What happens when calcium oxide is heated?

Metal oxides are, for the most part, very thermally stable, and do not easily decompose with added heat. Calcium oxide is one of those substances. Even the metal oxides will eventually decompose to the elements when heated to a high enough temperature. CaO glows when heated to about 2400 C, but does not decompose. It will require temperatures greater than that. By the way, this is the source of the term "lime light" when referring to celebrities.