A supernova is a short phase in the life of a very massive star. Only stars that start their life with more than about eight times the mass of the Sun can become a supernova. The Sun cannot turn into a supernova. Such a star becomes a supernova when the nuclear reactions inside the star cannot compensate anymore for the loss of energy to light and other radiation and neutrinos. Then the inner parts of the star collapse and the outer parts explode into space. The star then appears much (up to about 15 magnitudes or one million times) brighter for a few weeks. The outer layers that are launched into space form a so-called supernova remnant, of which the Crab Nebula is an example. It may be that the inner parts of the star explode as well, and that nothing is left of the star. It is also possible that a neutron star or a black hole is left.
The brightest stars in the sky that might turn into a supernova have magnitudes near 0, so if such a star goes supernova, then it can briefly get as bright as magnitude −15, which is just brighter than the Full Moon, but the Sun is about 25,000 times brighter still.
These stars are all quite far away, so there is no danger of the Earth being caught up in the explosion, or anything like that.
The brightest stars in the sky that might turn into a supernova have magnitudes near 0, so if such a star goes supernova, then it can briefly get as bright as magnitude −15, which is just brighter than the Full Moon, but the Sun is about 25,000 times brighter still.
These stars are all quite far away, so there is no danger of the Earth being caught up in the explosion, or anything like that.
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If the star was close enough, it could wipe out all life on earth. Luckily, none of the stars nearby look like they will supernova anytime soon.