Twinkle twinkle
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 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,
“Thankfully, Sirius B, which is a white dwarf, is too far from Sirius A to siphon off enough mass to become big enough to explode”
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 cau
