a metallic element of the calcium group, always naturally occurring combined, as in the minerals strontianite, celestite, etc. it is isolated as a yellowish metal, somewhat malleable but harder than calcium. it is chiefly employed (as in the nitrate) to color pyrotechnic flames red. symbol sr. atomic weight 87.3.-a radioactive isotope of strontium produced by certain nuclear reactions, and constituting one of the prominent harmful components of radioactive fallout from nuclear explosions; also called radiostrontium. it has a half-life of 28 years.