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Supernovae (SN) are huge explosions of stars at the end of their lives. They are so bright that they can out shine the light from whole galaxies. The first time astronomers saw a supernova was documented in 1054, a massive star in the Milky Way exploded about 6500 light years away. Today, we can still see the remnants of this explosion as a cloud of dust and gas, called the Crab Nebula.
So all Supernovae are explosions of stars, but within this there are some different groups whose explosion mechanisms are very different and the signature we observe vary greatly too. These different explosions can be grouped into two main categories (although there are still sub groups within these!). The two categories were originally created from observational data. One group exhibited hydrogen in their spectra and the others lacked it. Today we know that this observational difference is caused by very different types of explosions. Let’s take a closer look at these groups.
These are the explosions which exhibit hydrogen in their spectra. These occur at the ends of a massive star’s life. By massive we mean at least 8 times the mass of our Sun. Stars burn nuclear fuels in their cores. This produces huge amounts of energy and pressure which balances the gravitational pull, stopping the star collapsing. These stars are massive enough to fuse elements up to iron in their core. Massive stars burn through their nuclear fuel quicker than low-mass stars, so only shine for a few million years (a tiny length of time in the time scale of the Universe!). However, once they reach iron, fusion can no longer occur. When the star runs out of fuel, it cools and there is no pressure to counteract gravity. Thus, gravity wins and the star suddenly collapses. The collapse occurs so quickly that it causes huge shock waves outwards in the star, causing the outer part of the star to explode.
The result is a dense core left behind and a nebula (an expanding cloud of hot gas). This dense core can be a neutron star. The cores of the most massive stars become black holes – the densest objects in the Universe.
The Crab Nebula mentioned already is one of the most famous examples of a Core-collapse SN.
These explosions show no hydrogen in their spectra, but have an excess of carbon, silicon, calcium, and iron. These stellar explosions occur in binary star systems (two stars orbiting each other). One of these stars is a carbon-oxygen white dwarf, usually around the size of the Earth. A white dwarf is what stars similar to our Sun become at the end of their life when they have exhausted their nuclear fuel. The companion star can be a younger less evolved star, such as a main sequence star similar to our Sun or a red giant star. Red giant stars are bright, puffed up, low-to-medium mass stars nearing the ends of their lives.
The gravitational force from the white dwarf pulls material from its companion. Thus, the white dwarf gradually accumulates material in a process called accretion. This continues until the white dwarf reaches a critical mass, approximately 1.44 solar masses, known as the Chandrasekhar limit. At this point the star becomes unstable and explodes as a Type Ia SN.
Type Ia SN are famous as standard candles (distance markers in the Universe) because they all peak at approximately the same brightness. These standard candles help constrain the amount of Dark Energy. This is the mysterious energy in the Universe which seems to be pushing everything apart on the largest scales.
SN 2011fe was a Type Ia SN seen only ~23 million light-years away, in the Pinwheel Galaxy in 2012 and is one of the best observed Type Ia SN.
Supernovae are explosions of stars at the end of their lives. These are so bright they outshine whole galaxies. There are two main groups type Ia which are binary white dwarfs stealing material from a companion until they reach a critical mass. Or core-collapse SN which are the deaths of massive stars more than 8 solar masses.
This post was written by Dr Heather Campbell for Mission Astro.
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