He was among the first to challenge researchers claiming the achievement of fusion using magnetic pinch devices, showing instead that the neutrons being observed were not from high temperature fusion reactions, but rather from energetic ions hitting the chamber walls. He then collaborated with others like Jim Tuck and Marshall Rosenbluth to propose toroidal pinch devices that might prove to be more promising. In the late 1950s and early 1960s, he teamed up with (the ten-year younger) Harold Furth at Livermore where they were among the most productive of the early fusion researchers in the US.

While working on fusion at Livermore until 1964, Colgate remained active in weapons-related activities, including early attempts at nuclear test ban negotiations. His work on H-bomb explosions also led him to an interest in astrophysics and supernova events, on which he was to spend the bulk of his later scientific career.

At the time of his death, after a long illness, he was a Los Alamos National Laboratory Senior Fellow and member of the Laboratory's Nuclear and Particle Physics, Astrophysics and Cosmology Group. Los Alamos Director Charles McMillan said of Colgate, "His contributions to physics and national security science, including some very recent work, are broad, deep and exceptionally creative."

He is survived by his wife, Rosie, a son and daughter, four granddaughters, a great grandson and a sister. A "Remembering Stirling" reception is planned at Fuller Lodge in Los Alamos on December 15 at 5:45 PM.