Color-magnitude diagrams for all bright stars within 1,000 light-years of the sun as determined by Mt. Wilson astronomers in 1935. (Left) Diagram without the subgiants, in which it is difficult to connect the two clear populations. (Right) Diagram with the subgiants discovered by Walter Adams et al. The subgiants form a connective tissue between the sequences that inspired Allan Sandage and his colleagues to advance the theory of stellar evolution by exploring the physical properties of these stars relative to the other sequences.