Quite simply, word of mouth and then deep faith in those that preached.
High mass stars are far more massive than the sun, and therefore have a much larger gravitational impact. A star of 100 solar masses (100x larger than the sun) has a much shorter life than that of the sun because the forces that cause the star to expand and "burn" have to work harder to overcome the much larger gradational impact. While the sun has a life span of approximately 8 billion years; the life of a red giant of 100 solar masses would be at best a few hundred million years because gravity will overcome the counteracting force of fusion. (resulting in a supernova, black hole, white dwarf, or other astronomical anomaly.)
The answer is the Gettysburg address, this is the famous speech starting as, "four score and seven years ago...."
