A Universe of Galaxies
Einstein would soon have reason to discard the constant and call it the biggest error of his career. Discoveries made in the 1920's by the American astronomer Edwin Hubble of the Mount Wilson Observatory in California showed that Einstein's original conception of the universe had been correct after all. In addition, Hubble finally settled the question of whether the spiral nebulae are other galaxies.
Hubble studied stars called Cepheid variables, which periodically become brighter, dimmer, and then brighter again. The time between peaks of brightness can be a few days or several months.
In 1912, Henrietta Swan Leavitt, an astronomer at the Harvard University observatory in Cambridge, Mass., had made a key finding about the Cepheids. She discovered that the longer the time period between a Cepheid's peaks of brightness, the greater the star's true brightness. Cepheids with the same period—the same length of time from peak brightness to peak brightness—always have the same maximum true brightness. (A star's true brightness, as opposed to its relative brightness as observed from Earth, is the actual amount of light it gives off. A faraway star might have a higher true brightness than a nearby star, but because of its distance, it has a lower relative brightness.)
Leavitt's finding meant that Cepheids could be used as cosmic measuring posts. Because the brightness of an object varies with its distance in a known way, it would be a simple matter to calculate the distance of a remote Cepheid. This could be done by comparing the star's relative brightness to a nearby Cepheid of the same period whose distance had already been calculated by other means.
In 1925, Hubble reported that he had found Cepheid variables in spiral shaped nebulae and had calculated that the stars were much too far away to be within the Milky Way. Thus, the spiral nebulae were galaxies in their own right. Another class of nebulae, the ellipticals, with an oval shape but no spiral arms, were also soon recognized as being galaxies. The universe, astronomers now realized, consisted of billions of galaxies, each containing billions of stars.