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What Kinds of Stars End Their Lives as Supernovae?

Stars that are at least eight times as massive as our Sun are likely to end their life cycles as supernovae, but the most massive stars might not form supernovae at all.

The conventional wisdom about the life cycles of stars runs something like this:

  • Smaller stars burn smoothly for billions of years.
  • These smaller stars become white dwarfs.
  • Metals in stars accelerate supernova status.
  • Larger stars explode as supernovae.
  • Supernovae leave a neutron star or black hole.

There is general agreement among modern astrophysicists that stars like our own Sun from when vast clouds of hydrogen and helium collapse under their own weight. At the center of the collapsing gas, cloud pressure is so extreme that hydrogen atoms lose their electrons. 

Pairs of naked hydrogen atoms fuse together to form helium, releasing energy that counteracts the gravity causing the gas cloud to contract. The battle between the force of gravity and the energy released by nuclear fusion powers our Sun and countless stars like it.

During this “main sequence” of the life of a star like our Sun, which lasts about 8 billion years, the star steadily converts hydrogen into helium as it emits light and electromagnetic radiation. Our Sun is about 4.5 billion years old, so astrophysicists calculate that it has about 3.5 billion years left in its main sequence. But eventually, our Sun will run out of fuel.

How Stars Like Our Sun Die

Sun-sized stars happily burn hydrogen to make helium for billions of years, but eventually, since helium is heavier than hydrogen, they are left with a huge helium core. They don’t have enough hydrogen left to counteract the forces of gravity, so gases rush in to form a thin reactive zone around the helium core. 

For every action, there’s an equal and opposite reaction. Gases flowing toward the core of the star are balanced by gases rushing away from the core, and the star’s outer layers expand to make it vastly larger. With the enormous increase in its surface area, the Sun stops being white-hot and becomes red hot, making it a red giant.

In the case of our Sun, the outer edges of the Sun are likely to reach the current orbit of Mars. While the Sun is expanding, Earth will be dragged into its surface and disintegrate. 

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Long before that happens, the Earth would have been receiving so much energy as the Sun closed in on itself that oceans and atmosphere would evaporate into space, and life as we know it will cease to exist. 

Our Sun will continue for about a billion years as a red giant. Eventually, even the hydrogen in its outer layers will have been converted to helium, and the force of gravity will pull the helium back to its core. Helium will fuse into elements like carbon and oxygen but without the release of as much energy. Weak gravity will cause about half of the Sun’s mass to escape to form a planetary nebula, while the remainder of the Sun’s mass will contract into a white dwarf.

How Large Stars End Their Life Cycle

Astrophysicists theorize that stars of different sizes  reach different ends,

  • Every star starts by turning hydrogen into helium.
  • Red dwarfs never reach helium burning.
  • Red dwarfs shrink into helium-based white dwarfs.
  • Stars like our sun end as carbon-rich white dwarfs
  • Big stars undergo carbon fusion and explode.

Every newborn star fuses hydrogen into helium in its core. Red dwarfs not much bigger than Jupiter, stars the size of our Sun, and supermassive stars that can be hundreds of times more massive than the Sun all go through this first-phase nuclear reaction. The bigger a star is, the hotter its core gets, and the faster — even though the process takes billions of years — it burns its supply of hydrogen fuel. 

When stars of any size run out of hydrogen to fuel nuclear fusion, they contract and get hotter. They begin to fuse heavier elements, but these fusion reactions are limited to the size and gravitational force of the star.

Our Sun will eventually fuse groups of three helium atoms into carbon atoms. Giant stars will also create carbon from helium, but their core gravity is then strong enough that they will transform carbon into even heavier elements, oxygen, neon, magnesium, silicon, and sulfur, then turning into nickel, iron, and cobalt. 

Creating metals consumes more energy than it releases. The metallic core of a star 8 to 20 times as large as our Sun eventually will not produce enough energy to counterbalance the force of gravity. The aging large star will collapse in on itself, and a core-collapse or type II supernova will result.

  • A type I supernova involves a binary star system.
  • A type II supernova involves a single large star.

What Do People on Earth See When a Star Goes Supernova?

The collapse of a supernova on itself generates an enormous amount of energy. For a brief time, a single star gone supernova emits as much light as an entire galaxy.

In 1987, a supernova in a nearby galaxy called the Large Magellanic Cloud could be seen with the naked eye by people in Earth’s southern hemisphere. Astronomers are still studying this supernova as its shockwave as it travels through the interstellar dust.

On July 5, 1054, a supernova was recorded by documents written by astronomers in China, Japan, and the Islamic world and by a pictograph drawn by a member of the Pueblo people in Colorado. It appeared as a new, faint yellow star in what is now the Crab Nebula.

On April 30 and May 1 of 1006, a star exploded 7,200 light years away from Earth in the constellation Lupus. Astronomers of the time reported that it was bright enough to be seen during the day.

Gamma rays from these two supernovae are believed to have affected the chemistry of the Earth’s atmosphere, creating nitric oxide from the air that was trapped in bubbles of air found in an ice core sample in 2006.

Scientists calculate that a supernova exploding within 26 light-years of Earth would release enough energy to destroy half of the world’s ozone layer, leaving life on the planet unprotected from the Sun’s ultraviolet light. But astronomers have not found any stars likely to go supernova any closer than 500 light-years from Earth, so there is no cause to fear interstellar destruction. And there is a very good reason that supernovae are very rare.
 

Why Aren’t There More Supernovae?

There is a very simple reason that there aren’t more supernovae. There aren’t many stars that are big enough.

About 80% of the stars in our galaxy are red dwarfs. These stars have no more than 40% of the mass of our Sun and usually have less. Red dwarfs eventually shrink down to a ball of helium that doesn’t react any further.

Our Sun itself is larger than about 95% of the stars in our galaxy, and it is not large enough to form a supernova. Only about 1% of all of the stars in our galaxy are large enough to turn into supernovae, and these stars have other possible fates.

Some stars blow off enormous streams of matter but don’t become a supernova.

An example of this kind of “supernova imposter” is the nearby star Eta Carinae. It created a nebula over 300 years ago, but it continues to convert hydrogen into helium and is still shining today.

In an imposter supernova, something briefly causes the core to contract and heat up. As the core gets hotter, all kinds of nuclear reactions (not just a fusion of carbon to carbon) accelerate, and the star ejects matter into space without destroying itself.

Stars that are at least 20 times as large as our Sun can eventually go supernova, but they have at least two other possible fates. 

Supernova status occurs when a large star has a “just right” mass, although astronomers don’t know exactly what that mass is. When a star has more than this mass, say, 25 times our Sun’s mass, it may just disappear into a black hole. In 2017, astronomers expecting a star to go supernova unsuccessfully harnessed the combined power of the  Large Binocular Telescope (LBT) and NASA’s Hubble and Spitzer space telescopes to go looking for its remnants when it just vanished from sight, without ever going supernova.

Before this incident, astronomers had thought that black holes were only formed after the explosion of a supernova, but these observations seemed to confirm that huge stars can simply collapse on themselves without an explosion. The explanation for this event is that the star probably stopped producing enough radiation to balance the inward pull of gravity.

When enough mass gets compacted into a sufficiently small hole, an event horizon forms, and nothing can escape, not even light. Astronomers see a black hole.

Stars that are at least 100 times as massive as our Sun can experience the completely opposite fate. They can become a hypernova.

These stars give off so much energy as they collapse that photon split into pairs of electrons and their anti-matter counterpart, positrons. 

When electrons (matter) collide with positrons (anti-matter), they release a specific wavelength of gamma-ray energy. This energy heats the core of the star so that electrons and positrons are created even faster, so the entire giant stellar mass is blown apart.

This means that there are  four possible outcomes for a supermassive star:

  • A neutron star surrounded by gasses.
  • A black hole surrounded by gasses.
  • A massive black hole with nothing around it.
  • Just gases from a hypernova explosion.