trace amounts (about 1 nucleus in 10,000,000,000) still exist.
- Helium-4 + tritium = Lithium-7, which is stable, and again, trace amounts of it (again, about 1 in 10,000,000,000 nuclei) still exist.
- Helium-4 + Helium-3 = Beryllium-7, which lives about 53 days, and then decays into Lithium-7, which we observe.
- Helium-4 + Helium-4 = Beryllium-8, which decays back into two helium nuclei after about 10^-16 seconds.
This is a huge problem, because most of the Universe (at this point) is Helium-4 and Hydrogen! But because there are no stable mass 5 or mass 8 nuclei, the Universe gets stuck. It isn’t until millions of years later, when nuclei have become neutral atoms, trillions and trillions of atoms have collapsed into dust clouds, and the first stars begin to form, that we can start creating interesting elements.
Why? Because in the interior of stars, the density is suddenly high enough that 10^-16 seconds is long enough for a third Helium-4 nucleus to come along, and cause this to happen:
Finally! We can start forming heavier elements! It’s easy from here: Carbon + Helium-4 = Oxygen, Oxygen + Helium-4 = Neon, Oxygen + Carbon = Silicon, and you not only get the picture, you get all the elements capable of making this:
When these stars with all of these heavy elements in them die, they spread their elements all throughout their galaxy; everything on planet Earth, with the possible exception of some hydrogen, was once part of a star. Not only that, but every element on Earth, again with the exception of hydrogen, was made in a star. Including every element in your body.
As Carl Sagan said, “We are all starstuff.” Many of the stars you see in the sky will one day give their heavy elements up to the rest of the galaxy, and will lead to new worlds, and possibly new life. And perhaps this helps better explain why Fred Hoyle‘s discovery of how to make heavy elements in stars was so profoundly important.
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