- strontium, Sr, atomic number 38 - a soft silver-white or yellowish metallic element of the alkali metal group; turns yellow in air; occurs in celestite and strontianite
--1 is a kind of metallic element, metal
--1 is a substance of celestite; strontianite
--1 has particulars: strontium 90 - steradian, sr - the unit of solid angle adopted under the Systeme International d'Unites
--2 is a kind of angular unit
--2 is a part of sphere