Mainly non-astronomers were responsible for the development of the first physical theories of the structure and evolution of the stars in the second part of the 19th century. A. Ritter was just such an outsider for the astronomical experts. The complete extent of his work is in generally unknown. This paper shows the scientific discoveries for which Ritter was responsible and gives an impression of his personality. In his day, predictions of Ritter's theory could not be tested by observation. This (...) is one reason for the fact that his ideas did not receive enough notice among astronomers of the 19th century. (shrink)