What if a star explodes near Earth?

A hydrogen bomb detonated at your eyeball would be a billion times less bright than watching the sun go supernova from Earth, demonstrating how insanely powerful supernovae are.

• Supernovae are the biggest explosions in the universe • When visible in other galaxies, they are brighter than the combined light of hundreds of billions of stars • They appear so bright they seem to come out of nowhere

On October 8, 1604, astronomer Johannes Kepler observed a bright star that was brighter than all other stars in the sky and about as bright as the planet Jupiter. On moonless nights, it cast a shadow on Earth.

Kepler published his observations in a book called De Stella Nova (about a new star in Latin), thinking he witnessed a star's birth. The name supernova stuck even after scientists learned in the 1930s it was actually a star's violent death.

Stars exist in a stable balance where fusion in the core converts matter into energy, generating pressure that counteracts gravity. Stars are essentially propped up by their own light.

When fusion rate drops, temperature and pressure decrease, gravity compresses the star, which increases core temperature and pressure, increasing fusion rate again. This creates a stable, self-regulating system.

Stars have a finite amount of nuclear fuel that gets used up over time. More massive stars burn hotter and brighter but have much shorter lifespans than less massive stars.

• For 90% of a star's life, hydrogen fuses into helium • When hydrogen runs out, helium fuses into carbon for about a million years • Carbon fuses into neon for about 1,000 years • Neon fuses into oxygen for a few more years • Oxygen fuses into silicon for a few months • Finally, silicon fuses into nickel which decays into iron

Iron is the most stable element. Fusing it into heavier elements requires energy rather than liberating it. Both fusion and fission reactions ultimately end up at iron.

When the iron core reaches about 1.4 times the mass of the sun (the Chandrasekhar limit), gravity becomes so strong that quantum mechanics takes over.

Electrons run out of room to move and are forced into their lowest energy states. They become absorbed by protons in the nucleus, turning protons into neutrons and releasing neutrinos.

The core collapses at about 25% the speed of light. An iron ball 3,000 kilometers in diameter becomes a neutron star just 30 kilometers across, and the rest of the star caves in, hitting the neutron star and creating a huge pressure wave.

• Neutrinos normally interact so rarely with matter that 100 trillion pass through your body per second • It would take a light year of lead to give a 50-50 chance of stopping a neutrino • They interact only through gravity and the weak force

When electrons are captured by protons during core collapse, an unbelievable number of neutrinos is released—around 10^58. The supernova core is incredibly dense, about 10 trillion times denser than lead.

Despite their weak interactions, the extreme density of the supernova core traps some neutrinos and captures their energy, which is what makes a star go supernova.

A particle millions of times less massive than an electron that barely interacts with anything is responsible for some of the largest explosions in the universe.

• Only 1/100 of 1% of supernova energy is released as electromagnetic radiation (visible light) • About 1% is released as kinetic energy of exploding matter • The vast majority is released as neutrinos

Supernovae have enough energy to outshine a whole galaxy despite only a tiny fraction being visible light.

Neutrinos are the first signal detected from supernovae because they escape the core before the shockwave reaches the surface where light is generated. Neutrinos can arrive on Earth hours before photons, allowing astronomers to aim telescopes at the right part of the sky.

The narrator worked at a neutrino observatory during college on graveyard shifts, responsible for detecting significant increases in neutrino flux and alerting scientists to search for supernovae, though none were ultimately observed during that time.

Not all really massive stars explode as supernovae. Some form black holes during collapse instead, meaning they do not go supernova.

A white dwarf star can pull matter off a nearby star, and when its mass reaches the Chandrasekhar limit of 1.4 solar masses, it collapses and creates a supernova. Kepler's observed supernova in 1604 (20,000 light years away) was this type.

Because shocks are asymmetric, supernovae can eject neutron stars at very high velocities. One observed neutron star moves at 1,600 kilometers per second, thought to be caused by an asymmetric supernova explosion.

• Humans have observed supernovae for thousands of years • Ancient Indian, Chinese, Arabic, and European astronomers all recorded supernovae • They are quite rare—only about one or two per century in a galaxy like the Milky Way

In 1054, light from a supernova 6,500 light years away reached Earth and was recorded by Chinese astronomers.

The Crab Nebula is the giant remnant of radioactive matter left behind by the 1054 supernova explosion.

In the 1,000 years since the 1054 explosion, the remnant has grown to 11 light years in diameter.

Supernovae produce cosmic rays—particles (mainly protons and helium nuclei) that travel at very nearly the speed of light with tremendous amounts of energy.

• The closest stars besides the sun are the three stars in Alpha Centauri at 4.4 light years away • Stars do move around; on average, a star gets within one light year of Earth every 500,000 years

Within a light year of a supernova, the kinetic energy alone could possibly blow the atmosphere off Earth.

High-energy cosmic rays from supernovae break apart nitrogen molecules in the atmosphere. These then bond with oxygen atoms, which can break apart ozone molecules, potentially exposing Earth to dangerous radiation from space.

Scientists observe an increase in atmospheric NO3 concentrations coinciding with supernova explosions.

• A supernova within 30 light years is rare, happening maybe once every 1.5 billion years • Recent research suggests supernovae could be lethal all the way out to 150 light years away, making them much more common

A supernova occurred 150 light years from Earth 2.6 million years ago, visible to early human ancestors like Australopithecus.

• Scientists found traces of iron-60 in sedimentary rocks at the bottom of the Pacific Ocean in a layer deposited 2.6 million years ago • Iron-60 is extremely hard to make and is produced basically exclusively in supernova explosions • Iron-60 is radioactive with a half-life of 2.6 million years, so any iron-60 present proves a recent supernova

Some researchers hypothesize the 2.6 million year old supernova could be related to a mass extinction at the Pliocene-Pleistocene boundary that wiped out around 1/3 of marine megafauna.

Cosmic rays from supernovae hit particles in Earth's atmosphere, creating muons—charged particles like electrons but more than 200 times heavier.

The muon flux for years after the supernova would have been 150 times higher than normal.

Larger animals receive larger radiation doses from muons, which is why megafauna were disproportionately affected by the supernova event.

Animals living in shallower waters were more likely to become extinct compared to those at depth, where water provided protection from muons.

In the space between stars in the galaxy, there are on average around a million hydrogen atoms per cubic meter—essentially a perfect vacuum.

• For hundreds of light years in all directions around our solar system, there are 1,000 times fewer hydrogen atoms than average • The solar system exists in a low-density cosmic bubble as if material has been blown out

This cosmic void provides evidence for tens of supernovae that blew all the material outwards in the past.

There are cosmic explosions even more deadly than normal supernovae: gamma ray bursts.

Gamma ray bursts were discovered by Vela satellites on July 2, 1967, which were looking for Soviet nuclear tests but instead detected a large burst of gamma rays coming from space.

• Mergers of neutron stars • Core collapses of gigantic stars called hypernovae

Hypernovae are caused by stars at least 30 solar masses and rapidly spinning. Their collapse leads to an explosion 10 times more powerful than a regular supernova and leaves behind a black hole.

Gamma ray bursts from hypernovae channel most of their energy into beams just a few degrees across.

If a gamma ray burst occurred within 6,000 light years, it would decrease ozone levels enough to be catastrophic. A sphere with this radius contains hundreds of millions of stars.

On October 9, 2022, astronomers detected one of the most powerful gamma ray bursts ever measured, powerful enough to measurably affect how the ionosphere bounces radio waves—an effect similar to a solar flare.

The October 2022 gamma ray burst was located in a galaxy 2.5 billion light years away, demonstrating the immense power of these events.

• Astronomers speculate a gamma ray burst could have caused the Late Ordovician mass extinction 440 million years ago, which wiped out 85% of marine species • No direct evidence exists, but gamma ray bursts are common enough that there is an estimated 50% chance an ozone-removing, extinction-causing gamma ray burst has occurred near Earth in the last 500 million years

If a supernova or gamma ray burst went off near Earth now, it would be catastrophic.

We owe our existence to stellar explosions despite their danger.

4.6 billion years ago, a shockwave from a nearby supernova probably triggered the collapse of a cloud of gas and dust that gradually coalesced to form our solar system.

The sun, Earth, and all of us wouldn't be here today without the explosions of nearby stars.

Figuring out how supernovae explode required a combination of astrophysics, particle physics, computer science, and mathematics.

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