Supernova
The final stage in the life cycle of stars that are at least 10 times more massive than the sun. During this stage, a star explodes and emits a huge amount of light, which is brighter than the light of a billion suns. Because it is so bright, the star looks new. That is why this stage was named "supernova," which means "very new." After a short period of time, the light of a supernova star dims, and the star's remains become a neutron star.
Our Sun is in the "safe region" of Milky Way, (26,000 light years from the centre) where the the number of star in the given space cube is not so high and not so less. The central part of Milky Way has many times more number of star in the same volume. In this region, number of supernova bursts is very high.
Any star at 40-50 light years from us, if get supernova burst, can become the cause of "doom" on the Earth.
Supernova stars have redioactive iron-60 (half-life time : 15,00,000 years) in their centre. By the burst, they spread iron-60 in the nearer part of space of them. By calculating the age of the sample of iron-60, scientist can know whether that iron is of Earth or came from supernova.
Before 28,00,000 years, six supernova bursts were happened. If in future, this happens, it can harm the life on the Earth. German Scientist came to know this after sampling the rocks from Pecific Ocean.