Early theories of stellar structure and evolution

The origin of the theory of stellar structure and evolution can be traced to the understanding of the first law of thermodynamics. As a result of the experimental ingenuity of Julius Mayer (1814–1878) and, particularly, of James Prescott Joule (1818–1889), and the deep theoretical insights of Rudolph Clausius (1822–1888) and William Thomson, later Lord Kelvin (1824–1907), the two laws of thermodynamics were established in the early 1850s. In popular terms, they can be stated as follows.

1. Energy is conserved when heat is taken into account.

2. The entropy of any isolated system can only increase.

Applying the first lawto the stars, the source of energy could be attributed to the heat liberated when matter is accreted onto their surfaces. The kinetic energy of infall from infinity, which is equal to the gravitational binding energy of the material at the surface, is converted into heat when the matter hits the surface. A popular version of the theory involved meteoritic bombardment of stars as the means of providing the necessary energy release. This proposal contained, however, the serious flaw that the necessary flux of meteoroids would perturb the orbits of the inner planets and would also have resulted in a quite unacceptably high rate of meteoroid bombardment of the Earth.