When a star goes supernova, we can look at the elements ejected into space to see what had been created in the core of the dead star. Each element will have its own unique colour fingerprint, which can be detected by looking at the remains of supernovae – supernova remnants.
Each element will have a different colour spectrum based on the arrangement of the electrons around the nucleus. The electronic arrangement around the nucleus is unique for every element.
We can look at the colours in a supernova remnant to see what elements were made inside the star before it exploded. Above is a tiny part of the veil nebula, a supernova remant, with all of the elements shown. On the left is a supernova remnant viewed at three wavelengths, corresponding to those in the fingerprint of three elements; hydrogen (red), oxygen (blue) and sulphur (green).
The unique colours are given off when electrons are excited and then fall back to lower energy states. Electrons orbit the nucleus at fixed distances and can move from one level to another. When given energy, an electron can be excited to a higher energy state. The electron then decays back to a lower energy state and, as it does so, energy is given out in the form of light.
The colour of the light will have a very specific wavelength, depending on the energy levels of the higher and lower electron states. The difference in energy between the two levels will define what wavelength the emitted light will have. The energy levels are very precisely defined and therefore can accurately be used to identify specific elements.
Each element will have a different arrangement of electron energy levels. Nuclei with more protons will attract the electrons more strongly and so they orbit more closely. Within the same row of the Periodic Table, each subsequent electron is shielded slightly more from the nuclear charge and orbits slightly further away. The interplay of these two factors give each element a unique set of electron energy levels, and a unique colour and wavelength fingerprint.
In the picture below you can see the colours associated with a group of metals when they are burnt. The colours are unique and allow them to be easily identified.
Five different metals can be seen all with very different flame colours. The metals used above are lithium (Li), strontium (Sr), sodium (Na), copper (Cu) and potassium (K). The colours seen here would be exactly the same as seen in a supernova remnant. Sodium burns a bright orange colour, which is the same colour as street lamps, because street lamps use sodium metal in them to produce the light.
In the next and final episode, we'll take a look at neutron star mergers and you can take part in our quiz to test your knowledge on the series.
Dr Andrew Levan
"My principle research interests focus on gamma-ray bursts and supenovae. As the brightest explosions in the Universe by far, gamma-ray bursts can be seen across most of the visible Universe, and make powerful cosmological probes. My research into GRBs has two broad avenues. The first is to understand the progenitor systems for these extraordinary explosions, the second is to subsequently deploy them as probes of the Universe across cosmic time. GRBs are now mainly found by the Swift satellite, while my followup programme utilises the Hubble Space Telescope, Chandra X-ray Observatory and Spitzer Space Telescope, as well as numerous large ground based facilities (e.g. VLT, Gemini, WHT).
More recently I have become interested in other extreme transients events, including the tidal disruption of stars by supermassive black holes, and new kinds of extremely bright supernovae."
Dr Andrew Levan is Professor in the Astronomy & Astrophysics group in the Department of Physics at the University of Warwick.