At some point, the core can no longer support its own weight.The outer layers of the star, which had been held back by the core’s pressure, start to collapse inward.This collapse lasts only a few seconds, but it compresses the core to a million times its original volume.The temperature rises to a hundred billion degrees and the core is now hot enough to fuse nickel and iron into heavier elements.The core is now so hot and so dense that it can no longer hold itself up.It collapses to the size of a small city and the outer layers of the star fall into it.The energy released by the collapse causes a shockwave that tears through the star, blasting the outer layers into space.This is a supernova.
If a star exploded near Earth, the closest star being the Sun, it would be an incredibly powerful event. A supernova is the biggest explosion in the universe, and is so bright that it appears to come out of nowhere. Johannes Kepler famously observed a supernova in 1604, and it was as bright as the planet Jupiter. The process of a star going supernova is complex, and starts with the star running out of fuel and the core compressing until it is hot enough to fuse nickel and iron into heavier elements. This causes a shockwave that tears through the star, blasting the outer layers into space. When the iron core of a star reaches the Chandrasekhar limit of 1.4 times the mass of our sun, something wild happens. The electrons, which have run out of room to move, become absorbed by the protons in the nucleus and are forced into their lowest energy states. This causes the protons to turn into neutrons and release neutrinos. As a result, the core collapses rapidly at about 25% the speed of light, shrinking from a ball of iron 3,000 kilometers in diameter to a ball of neutrons just 30 kilometers across, forming a neutron star. With no outward pressure to hold it up, the rest of the star caves in, and the kinetic energy created by this is not enough to trigger a supernova. However, the neutrinos released in the process are, as an unbelievable number of them is released - around 10^58. Although neutrinos are usually thought of as particles that do not interact with matter, in a supernova they are trapped by the incredibly dense core, capturing their energy and triggering the explosion. Only 1/100 of 1% of the energy released is in the form of electromagnetic radiation, which is the light we can see, while the majority is released as neutrinos. This is why astronomers are able to detect neutrinos hours before they detect the light from a supernova. Not all massive stars explode when they collapse, and sometimes a white dwarf star can reach the Chandrasekhar limit and trigger a supernova. This is what happened in 1604, when Kepler observed a supernova 20,000 light years away. The asymmetric shocks of a supernova can also explain why some neutron stars move incredibly fast, with one observed moving at 1,600 kilometers per second. Despite only recently learning about how supernovae work, humans have been observing them for thousands of years. Ancient Indian, Chinese, Arabic and European astronomers all observed supernovae, but they are quite rare. In a galaxy like our Milky Way, consisting of 100 billion stars, there are only about one or two supernovae per century. A particularly amazing example is the supernova of 1054, when the light of a supernova 6,500 light years away reached the earth and was recorded by Chinese astronomers. If we look to where that supernova was recorded, we see the Crab Nebula - a giant remnant of radioactive matter, left behind by the explosion. In the 1,000 years since the explosion, the remnant has grown to 11 light years in diameter.
Supernovas produce a lot of cosmic rays - particles, mainly protons and helium nuclei, that travel out at very, very nearly the speed of light. They have a tremendous amount of energy. So at what distance could a supernova cause problems for life on Earth? The closest stars to us, besides the sun, are the three stars in Alpha Centauri. They are 4.4 light years away, but stars do move around and on average, a star gets within one light year of Earth every 500,000 years. So, what would happen if such a star went off?
Within a light year, you’re easily within a danger distance from just the kinetic energy. So even at that distance, you’re looking at possibly blowing the atmosphere off. But we would also have other problems to worry about. Supernovae create conditions that are hot enough to fuse elements heavier than iron. In the months after the explosion, these elements undergo radioactive decay, producing gamma rays and cosmic rays. Less than 0.1% of the energy produced by a supernova is emitted as gamma rays from these radioactive decays, but even this tiny percentage can be dangerous. At a few light years from a supernova, the radiation could be deadly, though most of it would be blocked by our atmosphere.
Now, the earth is protected from solar and cosmic radiation by our atmosphere, and specifically by ozone molecules, three oxygen atoms bonded together, but high energy cosmic rays from supernova can come down and break apart nitrogen molecules in the atmosphere, and then these bond with oxygen atoms, which can then break apart ozone, and so we can lose a lot of our ozone if there’s too many cosmic rays coming from supernova events, and that can expose us to all kinds of dangerous radiation coming in from space. We actually see an increase in atmospheric NO3 concentrations, coinciding with supernova explosions.
A supernova within 30 light years is rare, only happening maybe once every 1 1/2 billion years or so, but a recent article suggests supernovae could be lethal all the way out to 150 light years away, and so those would be much more common. We actually have evidence for a supernova that went off 150 light years from Earth 2.6 million years ago. It would’ve been seen by our early human ancestors, like Australopithecus, and we know this because there are elements present on Earth that could only have been deposited by a recent supernova. In sedimentary rocks at the bottom of the Pacific Ocean, scientists have found traces of iron-60, in a layer that was deposited 2.6 million years ago. Iron-60 is an isotope of iron with four more neutrons than the most common type of iron. Iron-60 is really hard to make; our sun doesn’t make it, nor is it produced, basically, anywhere else in the solar system. Iron-60 is made, basically, exclusively in supernova explosions, and iron-60 is radioactive. It has a half life of 2.6 million years. So every 2.6 million years, half of the sample decays into cobalt-60. So all of the iron-60 that was around during the formation of the earth, 4.5 billion years ago, has definitely decayed. So the iron-60 that the scientists measure is proof of a recent supernova. Scientists also measured trace amounts of manganese-53 in the same sediments, giving further evidence supporting the idea that recently there was an explosion of a nearby supernova. The supernova that happened 2.6 million years ago wasn’t catastrophic for our ancestors, but some researchers hypothesized that it could be related to the mass extinction, which is seen at the Pliocene-Pleistocene boundary in the fossil record around the same time. This extinction wiped out around 1/3 of marine megafauna. The idea is that the cosmic rays from the supernova hit particles in our atmosphere, creating muons, which are charged particles like the electron, only more than 200 times heavier. The muon flux for years after the supernova would’ve been 150 times higher than normal, and the bigger the animal, the larger the radiation dose it would’ve received from these muons, which is why megafauna were so disproportionately affected, and what’s more, the animals that lived in shallower waters were more likely to become extinct compared to the ones that lived at depth, where the water would’ve protected them from muons. Further evidence for these recent nearby supernovae comes from our place in the galaxy. You know, if you look in the space between the stars in our galaxy, on average, there are around a million hydrogen atoms per cubic meter. That may sound like a lot, but it’s basically a perfect vacuum but for hundreds of light years in all directions around our solar system, you find there are 1,000 times fewer hydrogen atoms. It’s like they’ve all been blown out somewhere, and our solar system is existing in this cosmic void, inside a low density bubble. So that is evidence for maybe tens of supernovae that would’ve blown all this material outwards, but there are cosmic explosions that are even more deadly than normal supernovae, gamma ray bursts. Gamma ray bursts were discovered by the Vela satellites, which were looking for Soviet nuclear tests but on the 2nd of July, 1967, the satellites detected a large burst of gamma rays, which were coming from space. There are two main sources of gamma ray bursts, mergers of neutron stars and the core collapses of gigantic stars called hypernovae. Hypernovae are caused by stars that are at least 30 solar masses and rapidly spinning. Their collapse leads to an explosion 10 times more powerful than a regular supernova, and it leaves behind a black hole. The gamma ray bursts caused by hypernovae channel most of their energy into beams which are just a few degrees across. If there was a gamma ray burst within 6,000 light years, it would decrease the ozone level enough that it could be catastrophic. To put this distance in context, a sphere with a radius of 6,000 light years contains hundreds of millions of stars. On October 9th, 2022, astronomers detected one of the most powerful gamma ray bursts ever measured. It was powerful enough to measurably affect how the ionosphere bounces radio waves. The effect on the ionosphere was around the same as a solar flare, but this gamma ray burst was located in a galaxy 2.5 billion light years away. Astronomers speculate that a gamma ray burst could have caused the Late Ordovician mass extinction, which wiped out 85% of marine species 440 million years ago. There is no direct evidence, but gamma ray bursts are common enough that it is estimated that there has been a 50% chance that there was an ozone-removing, extinction-causing GRB in the vicinity of Earth in the last 500 million years. So if a supernova or a gamma ray burst were to go off near the earth now, that would be pretty catastrophic but in an ironic twist, we kind of owe our existence to these sorts of explosions because 4.6 billion years ago, it was probably the shockwave from a nearby supernova which triggered the collapse of a cloud of gas and dust that gradually coalesced to form our solar system. So the sun, the earth and all of us wouldn’t be here today without the explosions of nearby stars. Figuring out how supernova explode was incredibly difficult. If you want to gain a better understanding of our universe, then check out Brilliant. It’s an interactive learning tool that uses a hands-on approach to teach a wide range of STEM concepts. With simulations, quizzes, and thousands of lessons ranging from the foundations of math to cosmology and quantum mechanics, Brilliant is the best way to learn something new every day. 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