The Abundance of the Different Elements in the Universe

Here’s another post I’ve written in preparation for an upcoming lecture. This topic is one that I’ll probably only mention in passing, but it’s one that’s interesting enough to warrant looking into a little more deeply.

The periodic table. (Credit: DePiep, CC-BY-SA-3.0)

On the periodic table of the elements are more than 100 elements. Some don’t exist in nature, only in atomic colliders on Earth. Others have fleeting existence only in the aftermath of a supernova (an exploding star).

Not all elements on the periodic table are present in the universe in the same amounts. Overwhelmingly, the most abundant element is hydrogen, followed by helium.

Abundances of elements in the universe. The vertical (Y) axis is in a log scale. That means that one unit up means ten times more abundant. Elements get heavier and heavier as you move right on the graph.(Credit: 28bytes, CC-BY-SA-3.0)

Hydrogen is the first element that formed following the Big Bang. The mathematical explanation for this is beyond my understanding – though I did hear it described once and it made perfect sense.

Many elements are formed in the burning centers of stars. The massive energy will fuse hydrogen atoms together to make helium. Then, when the hydrogen is gone, helium atoms fuse to make carbon, oxygen, and a few other elements. When the helium is used up, carbon and oxygen combine to make silicon, and later silicon combines with itself to make iron. Thus all elements from helium to iron (except lithium, beryllium, and boron).

Lithium, beryllium, and boron – three rather important elements – do not form by fusion. Rather, they form from the breakdown of heavier elements, like iron. This process is called spallation.

If fusion (and subsequent spallation) were the only means to make elements, the periodic table would end at iron. As I alluded to earlier, there is another process that creates elements, that of the supernova. When stars burn out, they can simply fade away. The more massive stars collapse upon themselves and explode with colossal energy. This energy causes a high flux of neutrons, which fuse with existing atoms resulting in the heavier elements, all the way to uranium and thorium and even beyond. This process is called ‘neutron capture.’

The heavier elements are rarer than the lighter elements because the neutron-capture events required to create these elements are less common.

To summarize:

Hydrogen was formed during the Big Bang.

The elements helium through iron, except, lithium, berillium, and boron, are formed by fusion within stars.

Lithiium, beryllium, and boron are formed byt the decomposition of heavier elements – a process called spallation.

Elements heavier than iron form in the aftermath of supernovae, in a process called neutron capture.

Any questions?

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  1. Pingback: De l’astronomie √† l’entrepreneuriat : t√©lescopes spatiaux dans les rayons gamma, X et UV et ondes radio

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