Clear Science!

We saw that at the beginning of the universe, the only elements present were the most simple ones: Hydrogen, Helium, and some Lithium and Beryllium.

Over billions of years, nuclear reactions in stars made heavier elements. Some of those include Carbon, Nitrogen, and Oxygen. These elements make up most of life on Earth.

They came from stars, and ended up here. They became part of the cycle of life. Some of them became you. You are made of star stuff.

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100% Star Stuff available at ThinkGeek

Where is the temperature high enough to allow nuclear reactions? One important answer is: inside stars.

We showed how two light elements (H) could combine to form a heavier element (He). There are many such reactions, which can produce heavier and heavier elements. (“Heavy” meaning having a higher number on the periodic table.)

Above is the triple-alpha process, which is how carbon is produced from 3 helium atoms. All carbon in the universe was likely made this way. Inside stars.

This is called stellar nucleosynthesis. (Stellar = star, nucleo = nucleus, synthesis = making.) Wikipedia has an excellent list of reactions.

If the early universe had pretty much just hydrogen and helium, how did the heavier elements get made?

Typical chemical reactions happen when atoms swap or share electrons. However, at very high temperature, reactions can happen that involve the particles in the nucleus of an atom (protons and neutrons). So we call these reactions nuclear reactions.

For example, if two hydrogen atoms combine, and they each have a proton, then the resulting atom can have two protons. Protons determine the element’s identity. You started with H and ended up with He. 

We said the periodic table right after the Big Bang (about a minute after) was very simple, and had H and He (and a dash of Li and Be).

You might be skeptical (the Clear Science staff always are!): How can you possibly measure that? It’s just a guess right?

You actually can measure it. To study the early universe, you either look at things that are very old or are very far away. In fact, if you look very far away with a telescope, you are also looking back in time. Do you know why? 

We talked about how in the universe the most abundant elements by far are hydrogen and helium. (We’re thinking about what Carl Sagan means by “We’re made of star stuff.”)

Consider this: we all know what the periodic table looks like. However, in the moments right after the Big Bang, the periodic table looked much different. In fact it looked very simple. Easy to memorize! (Can we all go back and have chemistry class one minute after the Big Bang please?)

PS at the very instant of the Big Bang, there were no elements.

You are made of elements, along with everything else on Earth. You’re mostly oxygen, carbon, hydrogen, and nitrogen.

However, on all of Earth, it’s a bit different. Oxygen is still #1, followed by silicon, aluminum, and iron.

Let’s be more extreme, and think bigger: in the cosmos, it looks very different. Hydrogen is by far the most abundant element, followed by helium. Everything else is only a very tiny fraction.

Remember: Life is part of the Earth, and the Earth is part of the universe. Carbon, for example, is very concentrated in living things on Earth, but is (in the grand scheme of things) a very rare thing.  

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“The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of star stuff.”

— Carl Sagan

One of Carl Sagan’s favorite things to say was “We are made of star stuff.” And it’s true. For a minute, should we dwell on what he meant by this?