Stars evolving and dying? These may be strange concepts to many readers, since most of us think of stars as symbols of permanence.
Compared to a human lifetime, most stars appear permanent. But in the course of millions to billions of years, they do evolve and age, and eventually they die. The most massive stars end their lives violently and spectacularly. Less massive stars die more peacefully. Examples of the former are supernova explosions, which are among the most energetic events in the heavens. Examples of the latter are the formation of planetary nebulae, in which, over the course of thousands of years, a star sheds its outer layers.
Both of these events -- supernova explosions and planetary nebula ejection -- leave behind stellar remnants. In the case of supernovae, the remnant is a neutron star or a black hole. In the case of planetary nebula ejection, it is a white dwarf.
White dwarfs, neutron stars, and black holes are the dead cinders of former stars. But if they have a close stellar companion from which they accrete fresh matter, they may get rejuvenated and undergo further outbursts.
This article describes the evolution of stars and the dramatic events that await them during their final years of glory.
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