Supernovae: The Universe’s Most Spectacular Endings

The origin of the Supernova

The term supernova originates from 1604, when popular astronomer Johannes Kepler observed a new star in the night sky. At its brightest is was able to outshine Jupiter and was even visible during the day. He would then coin the term “De Stella Nova” (On the new star) and would publish a book by it. However, Kepler was in fact witnessing the death of a star instead of the birth of one. [1]

Image source: Public Domain

What happens in a Supernova?

A stars existence is a constant balance between gravity and heat. Gravity acts inwards towards the star’s core and the heat produced by fusion of elements acts outwards. Supernova occur when iron is fused at the core of a massive star (more than eight solar masses). A star is unable to fuse elements heavier than iron because the fusion on iron is endothermic rather than exothermic (takes in energy rather than give off energy). As more iron is produced and the star uses up its fuel the outward pressures weaken. When enough iron is formed gravity quickly compresses the star in a matter of seconds. And with this the supernova has begun. This results in the star having a very high density which increases the potential energy.[2] Some of this potential energy is used to produce a shockwave which blows out the stars outer layers. With this shockwave, nearby temperature rapidly increase leading to the formation of heavier elements such as gold, silver, and uranium. [3]

Image source: The outward pressure of fusion balances the inward force of gravity. https://cnx.org/contents/22C4lQHJ@8/Nuclear-Fusion

While a lot of energy is used to expel these outer layers only about 1% of the energy emitted by the supernova is in the form of kinetic energy. Furthermore about 0.01% of the energy is emitted as light [4]. The rest of the energy emitted by the supernova is emitted in the form of neutrinos. Neutrinos have very weak interactions with mass. They only interact using the two weakest forces of the fundamental forces (gravitational and weak force). This allows them to pass through the star’s dense centre before the photons of visible light. We therefore use neutrino detectors to detect supernovae in our galaxy and we then point our telescopes in the direction the neutrinos arrive from [5].

What would happen if the sun went Supernova?

Now what if the sun when supernova? Spoiler alert, it won’t. But if it had 8 times its mass it would be likely to explode in a supernova at the end of it’s life. For the sake of simplicity we will only consider a core collapse supernova. First of all the, average supernova has a luminosity 6 billion times that of our sun [4]. We could expect the ozone layer to be practically obliterated by the UV and Gamma rays emitted by the supernova. We could expect the shockwave from the explosion to evaporate the oceans, along with melting much of the surface materials of the side facing the sun at the time of explosion. The temperature of the Earth’s surface would increase to an unhospitable temperature. Finally, with the sudden decrease in mass of the sun, the earth would no longer be bound to its orbit and its trajectory would change. Fun right? Fortunately we can assure ourselves that this will never happen. However, what is a safe distance for Earth from a supernova such that it wouldn’t be affected by it? According to a recent study based on data from the Chandra X-Ray Observatory , a supernova would have to be within 160 light-years of Earth before we would feel its damaging effects. Formerly, it was believed a supernova would have to be within 50 light-years of Earth to impact our planet [6]. So we can consider ourselves safe for at least our lifetimes.