A Star is Born: How Stars are Created and How They Work

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Podcast Transcript

It is estimated that within the observable universe there might be as many as a septillion stars. While each of them is far larger than the Earth, they all differ in terms of age, size, color, and composition. 

Despite being very far away, we know a surprisingly large amount about them through observation and an understanding of the basic units of matter.

Learn more about stars, how they are born, and how they die, on this episode of Everything Everywhere Daily.


When humans first looked up at the night sky they could see lights, but they had no clue what they were. 

The first theories held that they were either holes in the firmament with light shining through, or that they were lights that were somehow fixed in the sky….except that there were a few of those lights that actually moved.

Whatever they were, they were very clearly different than the sun and the moon.

Except that some ancient Greek and Islamic astronomers thought that the stars might just be the same thing as the sun. 

This idea was revived in 1584 by the Italian astronomer Giordano Bruno. Bruno not only thought that the stars were like our sun, but that they also had planets like Earth, including possibly people just like us. 

His theories were mostly rejected by his contemporaries and he was later tried for heresy, but within a century the idea that stars were like our sun took hold. 

But if the stars were like our sun, what exactly was the sun? 

For the longest time, people thought that the sun was a ball of fire. To be honest, it wasn’t a totally unreasonable assumption given what people knew at the time. It was hot like fire and it was a similar color to fire. 

As science advanced and we learned more about things like heat, energy, and the size of the sun, some things just didn’t add up. 

One 19th-century scientist, in particular, Hermann von Helmholtz,  tried to figure out what made the sun give off energy. 

First, he assumed that the sun was on fire and it was literally undergoing combustion just like a fire burning coal as its fuel source. After doing the math, he realized it would burn out relatively quickly, in about 1,500 years. 

This didn’t even square with biblical theories which held that the Earth was only 6,000 years old, let alone the evidence mounting from geology which said the Earth was millions of years old. 

Another theory he came up with assumed that space was filled with matter, and that matter was constantly crashing into the sun which was responsible for the endless supply of heat. However, that also didn’t pan out as it would require 100 trillion tons of mass to slam into the sun every hour to produce that much heat.

A final theory was that the sun was constantly shrinking. He estimated that the contraction of the sun by its enormous gravity would provide the energy to make it shine. He calculated that the sun would only have to contract in diameter by 1 inch, or 2.5 centimeters every three hours.

This at least got the sun to a point where it could shine for millions of years. However, the theory still had problems. It would mean that the sun was once much larger and that it was constantly getting smaller. It still also didn’t jibe with the age of the Earth.

Nonetheless, this was the accepted theory for much of the late 19th century because there wasn’t a better theory. 

There must have been something else that was making the sun give off energy that answered all of the outstanding questions, but no one knew what it was. 

At the same time, the science of spectrography was developed. Through spectrography it was discovered that elements gave off a very identifiable spectrum of light when heated. 

From this, it was possible to analyze the light from the sun, and it was determined that it was mostly hydrogen and helium. 

So the sun was made of the same stuff we had on Earth, but it was producing energy from an unknown method.

In 1904 Ernest Rutherford posited that it was radioactive decay that was producing all of the heat. 

Things became clearer when Albert Einstein published his famous E=mc2 equation. This stated that mass could be converted into energy. A LOT of energy. Enough energy that the sun’s energy output just might start to make sense. 

The guy who put it all together was a British astronomer by the name of Arthur Eddington. 

Eddington wondered what was happing inside the core of the sun. He calculated what the pressure and temperature would be deep inside the sun. 

He realized the pressure would be enough to turn hydrogen into a solid, and the temperatures would be almost 27 million degrees Fahrenheit or 15 million degrees celsius. 

At those pressures and temperatures, it is enough for the hydrogen atoms to fuse together to create helium. The source of the sun’s energy it turned out was nuclear fusion. 

Since Eddington’s theory, there has been a great deal of experimental and observational evidence, of both the sun and other stars, such that we now have a pretty good idea of how stars are made and what happens to them. 

So, what exactly is the process? 

It all starts with hydrogen. 

As I mentioned in my episode on hydrogen, the vast majority of all matter we can observe in the universe is hydrogen. 

A star will start out as a massive cloud of hydrogen, and a small amount of helium. All other elements are in such small amounts that I’ll ignore them for the purposes of illustration. 

Even though the hydrogen is in the form of a very diffuse gas, it still exerts a gravitational force. Over time, this gas can coalesce via gravity into a massive sphere.  There are astronomical images of nebulas that are known as the cradles of stars. They are massive clouds of gas where stars are produced.

The key thing in the life of any star, and the thing which will determine how hot it burns and how long it lives is its mass. Mass means everything. 

Paradoxically, the more mass a star has the shorter its life. More on that in a bit. 

As the hydrogen sphere begins to contract, the pressure becomes greater as do the temperatures deep inside. 

Eventually, if there is enough mass, the temperatures and pressure will be enough that fusion will occur. Hydrogen atoms will be fused to make helium and it will give off a tremendous amount of energy. 

The amount of mass that is necessary to start fusion is approximately 7.5% the mass of our sun, or about 80 times the mass of Jupiter. A star without enough mass to start fusion is called a brown dwarf. 

Stars with about half the mass of our sun or less are called red dwarfs. They are the most common type of star in the universe and they can be exceptionally long-lived. 

One question that was raised early on was what stopped a star from continually collapsing inward due to gravity. 

The reason why the size of the sun is relatively stable has to do with something called hydrostatic equilibrium. Matter expands when heated. While gravity is pushing inward, the heat of the matter inside the star pushes outward. 

If the star keeps contracting, it will accelerate fusion, creating more heat, expanding the star outward until it reaches an equilibrium. 

The energy which is being released inside of a star can take a very long time to reach the surface. A photon released near the core might take as long as 10,000 to 170,000 years to reach the surface. This is mainly due to the density of the matter it has to go through. 

I mentioned that larger stars actually burn quicker. This is due to gravity. The more mass a star has, the greater the gravity, the more fusion will occur, and the faster it will burn. 

The mass and temperature will also be reflected in the color of the star.

Astronomers have developed a classification system for stars according to their color and temperature. 

Going from hottest to coolest, the classifications are: O, B, A, F, G, K, M.

This is known as the main sequence. 

It sounds rather out of order, but there are several mnemonic devices for remembering it. The classic one is “Oh be a fine girl, kiss me”, but there are many others including “Overseas broadcast: A flash! Godzilla kills Mothra!”

Within each letter category, there are also numbers. Our sun is classified as a G2 star. A G-type star can burn for billions of years. 

O stars are the hottest and are blue. They are the rarest stars in the universe because they usually aren’t around very long. An O star might only have a life of a few million years before it explodes as a supernova. 

The sun turns 600 million tons of hydrogen into helium every second. 

Even though the sun is big, the amount of hydrogen is finite. What happens when it runs out of hydrogen?

Then helium starts to fuse into carbon. This is a multistep process, known as the triple-alpha process, as helium has 2 protons and carbon has 6. 

When this happens, our sun will turn into a red giant. The core condenses which is necessary to get enough pressure to fuse helium, but the upper layers will expand from the heat. 

What happens when there is no more helium? The fusion of heavier elements will continue depending on the mass of the star. If there isn’t enough mass, then fusion will eventually stop. 

Regardless of how big the star is, however, there is a limit. If you remember back to my episode on uranium and plutonium, you can get energy by splitting those large atoms via nuclear fission.

You can get energy from light atoms like hydrogen via fusion. There is a point in between where you will no longer get energy from fusion or fission. You will need to put energy into the system to split or fuse atoms, and no energy will be given off. 

That point is the element iron. 

Once a star is left with iron, it is game over for fusion. 

At that point, the star will no longer produce heat from fusion and there isn’t anything stopping the star from collapsing inward. 

What then happens will depend on the mass of the star.

The end fate for our sun is what is called a white dwarf. It will lose much of its mass in the process, but it will shrink down to a very small size. The only thing stopping it from collapsing further is something called electron degeneracy pressure. 

The best way to describe electron degeneracy pressure is that the forces inside the atom at literally the only thing preventing further collapse. 

For a white dwarf to be created it has to have 1.44 solar masses or less. This is known as the Chandrasekhar limit, named after the Nobel Prize winning physicist Subrahmanyan Chandrasekhar.

If a star is greater than the Chandrasekhar limit, electron degeneracy pressure will be overcome. The forces inside all the atoms will cause the electrons to fuse into protons to become neutrons. The whole star will become a neutron star. 

Neutron stars are incredibly dense. Imagine stars larger than our sun condensed down to only 10 kilometers or 6 miles in diameter. They are like a giant atomic nucleus. 

The thing preventing a neutron star from collapsing further is neutron degeneracy pressure. This is the very force of an atomic nucleus keeping it from collapsing. 


However, this too has a limit. If the mass is more than about 2.1 solar masses, it passes the Tolman–Oppenheimer–Volkoff limit, and at that point, there are no known forces in the universe that can support the star, and it becomes a black hole. 

The process of collapsing into a white dwarf, neutron star, or a black hole can result in a supernova, which is what will happen if a star is about 10x the mass of the sun. This happens from the shockwaves from the collapse, so the resulting body has only a fraction of the matter from the original star. The rest of the mass is explosively expelled outward. 

There is one final stage that might theoretically exist for white dwarf stars. A black dwarf. 

When a star collapses into a white dwarf, there is still a lot of residual heat even though there is no more fusion. Over time, a very long time, all of that heat will eventually dissipate, and the result will be a very dense body that is cold. 

This is theoretical because the time it would take to cool down to that level is longer than the current age of the universe, and if they did exist, they’d be impossible to see. 

Stars are the engine that drives the universe. Our sun is responsible for all life as we know it. All of the heavy elements, energy, and fantastic stellar objects we know of all came from stars. 

And the stars are just the result of incredible amounts of smushed together hydrogen. 


Everything Everywhere Daily is an Airwave Media Podcast. 

The executive producer is Darcy Adams.

The associate producers are Thor Thomsen and Peter Bennett.

Today’s review comes from listener firstborn over at Apple Podcasts in the United States. They write:

Well researched & informative The best way for me to review Everything Everywhere Daily is to share my experience listening to this pod one morning after a tough one-hour workout. Feeling tired, and muttering under my breath, I played the episode about the 1904 Olympic marathon. Somewhere between the descriptions of the Cuban man, who paid his own way, and planned to run the marathon in street clothes, and the mix of alleged “performance enhancers” given to the eventual winner, I concluded that my one-hour workout had been a walk in the park. To learn everything NOT to do when running or staging a marathon, listen to that episode.

Thanks, firstborn! Indeed, the 1904 Olympic marathon was probably the worst event in Olympic, and probably athletic, history. Even extreme events today don’t actively try to poison you. 

Remember, if you leave a review or send me a boostagram, you too can have it read the show.