We’ve explored what would happen if our Sun went supernova [link]. Spoiler: while it would be lethal enough to necessitate evacuation to Pluto or beyond, it’s not the biggest headache for humankind today. But what if another star, not very far from our Solar System, exploded? We’ve seen that a supernova explosion could pose a risk of life even at interstellar distances.
According to the Gaia catalogue [link], we have almost 400 stars in the immediate neighbourhood (10 parsecs, approx. 33 light years), 5400 stars within 25 parsecs (80 light years) and a mind-boggling 300,000+ stars within 100 parsecs (325 light years). Even with such elevated figures, it is very unlikely that this would be the full list: some of the stars are hard to detect, for instance the dimmer white and other dwarfs, and high-tech astronomy is relatively recent, so we simply haven’t had time to find them all. So of this myriad stars, are there supernova candidates too close for comfort?
The two main types of supernovae: massive stars (Type 2) and white dwarfs (Type 1)
Massive stars – core-collapse supernovae (type 2)
We all know that supernovae produce helluva big explosions, but the mechanism leading to the explosion can be quite different depending on the star. The best-known supernova type is the massive star, at least 8-10 times the mass our Sun, fusing heavier and heavier elements together until it runs out of suitable raw material to produce energy, and it collapses under its own weight. This core collapse is a phenomenally rapid and violent process, liberating more energy than some stars produce in their lifetime, and can produce such brightness that it could be easily observed from a distance of millions of light years, in galaxies far, far away.
The more massive the star, the shorter its lifespan, and the more violent the explosion. A star that is the size of our Sun can go about fusing hydrogen into helium and shining brightly for billions of years. Much more massive stars would burn through their hydrogen quickly and then fuse helium and heavier elements, evolving into supergiants in the process. Supergiant stars are 30-100 times larger than our sun, and they are much, much brighter. They are typically red or blue: red supergiants have low surface temperatures (hence the colour red), while blue supergiants are among the hottest stars in the Universe.
If blue supergiant star sounds exotic to you, no wonder – they are called “rock-and-roll stars” because they burn bright, live on the edge and die young. This short lifespan makes it difficult to catch and study them. They come in many varieties: some are called supernova impostors, because they produce such giant eruptions that they are mistaken for supernovae; some rotate at such extreme speed that they distort from sphere to oblong; and some are more ordinary and stable, like a rock star settled with their family. Whatever colour, typical supergiants use up helium, carbon etc. in their cores in a few million years, and then explode as supernovae. The remnants would end up as neutron stars (with a very small, very dense core of neutrons) or the most massive ones, as black holes.
Binary white dwarfs – thermonuclear supernovae (type 1)
White dwarfs are one of the final stages of a star’s life cycle. Typically they have evolved from a not too massive main sequence star (like our sun): after billions of years, the star would become a red giant, fusing helium, and then, after running out of helium, it would stop fusion and shrink into a white dwarf. A white dwarf in effect is a dead star, roughly the size of Earth, glowing bright and white due to the residual heat, but without any nuclear fusion occurring in its core. It emits light, but it’s dimmer than in the previous phases. Eventually, they fade into black dwarfs.
But this is not always the end of story for white dwarfs. White dwarf stars that form part of a binary system can go out in a more spectacular manner. In such a system, the white dwarf could start “stealing” hot gaseous mass from its companion.
If enough hot gas piles up on the surface of the white dwarf, its core can re-ignite, and the resulting thermonuclear explosion would blast the star to bits, producing a bright supernova. Despite the humbler origins, white dwarf supernovae are brighter than many core-collapse supernovae. Even more interestingly, they show a certain uniformity in peak brightness, which makes it possible to use them to calculate interstellar and intergalactic distances.
As usual in life and the universe, the supernova typology is more complex, and both type 1 and type 2 have various subcategories (you can read further e.g. on this link [link]. But our main interest here is not classification, but risk – what kind of threats would different supernovae pose to life?
Long-distance dangers of supernovae
We’ve seen that the heat and blast impact would only be lethal within the same solar system, on the inner planets. The massive volume of energized neutrinos from the explosion could pose a risk further away, but even the most pessimistic estimates limit the harm at around 10 light-years. On the other hand, some phenomena – in particular gamma rays, X-ray and ultraviolet radiation – could be harmful to Earth from much greater distances.
Gamma ray bursts
Gamma rays are probably the most fearsome of the long-distance effects. They are electromagnetic radiation at the very edge of the spectrum, with the shortest wavelength and highest energy. They are produced by the hottest and most energetic objects in the universe – think of neutron stars and pulsars, black holes, and yes, supernova explosions. Gamma-ray bursts a particularly fearsome-sounding variant, originating from superluminous supernova explosions. These radiation bursts are extremely energetic (a typical burst releases as much energy in a few seconds as the Sun will in its entire 10-billion-year lifetime) and extremely rare (a few per galaxy per million years). So far, all observed gamma-ray bursts have originated from outside the Milky Way galaxy. According to hypothesis, a gamma-ray burst in the Milky Way, pointing directly towards the Earth, could cause a mass extinction event.
Fortunately, this is highly improbable. First, the gamma-ray burst would need to happen within our own galaxy, at a distance of 5000-8000 light years, to represent any real threat. As Earth is located in Milky Way’s quiet suburbia, in a low density region far from the crowded and overactive central regions, the chances of such an energetic event at this distance is very low. Second, gamma-ray bursts are narrowly focused, with most of the explosion energy aligned into a narrow beam. This narrow beam would need to point at Earth to cause significant damage – which, given the size of the galaxy, would really take some rotten luck. Until now, we have not identified any future candidate for a gamma-ray burst in the Milky Way.
Still, what would happen if a supernova exploded at a distance of 5000 light-years from Earth, and its gamma rays beamed directly at Earth? The result would likely be a mass extinction event. The gamma radiation would destroy a significant portion of our atmosphere, specifically the ozone layer, and deplete our protection from cosmic rays. The water and land surface would experience high doses of radiation that would damage cells, cause cancer and mutations, and in the worst case scenario, could exterminate most existing species.
Ionizing and ultraviolet radiation
While gamma-ray bursts are exotic and rare enough to cause nothing more than scientific curiosity, any garden-variety supernova could produce harmful effects in the form of ionizing radiation (gamma rays and X-rays) and ultraviolet radiation. While ionizing radiation can directly destroy cells, the real threat on global scale is not so much to individual human bodies, but to the atmospheric composition, and to the basis of the food chain. A massive extinction of oceanic plankton would result in a chain reaction, leading to the extinction of a great number of species.
Studies looking at past supernova events and their effect on Earth estimate the dangerous range of X-ray impact around 100-160 light years. Others using atmospheric models calculate that a Type II supernova closer than eight parsecs (26 light-years) would destroy more than half of the Earth’s ozone layer.
Astronomers have a list of core-collapse supernova candidates, such as the red supergiants Antares and Betelgeuse, and blue supergiant Eta Carinae, but happily, none of them are closer than 500 light years. With Type I supernova, it’s a slightly different story. As they arise from dim white dwarf stars, the likelihood of astronomers not spotting the star before it explodes is much higher. The closest known candidate is IK Pegasi (HR 8210), located at a distance of 150 light-years, but observations suggest it could be as long as 1.9 billion years before the white dwarf accrete the critical mass to produce a supernova.
Still, the unpredictability of white dwarf supernova closer to our planet remains a slight risk. While astronomers continue watching the skies and studying star systems to identify this kind of potential threat, other scientists are digging the ground under our feet to find answers. There is already some evidence, though not unequivocal and in all cases, much debated, that past extinctions could have been caused by supernovae.
Did supernovae cause past extinctions? [coming up]
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