In the 19th century, the prevailing scientific view on the source of the Sun's heat was that it was generated by gravitational contraction. In the 1840s, astronomers J. R. Mayer and J. J. Waterson first proposed that the Sun's massive weight would cause it to collapse in on itself, generating heat. Both Hermann von Helmholtz and Lord Kelvin expounded upon this idea in 1854, suggesting that heat may also be produced by the impact of meteors on the Sun's surface. Theories at the time suggested that stars evolved moving down the main sequence of the Hertzsprung-Russell diagram, starting as diffuse red supergiants before contracting and heating to become blue main-sequence stars, then even further down to red dwarfs before finally ending up as cool, dense black dwarfs. However, the Sun only has enough gravitational potential energy to power its luminosity by this mechanism for about 30 million years—far less than the age of the Earth. (This collapse time is known as the Kelvin–Helmholtz timescale.)