Stars may explode, releasing an enormous amount of energy and ejecting incandescent gases at extremely high velocity. In the space of a few months, novae may release the same amount of energy radiated by the sun in 10,000 years; supernovae release the same amount of energy released by the sun in 1,000 million years. It is highly probable that life on earth would be completely destroyed if a star close to the solar system were to explode in this way.
Stellar explosions are infrequent; novae occur in the galaxy at the rate of 20 to 50 per year and supernovae occur at an estimated rate of one per galaxy per 360 years. Galactic novae are concentrated in a band 10 degrees each side of the plane of the galaxy and are densest toward the centre (our solar system lies towards the perimeter). The probability of the earth being affected is therefore very low. It has however been estimated that a supernova explosion will occur once in every 50 million years within 100 light years of the earth. Even at this distance the effect upon the atmosphere would be certainly catastrophic and sufficient to cause mass extinction of many species.