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What causes a star to go supernova?

~Yagyasha Rastogi

Before diving into what causes a star to go supernova, let’s understand what a supernova is, A supernova is a celestial phenomenon in which a stellar-mass explodes emitting a large amount of energy. This event occurs during the last evolutionary stages of a massive star or when a nuclear fusion is triggered in a white dwarf. Astronomers divide supernovae into mainly two categories, type I and type II, based on the way they explode. Type I supernovae are approximately three times more luminant than type II supernovae, they also expand more rapidly than the latter. When a star gains matter from a nearby star until a nuclear chain reaction starts, it is categorized as a type I supernova. Whereas, in type II reactions star runs out of fuel and collapses under its own gravity. Now that we know what supernovas are, let us figure out what causes a star to go supernova,  There are several different reasons for a star to go supernova, one of the most common studies ones is “Core-Collapse” supernovae (or type II supernova). This type of supernovae occurs at the later stages of a star’s life. Massive (having a mass of about 8 solar masses) stars have fuel burning in their cores, which produces heat that in turn provides an outward force against the inward gravitational forces. These forces are balanced for all of the star’s life, and the star remains stable. However, as the star starts running out of fuel, the outward force decreases. Gradually, the star develops onion-like layers with heavier elements at the core and lighter ones on the outer layers.  When the core’s mass exceeds a limit (Chandrashekhar limit), the core collapses on itself causing the star to go supernova. This collapse emits a large amount of energy and matter. After the core collapses, all that is left is a dense core and hot gas called a nebula. If a star is very massive, the core forms a black hole. Otherwise, the core becomes an ultra-dense neutron star. However, not all stars are capable of going supernova, there is a condition that decides the fate of a star. For a star to go supernova, its core should cross a threshold mass as discussed above. This critical mass is called the Chandrashekhar limit. Stars whose core does not pass the Chandrashekhar limit, do not go supernova. Once they run out of fuel, the force of gravity does shrink them, but they can settle at a radius of a few thousand kilometers. These stars are very dense and are called white dwarfs. We expect the Sun to become a white dwarf at its final stage.  Here are some beautiful pictures of some supernovas and nebulas:



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