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Each different element emits light in a different, very specific set of wavelengths. When we look at the spectrum (the "rainbow") produced by any hot object, including stars, we see a unique fingerprint of colours depending on the relative abundance of the elements in it.
This website converts those light frequencies (in nm) into sound waves (in Hz) so you can "hear" the spectrum of different elements.
These frequencies should be exactly the same for any given element, anywhere in the universe, so when we see (or hear!) a difference we can use the Doppler effect to measure how fast it's moving towards us (blue-shift, or a higher pitch), or away from us (red-shift, or a lower pitch). The Doppler shift on this page does not correctly map to astronomical Doppler effects, mainly to keep the sounds within a reasonable range of human hearing.
Each tone is made up of the four strongest lines in a spectrum...many others exist, but it can get a bit "screetchy"!
To do: Fix mobile issue where it stops working after 3 notes. Allow real stellar data to be imported with amplitude based on abundance, add all elements up to iron, more detailed FAQ (e.g. blue/redshift imbalance)...give me a break, only two days into this ;)
Background image: Galaxy 1068 is shown in visible light and X-rays in this composite image. High-energy X-rays (magenta) captured by NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, are overlaid on visible-light images from both NASA's Hubble Space Telescope and the Sloan Digital Sky Survey.
Credit: NASA/JPL-Caltech/Roma Tre Univ.