I was watching John Michael Godier and he said that a possible technosignature that could indicate alien life would be a star that had an unnatural composition. I know the heaviest element a star can fuse naturally is iron but scientists have discovered Przybylski’s star which contains traces of plutonium. I personally think it’s NOT a technosignature and I am thinking of writing a story and I was wondering what element would never exist in a natural star and would have to be created artificially. Does anyone know what that element could be?

Stars are formed from materials that already existed in accretion disks. The later the star generation, the more heavy elements the accretion disk contains.

At the end of its life, the Sun will consume Mercury, Venus, and Earth, so most of Earth's existing atoms here will become part of the solar red giant.

It would have to be an element that would not be made by the normal processes of the star. Such as a radioactive element, heavier than iron, that has a short enough half-life that it would be essentially gone in the time it takes the star to form.

Plutonium isn't a candidate because it can form from Uranium, which has a multi-billion year half-life. And this same neutron-absorption process can make just about anything (including Technetium). So what you would have to have is an unusual composition, such as too much of an isotope, or too little, which can't be accounted for with the processes that naturally occur in a star.

 

edit: one possibility is reactor waste products. But you'd have to dump a lot of that into the star to get it to register. Our sun has almost 10^28 fusion reactions per second. Sending a few hundred tons of radioactive waste into it is a very small blip, but perhaps detectable. Too much Cs-135, for example (half-life of 2.3 million years), or, less specifically, too much of all of the isotopes that one finds in spent fuel rods.