Chapter 2 – Accretion

Defined Words: atomic nucleus, hydrogen, helium, accretion, fusion, star, supernova, planet


You just learned why a proton and an electron hang out together. Their opposite charges cause destructive interference, lowering the overall harmonic energy. An electron revolves around a proton and due to their lack of charge, neutrons can settle in next to the proton in the center of the atom. We call the center of an atom the atomic nucleus.

After the Big Bang, atoms of 1 proton and 1 electron formed as a stable composition of particles, we called it hydrogen. Another type of atom with 2 protons, 2 electrons and 2 neutrons was even more stable than hydrogen, we called it helium.

In the beginning, only hydrogen and helium formed. In fact, roughly 75% of the universe today is made up of hydrogen atoms. However, you’ve probably heard of plenty of other elements other than hydrogen and helium; here’s how the rest of them formed.

Just after the Big Bang, energy condensed into strings and some had the attribute we call mass and are affected by gravity. Gravity tells us that objects with mass attract each other: the more massive an object and the closer the objects are the greater force of attraction. Now we are going to follow a single hydrogen atom.

Let’s say our hydrogen atom is just hanging out in a massive cloud of hydrogen atoms and another one passes nearby. Gravity will cause the two atoms to move closer to each other until they eventually “stick.” The pair of hydrogen atoms now has twice the mass as each individual atom so now it has twice the attractive power for more atoms that pass by.

Now our hydrogen and its pair will attract more atoms, which will create a positive feedback loop that increases the chance of even more atoms being attracted to that object. This process is called accretion and it is the process that formed all stars, planets, and galaxies in the universe. Think of how a snowball gets bigger when it’s rolled along the snowy ground.

These collisions are happening in space, so particles coming from multiple random directions will cause the entire object to spin. A disk of atoms forms from this gravitational force, pulling more atoms to the center. To imagine this, envision a chef tossing pizza dough. The reason the dough gets flat is the same reason our solar system is in a flat plane and why our moon revolves around our planet in a flat plane.

We’ll reunite with our hydrogen atom in the center of the object and by this time, there are hundreds of billions of other atoms piling on top squeezing from all directions. With enough pressure inward from the gravity, our hydrogen atom pushes past the electron barrier between it and the atom next to it. The proton of our hydrogen atom joins the proton of the other atom in a single atomic nucleus.

This process is called fusion and it releases a huge amount of energy in the form of photons. The energy release is equal to the difference in energy between hydrogen and helium, since helium is a more stable atom. To put that energy in perspective, every second our sun puts out almost 2 trillion times the amount of energy released by the most powerful bomb humans have ever created.

The released photons are absorbed by nearby hydrogen atoms; this raises their energy enough to fuse with another hydrogen atom. This causes another positive feedback loop as one fusion reaction spurs on more. The energy release pushes away many of the elements in the disk, clearing some room around the newly formed central object. We call this a star.

Stars are always in equilibrium between the gravitational force inward and the outward force of the fusion explosions at the center. Eventually the fuel of hydrogen atoms will be used up and only helium will be left.

At this time, the star is no longer experiencing the outward force of the explosions and gravity will cause the star to collapse on itself. The star will become denser and able to fuse helium, releasing even more energy and causing the star to expand again

The size of the star directly affects the star’s destiny. It will either expand so large that gravity cannot keep it together or it will continue fusing heavier and heavier elements until it cannot fuse anymore.

When a star is out of fuel for fusion explosions, it collapses until it can’t get any smaller and then rebounds. The rebound, called a supernova, causes the star to spew all its guts out into the surrounding space, releasing the elements formed within. The released elements will be accreted again into new stars, bound by gravity to repeat the process.

Sometimes, the star forming doesn’t clear away all the material in its disk after the first fusion explosion. Gravity causes the atoms revolving around the star to accrete by the same process. The new objects accrete at distances to maintain equilibrium between the gravitational pull of the star inward and the forward momentum of the object revolving around it.

When the object is able to clear the entire path of its orbit around the star, we call it a planet.

Heavier elements will stay closer to the star while the lighter elements are sent farther away. This is why the planets close to our sun are rocky while the ones farther away are gas giants.

The simple process of accretion, caused by gravity, is the reason our universe is full of stars, planets, and galaxies. Accretion is also the reason any elements beyond hydrogen and helium exist at all. Every other type of atom formed in the heart of a star, which means that the pieces that make up you and me and everything that’s ever existed were born within the bellies of ancient stars.


<Chapter 1 – Origin of the Universe

Chapter 3 – Geochemical Cycles>

Origin Story

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