Most celestial events unfold over thousands of years or more, making it impossible to follow their evolution on human timescales. Supernovas are notable exceptions, the powerful stellar explosions that make stars as bright as an entire galaxy for several days. Although they are very rare – only a few such explosions take place every century in a typical galaxy – supernovas can be seen with the naked eye if they are reasonably nearby. In fact, when supernovas were discovered they were thought to be new stars appearing in the sky – ‘nova’ means new in Latin. Astronomers have recorded supernovas long before a theoretical understanding of these events as stellar explosions was developed in the 20th century. The most ancient documented record dates back to 185 AD, when Chinese astronomers saw a ‘guest star’ that remained visible for several months, in the vicinity of the two stars Alpha and Beta Centauri. The material ejected during these explosions sweeps up gas and dust from the surroundings, creating picturesque supernova remnants that can be observed long after the explosion. Modern astronomers believe that the object shown in this image, the supernova remnant RCW 86, is what remains of the supernova that was discovered in 185 AD. The blue and green glow at the edges of the bubble represents X-ray emission from hot gas, heated to millions of degrees by shock waves generated after the explosion. The diffuse red glow marks infrared emission from warm dust in the interstellar medium around RCW 86. Sprinkled across the image, in yellow, are young stars that shine brightly at infrared wavelengths. This image combines X-ray data from ESA’s XMM-Newton and NASA’s Chandra X-ray Observatory (combined to form the blue and green colours) with infrared observations from NASA’s Spitzer Space Telescope and Wide-Field Infrared Survey Explorer (yellow and red). The supernova remnant RCW 86 is some 8000 light-years away.

The death of stars

Stars inevitably run out of fuel to burn in the nuclear fusion reactions that burn in their cores and when this happen there are a number of spectacular events that occur in their death throes. As a star goes through its main sequence stage it burns hydrogen into helium through nuclear fusion. As it starts to generate lots of helium this falls to the centre of the star because it is heavier than hydrogen. Once enough helium is created the temperature and pressure increase enough for helium to star fusing into carbon, which again falls to the centre of the star. This process continues creating elements up to iron in shorter and shorter timescales. No elements beyond iron are created during the thermonuclear fusion in stars because the temperatures and pressures needed cannot be reached. You need to add additional heat to fuse elements create then iron, but clearly there are elements heavier than hydrogen, e.g. all the precious metals, so where do these come from?

For stars that are about 25 solar masses (25 M⊙) or more the star will create an iron core which generates a huge gravitational force pulling in the entire star towards the centre. Once the iron core starts to exert it’s massive gravity and silicon burning finishes, the star suddenly stops producing energy to hold itself up and it collapses into the core. Bearing in mind the star is about 25 times more massive than the Sun it collapses in a really short time, about 1/4 of a second, only slightly longer than it takes you to blink. As the material that is outside the core collapses it releases gravitational energy which is turned into heat and neutrinos. When the collapsing material hits the solid iron core it bounces off increasing both the temperature and pressure significantly and in the moments after the bounce further nuclear fusion takes place because of the sudden temperature and pressure increase, creating the elements beyond iron and scattering them out into space.

At the moment of the bounce the collapsing pressure forces the protons in the core to overcome the pressure holding their elections in orbit, squeezing them together creating neutrons which are packed tightly together with no gaps (think of a box full of marbles and the way they stack up next to each to each other.) This creates a neutron star, incredibly dense with some bizarre properties such as extremely fast rotation, thousands of times a second, and the interior is thought to be a superfluid and a superconductor due to the intense gravitational pressure.

Stars with more than 25 solar masses create a neutron core so big that even though the neutrons are packed as tightly as possible it still can’t resist it’s own gravitational attraction and it collapses further into a singularity, more commonly called a black hole. 

Table 1 below shows how the temperature and density increase, and timescales shrink during the life cycle of a 25 solar mass star until it collapses, bounces and explodes into space. As you can see the temperature and density increase significantly, 12 orders of magnitude for the density, and the stages burn for shorter and shorter times, from 7,000,000 years burning hydrogen to 1 day burning silicone, over the course of the stars lifetime.

Table 1 - Evolutionary stages of a 25 solar mass star

This is a picture of a supernova remnant RCW 86, which is about 8000 light years away and you can see how violent these explosions can be. You definitely don’t want to be too close when a star dies!

Image Copyright: ESA/XMM-Newton & NASA/Chandra (X-ray); NASA/WISE/Spitzer (Infrared)