Ernst Öpik's fundamental ideas on the structure of red giants

As early as in 1923, the Estonian astronomer Ernst Öpik indicated in a pioneering investigation that the only source of stellar radiative energy capable of sustaining the observed stellar luminosities for billions of years must be the process of nuclear fusion in stellar cores, transforming hydrogen into helium, carbon and so forth up to iron. He was the first to introduce in 1938 the so‐called compound unmixed stellar models with a helium stellar convective core and a hydrogen envelope in radiative equilibrium lying on the top of it. Contrary to the universally adopted view of Eddington who firmly believed that rotationally induced convection would inevitably lead to a full mixing of the stellar matter, calculations of Öpik confirmed the validity of compound models and indicated that upon the exhaustion of hydrogen the core will start contracting whereas the hydrogen envelope will expand. The structural differences between giant and main sequence stars were interpreted in this way, and stellar evolution along the H‐R diagram explained in broad features. Öpik was ahead of Fred Hoyle and Martin Schwarzschild who reached similar conclusions only 15 years later. Here we analyze various implications of Öpik's research dedicated to the internal structure of red giants for the treatment of the advanced stages of stellar evolution. It took almost half a century before the priority of this discovery by the Estonian astrophysicist found a deserved recognition.