galaxies. We have no real metaphor for their power  – if the sun were to magically go supernova it  

would feel like you were being hit by the energy  of a nuclear explosion, every second. For weeks. 

While supernovae are the engines of creation,  forging the elements that enable life, they also  

burn sterile whole regions of galaxies.  So what would happen if one hit earth?

There are roughly speaking two ways to  make a supernova. Either the core of a  

massive star implodes, or, less common, a  white dwarf gains mass to the point where  

it ignites explosive nuclear fusion. The  outcome is the same: a supernova explosion. 

If we think of an explosion on earth, we  think of something that happens fast and ends. 

But a supernova is more like a volcanic  eruption followed by a tsunami. At first  

there is a colorful ball of hot, expanding gas,  creating a spectacular cloud that will shine for  

about a month – but then it doesn’t stop. Hot  and dangerous gas rushes outward at speeds of  

10,000 km/s through the near vacuum of space,  sweeping up the sparse gas of the galaxy.  

This wall of gas expands for tens of thousands  of years and will eventually span up to dozens  

of light-years until it finally cools off, and  disperses its substance back into the galaxy. 

So what if this star tsunami hits us? Well,  the damage depends on how far away it is.

Stage 1: Thousands of Light Years away 

Humans have witnessed dozens of supernovae but  all of them were thousands of lightyears away.  

They appeared as new stars, some outshining  the moon, twinkling for a few weeks and  

disappearing. Aside from looking very pretty  at this distance they don’t do much to us.

Stage 2: 300 Light Years away Things begin to get a tiny bit  

icky once a supernova occurs around 300  lightyears away. We can expect one this  

close to us every few million years:  a single star giving the night sky  

an eerie glow like twilight. And while this  is far and dim enough not to do harm to us,  

they can affect the earth. At these distances  it is like being hit by the last weak waves  

of the star tsunami. Not strong enough  to do real damage but still noticeable. 

In fact, we know that over the past 10 million  years multiple supernovae have struck Earth from  

these distances because we can find radioactive  isotopes of iron deep in the rocks and sediments  

at the bottom of the ocean. Amazingly, these  supernovae around the solar system have cleared a  

1000 light-year wide pocket of space that is  called the 'Local Bubble'. They blew away the  

interstellar gas and dust, creating a lumpy wall  of gas that is now a cradle for star formation.

Stage 3: 150 Light Years Away Once a supernova happens much closer  

than 300 light-years, we’re approaching  the zone where it does real damage. 

Stars have extremely powerful magnetic fields.  When they die, the tsunami of dead star actually  

retains a lot of this magnetic energy, woven  through the shockwave that expands outwards.  

In this highly magnetized cloud,  we get conditions like in a huge  

particle accelerator that is accelerating  charged particles like protons, nuclei,  

and electrons to immense speeds. Which means we  have an expanding cloud that is shooting deadly  

radiation in all directions, long after the bright  light from the initial explosion has faded away. 

If a supernova happens too close by, waves  of these ‘cosmic rays’ will wash over the  

solar system for thousands of years. While  we’re mostly protected on earth’s surface  

by the atmosphere and ozone layer, the influx  of extra radiation will still increase cancer  

and mutation rates. Not enough to cause a  mass extinction but it will be noticeable. 

Spaceflight would become  impossible in the solar system,  

as astronauts would not survive  the waves of radiation for long.  

We don’t know exactly how bad this would  be, but a supernova that is close enough may  

trap our species on earth for generations, maybe  thousands of years. It only gets worse from here.

Stage 4: Closer than 100 light years Within 100 light years, things get bad,  

as a supernova disrupts our climate in  ways that we don't fully understand yet.  

There are a few unpleasant things  happening all one after another: 

First, the high energy photons arrive from the  explosion, followed by many decades of radiation  

from the radioactive tsunami, both of which  seriously damage the ozone layer, earth’s shield  

against harmful radiation. The ozone layer absorbs  ultraviolet radiation by breaking apart ozone, O3,  

into O2 and a free oxygen atom, which later  reforms back into another ozone molecule.  

But the supernova radiation breaks up Nitrogen  molecules that gobble up the free oxygen,  

breaking the cycle and depleting the ozone  layer quickly: Without a radiation shield  

everybody living on the surface is exposed  to very high levels of UV radiation from our  

sun – cancer rates would skyrocket and just going  outside during the day could be life threatening. 

The extra radiation would also kill a lot, if not  most of the plankton in the oceans that live near  

the surface and are the basis for the marine  food chains – leading to a mass extinction. 

Worse still supernova radiation would ionize gas  in the atmosphere, which means that it would punch  

through molecules and knock electrons off nuclei,  leaving them charged. These charged nuclei then  

act as seeds for water vapor to gather and form  massive global clouds. In the worst case they  

would reflect enough sunlight to trigger an ice  age. In fact, it's thought that the ice age 2.5  

million years ago was caused by a supernova. Some  scientists even think that a supernova about 60  

light years away might have been the cause for the  Devonian mass extinction 350 million years ago.

But wait, there is more. The electrons punched  free by the radiation, form enormous electric  

avalanches – or in other words: lightning. Earth  is hit by some of the worst thunderstorms in  

millions of years. The intense lightning causes  global wildfires that consume forests and crops,  

devastate cities, disrupt our electrical  grids and global supply chain. All while  

a decimated ozone layer leaks deadly radiation. While in the past, the ecosystem may have bounced  

back from a nearby supernova after a few thousand  or million years, there’s no guarantee modern  

civilization can take a hit of this magnitude.  Food shortages, skyrocketing prices, and wars,  

as nations struggle to not be consumed by chaos. So a supernova this close would at the very least  

do significant damage for hundreds or thousands  of years, if not end our modern civilization, and  

with it millions or even billions of lives. Still,  humanity would likely survive and could recover.

Stage 5: Closer than 25 light years A supernova closer than 25 light years  

means that we’re in its ‘kill radius’ where  a mass extinction is all but guaranteed.  

Probably about half of the ozone layer would  be destroyed, and massive climatic disruption  

on a scale we have never witnessed would ravage  earth. Entire ecosystems would swiftly be wiped  

out by radiation, as global wildfires envelop the  planet. All the things described before happen,  

but way more intensely and much faster. A few  people might survive for years in bunkers,  

if they have food supplies, but the world  they return to will be devastated and  

hostile to life for hundreds of thousands of  years. Human extinction is extremely likely.

Final stage: 4 light years Being any closer to a supernova is very  

unlikely because space is big. But the effects  would be extreme. Even from 4 light years away,  

the distance to Alpha Centauri, a supernova would  be almost as bright as the sun in the sky. While  

casting two shadows could be fun for a few hours,  within days the earth’s surface gets as hot as a  

sauna, baking the surface for weeks until the  explosion fades. The surface of earth burns,  

scoured of life. Even the oceans aren't  safe: the massive amount of radiation  

that follows burns away the ozone layer,  killing everything that sees sunlight. 

It would be the largest extinction event in  history, reducing life to a few survivors  

in the deep sea and critters in the deep  soil. Life basically has to start over.

Conclusion: How worried do you need to be? So should you worry? No! Fortunately,  

there are only a handful of stars that may explode  within 1000 lightyears of earth and none are close  

enough to be a serious threat. Even better, these  stars will probably not go supernova for many  

millions of years. So you are safe. But there’s  no guarantee for the far future. As stars orbit  

the galaxy, our descendants may find themselves  dangerously close to a supernova – but by then  

a far more advanced and wiser humanity will  hopefully be able to just move out of the way. 

In any case, you can sleep well  tonight under the beautiful night sky.

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