What would we see, and how would Earth be affected, if Sirius, the brightest star in our night sky, were to go supernova? Would it be over in a blink or a decade? And would it be visible in daylight?
Mike Follows
Sutton Coldfield, West Midlands, UK
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Sirius is a binary star system that is only 8.611 light years from Earth, comprising a main sequence star of a type larger than our sun and a white dwarf. If one went supernova, there is little doubt that it would be visible even in daylight and would certainly cause a mass extinction on Earth, so you wouldn’t be around for long to admire it. Supernovae can briefly outshine the rest of their galaxy and stand out from their background stars for months.
Supernovae come in different varieties. Stars that begin life with at least eight times the mass of our sun explode when they run out of fuel. This is known as a type II supernova. A white dwarf that is part of a binary star system can explode in a type Ia supernova. This occurs when it sequesters material from its companion, typically a red giant, and reaches a critical mass known as the Chandrasekhar limit, which is about 1.44 times that of our sun. Thankfully, Sirius B, which is the white dwarf in the Sirius binary system, is thought to be too far from its companion, Sirius A, to siphon off sufficient mass for this.
The appearance of a supernova and its effect on Earth life depends on its mass and distance from us. In the Milky Way, there is typically one supernova every 50 years. We would expect them to occur closer to us as Earth passes through the spiral arms of our galaxy, as these regions contain more short-lived massive stars.
The potential damage from a supernova gets delivered in instalments. Neutrinos carry away most energy from a supernova and they pass harmlessly through matter. Their arrival coincides with gamma rays, X-rays and visible light, which increases the brightness of the exploding star by a factor of 10 to 100,000. If the supernova were within 20 light years, this would destroy up to half of our ozone layer, leading to a mass extinction event.
It can take thousands of years for matter from the blast, including the particles that make up cosmic rays, to arrive. Although they only account for 10 per cent of the supernova energy, cosmic rays could cause a mass extinction from up to 50 light years away.
A supernova up to 100 light years away may have been responsible for the climate cooling that took place at the start of the Pleistocene epoch around 2.7 million years ago. This coincides with the change in habitat in Africa, when grasslands spread and forests declined, which may have favoured the appearance of Homo habilis, the first species in our genus, around the same time.
A supernova 41,000 years ago and 250 light years away probably resulted in the The gamma radiation and neutrinos would have taken just 250 years to reach us. It took another 7000 years for radioactive iron-rich grains from the blast to pepper Earth, seen as tiny impact craters on mammoth tusks.
Mercifully, there are no obvious supernova candidates close enough to threaten life on Earth.
Herman D’Hondt
Sydney, Australia
Let me start by saying that, with a mass of only about twice that of the sun, Sirius [A] isn’t expected to go Such stars need about eight solar masses to blow.
If it could and did go supernova, it would definitely be visible in daylight. The most recent such explosion in the Milky Way was observed by Johannes Kepler in 1604. It was visible in daylight, even though it was about 20,000 light years away from Earth. As Sirius is only about 8.7 light years from us, it would make an impressive display. The actual explosion takes only seconds, but the brightness would seem to increase over several days, and then gradually get dimmer over months. The duration varies with the type of supernova.
To be safe from a supernova we would need to be at least 150 light years away. Any closer, and we would start feeling the effects. At 30 light years, X-rays and gamma rays from the blast can destroy Earth’s ozone layer, which protects life from solar UV radiation. This would cause mass extinctions.
Considering the frequency of supernovae, this has probably happened a few times. It is estimated that around 20 have occurred within 1000 light years of Earth in the past 10 million years, and a supernova at just 30 light years is expected every 250 million years. There is evidence, through the discovery of the unstable iron-60 isotope in deep-sea and lunar deposits, as well as in Antarctic ice, that one happened relatively close to the solar system between 2 and 3 million years ago. This form of iron is made in some stars and flung out by such explosions. This particular supernova is estimated to have been about 200 light years away, but may still have led some large marine animals to go extinct.
Edward Anderson
via Facebook
There would be sirius implications!
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