Why Do Stars Explode?

Why Do Stars Explode?

I remember standing outside with my friends as a child and gazing up into the night sky, looking for falling stars to make a wish. We would lie on the ground half the night staring up at the sky but rarely glimpse a shooting star. When we did, I didn’t think they exploded. I just thought they died. Yet now I know that they weren’t falling stars we saw and that stars really do explode after all. I wonder why stars explode?

Stars burn brightly for billions of years. They are made up of hydrogen and helium and at the core of the star are intense high pressures and temperatures. This causes the hydrogen atoms to smash together, forming helium and a ton of energy that heats up the gas. The heating of this gas is called nuclear fusion, and the energy that comes from this fusion creates an outward force of pressure.

If the star is at the end of its life, there is no more fuel to burn, so some of its mass flows into its core. When this happens, the core is so heavy it can’t handle the gravity, and it collapses in on itself, causing the star to implode and then eventually explode, causing something called a supernova.

What’s a Supernova?

What's a Supernova?

A supernovaOpens in a new tab. is what happens when a star is about to die. It ends in the most massive explosion anyone has ever witnessed. It is known as the colossal explosion of a star. Supernovae can temporarily outshine entire galaxies and have the potential to radiate more energy than our sun will in its entire lifetime.

Supernovas are the primary source of heavy elements in the universe.

There are two types of a supernova – Type I Supernova and Type II Supernova. An example of a Type I Supernova would be a core-collapse, and a thermal runaway is an example of a Type II supernova.

Core-Collapse Supernova

The core-collapse supernova is what I described above, where a star, which is at least five times the mass of our sun, is in its end-of-life state.

When this massive star burns nuclear fuel in its core, it produces so much heat that the pressure generated by the heat pushes outward against the forces of gravity, which pulls inward on the star. For most of the star’s life, the star is in something called hydrostatic equilibrium. This means that all the interactions that are happening in the core keep producing enough energy to keep the star puffed and shining bright and are in balance with the gravity, which keeps it from collapsing. The star remains in balance between two opposing forces. The star’s gravity works toward squeezing the star into the tiniest, most compressed ball possible. At the same time, the nuclear fuel that is burning in the core creates increasing pressure as it heats up.

As the star uses up the fuel, it begins to cool down. This causes the outward pressure to drop. Once the pressure drops low enough in a massive star, gravity takes control, and within seconds, the star will collapse.

When something one million times the mass of the Earth collapses in just 15 seconds, it creates a massive shock wave, which then causes the outer part of the star to explode. The explosion is a core-collapse supernova.

After the explosion, all that remains is a very dense core and a growing cloud of hot gas called a nebula. The remaining core will continue collapsing under its own gravity and will turn into one of two objects. It will either become a neutron star, or it will form into a black hole.

A neutron star is a celestial object born from the explosive death of a massive star. It has a very small radius, and high density made up of closely packed neutrons.

Thermal Runaway Supernova

When two stars are orbiting one another, and one of the stars is a white dwarf that is at least the size of the Earth, it can cause a thermal runaway supernova. A white dwarf is the remains of a sun-sized star that has run out of fuel. When one white dwarf crashes into another or pulls too much debris from its nearby star, the white dwarf can explode. This is the other kind of supernova.

Are Supernovas Very Common?

Supernovas are considered rare. Less than one percent of all stars are actually of a big enough size to form a supernova. Some astronomers estimate that in a galaxy the size of our Milky Way, there should be one or two supernovas every 100 years.

In 1006, people witnessed what was considered the biggest recorded supernova. They saw an amazing light show that appeared one night without warning. Called a “guest star” by Chinese astronomers, it was brighter than a crescent moon and was seen during the day as well as the night. It continued to shine for months until, by the end of the year, it had dimmed and could no longer be seen.

Can Scientists Predict When the Next Supernova Will Happen?

Presently it’s not possible to predict a supernova, though astronomers do have some guesses. They are able to decipher when certain stars appear to be in the preparation mode of exploding into a supernova, but they are unable to predict exactly when that will happen.

However, they have been looking at a particular exploding star they’ve named Supernova Requiem. If their predictions are accurate, they believe the light from this supernova explosion will be able to be seen by telescope around the year 2037. You see, the Supernova Requiem has already exploded and was originally seen in 2016. To understand this, you would need to be familiar with light-years.

The Light Years of a Near-Earth Supernova

A near-Earth supernovaOpens in a new tab. is an explosion that occurs close enough to the Earth to have a noticeable effect on the Earth’s biosphere. This would place the supernova around 30 to 1000 light-years away.

If you read my article, Why Do Stars Twinkle?, you may remember that the closest star to Earth is 4.24 light-years away. This would equal 5.88 trillion miles and means that it takes the light that many miles to travel before we would see it from Earth.

If a supernova explodes and is 30 light-years away, it would take 30 years for us to be able to see the explosion on a telescope. With the Supernova Requiem, the distance of the exploded star is so great that astronomers believe the light from the explosion won’t be visible from Earth until around 2037.

This Supernova was also spotted in 2019 but was located behind a giant galaxy cluster and thus, was not visible. However, the galaxy cluster had enough gravity to bend and magnify light from the supernova, and this gravitational lensing split the light from the supernova into multiple mirror images.

Astronomers estimate that the supernova light will return based on results from computer models of the cluster.

Scientists Witness a Supernova Explosion

Scientists Witness a Supernova Explosion

For the first time ever, scientists watched the massive explosion of a red supergiant named SN 2020tlf, located about 120 million light-years from Earth. Researchers actually saw the star explode with blazing flashes of light among the globs of gas that burst from the star’s surface. It was even bigger than they anticipated.

They had been watching it for more than 100 days before it finally collapsed. Researchers called this discovery a major breakthrough in understanding what happens to massive stars moments before they die.

How Were Scientists Able to See the Supernova Explosion?

Scientists first saw the star flicker with bright flashes of radiated gas using two telescopes in Hawaii – the University of Hawaii Institute for Astronomy Pan-STARRS1 telescope and the W.M. Keck Observatory on Mauna Kea.


The Pan-STARRS1 (PS1)Opens in a new tab., which stands for Panoramic Survey Telescope and Rapid Response System, is a 1.8-meter diameter telescope. It is equipped with the first world’s largest digital camera with almost 1.4 billion pixels and sits near the summit of Haleakala on the island of Maui. Since its invention, a similar telescope, Pan-STARRS2 (PS2), has been constructed adjacent to PS1. It has an even larger digital camera, almost 1.5 billion pixels.

The Pan-STARRS1 is the leading Near-Earth Object discovery telescope.

The W.M. Keck Observatory Telescopes

The W.M. Keck Observatory Telescopes

The W.M. Keck Observatory telescopes are some of the most scientifically productive telescopes on Earth. They are placed atop Mauna Kea on the Island of Hawaii and sport advanced instruments such as imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems.

Final Thoughts

Hopefully, you have a better understanding of why and how stars explode. The inward pressure of gravity and the outward pressure of the heated core keep stars alive for a very long time. But once fuel runs out and the star loses its equilibrium, it is near its final stages of life. And if it is big enough, the final result is the massive explosion of a supernova.

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Alexandra Christensen

Alexandra Christensen is a freelance writer and editor. When she is not working on an assignment, she can be found hanging around with other writers on Medium.com/@alexandra_creates where she writes mostly about raising foster and adopted kids and those with invisible disabilities.

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