THE INCREDIBLE EXPANDING UNIVERSE
The first decade of the twentieth century saw several discoveries that would profoundly affect astrophysics. In 1905, Danish astronomer Ejnar Hertzsprung and American astronomer Henry Norris Russell investigated the relationship between a star's color and temperature and its brightness. Russell's work on the evolution of stars led to the notion of red giants and white dwarfs. Charts that plot a star's color against its brightness are now called Hertzsprung-Russell, or H-R, diagrams in their honor.
In 1912, Henrietta Leavitt at the Harvard Observatory discerned a relationship between the period of brightness fluctuations and the true brightness of a class of stars called Cepheid variables. She noticed that Cepheids with slow cycles were brighter than those with fast cycles. Using Leavitt's period-luminosity law, Harlow Shapley calculated the distances to Cepheids in globular clusters. His results proved that the Milky Way was far larger than previously thought and that the Sun was not at its center.
Still under debate were the distances to, and the nature of, the "spiral nebulas," which most astronomers believed belonged to our galaxy. A series of observations, begun in 1912, showed that most spiral nebulas were receding from us. In 1924, American astronomer Edwin Hubble, scrutinizing the Great "Nebula" in Andromeda (M31), was able not only to make out individual stars in the nebula, but to determine that some of them were Cepheid variables. He then calculated that the nebula lay almost a million light-years away—a vast underestimate, as it turns out, but unquestionably beyond the bounds of our own galaxy.
But the biggest surprise came when Hubble used spectroscopy to analyze the velocities of galaxies, finding that the farther away a galaxy lay, the faster it was receding. This could best be explained, Hubble concluded, by presuming that the universe was expanding like a balloon. As time progresses, the universe's size increases, a possibility that had been predicted by Albert Einstein in his 1917 General Theory of Relativity.