Deep inside a massive star, before it explodes, layers sit on top of each other like the layers of an onion. Sometimes two adjacent layers, the carbon and oxygen layers, mix together. It's like how layers of broth and vegetables in a pot of soup blend when you stir them with a spoon. This mixing changes which chemical elements the star will eject into space when it explodes.
Scientists compared eight sets of explosion models with observations of the supernova remnant Cassiopeia A. This remnant is like a crime scene photograph, allowing us to reconstruct what happened inside the star. Models with layer merging yield a high argon-to-neon ratio (greater than 0.1) because neon is destroyed while argon is produced. Other models give a lower ratio. Observations in X-ray and infrared wavelengths match the merging models best.
The same approach was applied to supernova 1987A. There, based on the shift in titanium-44 lines, layer merging was likely not the main source of this isotope. This means different supernovae can have different histories: in some, layers merged; in others, they didn't. This helps astronomers read the chemical history of the universe more accurately.
Titanium-44, the isotope in question, is radioactive and decays with a half-life of about 60 years. Astronomers use its glow in supernova remnants as a clock to determine the age of the explosion.