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What is a Supernova?

In general terms, a supernova is the scientific term for an exploding star. This usually occurs when a large star runs out of nuclear fuel and “dies.” Stars are powered by nuclear fusion, and when this fuel runs out, they might explode. Supernovas are the largest explosions known to mankind. Usually, supernovas emit incredibly bright rays of light and energy that can be seen hundreds of lightyears away. 

Exploding stars at the end of their lives may seem like a simple explanation for supernovas. In reality, supernovas are actually much more complicated than just exploding stars. There are a variety of factors that need to be correct for a supernova to occur. At the end of their lives, different-sized stars may have different types of supernovas or no supernova at all. 

This article breaks down what supernovas are. This includes what conditions are needed for a supernova, what exactly causes a supernova, the types of supernovas, and how often they occur. 

What conditions are needed for a Supernova?

Supernovas occur when stars explode at the end of their life cycle, but not all stars have the right conditions for a supernova to occur. The main determinant of whether or not a supernova occurs is the mass of a star.

The mass of a specific star is usually what determines how it will die. Stars with the right mass will explode into a nuclear supernova, while stars that are either too big or too small will not.

In order to become a supernova, a star has to be huge. Compared to us, all stars may seem huge. However, in order to become a supernova, a star has to have at least five times the mass of our sun. This means that, luckily, our sun won’t explode into a fiery supernova anytime soon. 

When a star is too small for a supernova, like our sun, something different occurs when it “dies.” Because it lacks the mass for a supernova, a star like our sun will swell into a “red giant” when it runs out of nuclear fuel. Then, it will turn into a “white dwarf.”

What causes a Supernova?

Supernovas occur when a star “dies,” but what exactly does this mean?

The answer has to do with how stars are powered: nuclear energy. Stars are actually powered in similar ways to the nuclear power plants that now dot our globe. At their cores, stars burn huge amounts of nuclear fuel. This produces a massive amount of energy. Accompanying this energy is a massive output of heat. 

Similar to our earth-bound nuclear reactors, these nuclear processes are stuck in a very delicate balance. As humans have found out from nuclear disasters like Chernobyl, when that delicate balance is disturbed, nuclear reactions can go haywire and cause devastating effects. 

Stars don’t have an unlimited supply of fuel, and unlike our nuclear reactors, they cannot be refilled once emptied. When a star either runs out of fuel or some external factor throws off the conditions of the star, a deadly chain reaction ensues, and a supernova usually occurs shortly after.  

Supernova happening

The two types of Supernovas

There are actually two types of supernovas, each with very different causes and results. They are known as Type I and Type II supernova. 

Broadly, a Type I supernovae occurs when two stars are orbiting one another, and one star is roughly the size of the earth and categorized as a “white dwarf.” These two stars interact and occasionally explode in a fireball. 

On the other hand, a Type II supernovae occurs when a massive star runs out of nuclear fuel, causing conditions inside the star to change. Due to gravity, the star will eventually collapse inwards on itself and create a massive explosion.

Type I Supernova

More specifically, Type I supernovae occur from white dwarf stars in certain close binary star systems. White dwarf stars are stars who are “dead” but are too small to explode into supernovae, like our sun. As mentioned previously, when these stars run out of fuel, they eventually cool down into “white dwarf” stars. 

Even though they are no longer active, white dwarf stars can still pull gas from nearby stars. This is what causes a Type I supernova to occur. As the two stars interact near each other, the white dwarf will slowly accumulate gas from the companion star on its surface. When this happens, the white dwarf star becomes more and more compressed. 

After a while, this compression sets into motion a nuclear chain reaction that causes the white dwarf star to explode into a supernova.

Type I supernova can be further broken down into the categories of Type Ia, Ib, and Ic. Types Ib and Ic are mostly the same as Type Ia, but the conditions are slightly different in terms of how much hydrogen is left on a star.

Type II Supernova

Type II supernovae are what most people visualize when they think of a supernova. In this instance, stars eventually run out of hydrogen and helium fuel at their cores. This causes the star to collapse under its own weight. 

The nuclear forces at play in a massive star create enough heat and pressure to counteract the forces of gravity being imposed on the star. When the star runs out of fuel, this heat and pressure stop, and gravity wins. 

Before the star implodes, heavy elements build up at the center of the star until it hits the “Chandrasekhar limit.” Once this limit is hit, the star begins to implode. After implosion begins, it will reflect off of the star’s core and explode out into space, forming the giant clouds associated with supernovae.

Type II supernovae can be further categorized by the intensity and duration of light that they produce. The light from Type II-L supernovae declines steadily, while the light from Type II-P supernovae lasts for much longer.


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How often do Supernovas Occur?

Supernovae that humans can see occurring are very rare. In fact, scientists have only been able to witness a few supernovae occur. However, supernovae leave giant clouds of light and debris behind that give scientists clues into how often they really occur.

In the milky way, it is thought that a supernova will occur roughly once every fifty years. Scientists think supernovae occur around once every ten seconds in the entire universe. 


Supernovae occur when stars “die” and explode. This only occurs in stars that have the right mass. Stars that are too large may turn into black holes, while stars that are too small will turn into white dwarf stars. 

There are two types of supernovae: Type I and Type II. Type I supernovae occur when a white dwarf star and another star interact. The white dwarf pulls gas from the other star until there is too much mass. Then, the white dwarf explodes. In a Type II supernova, a large star runs out of nuclear fuel, which means it can no longer fight back against gravity and implodes.

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