Edwin Hubble

Edwin Hubble is known for revolutionizing our view of the universe. Before Hubble, everyone believed that the universe was composed of one vast island of stars, the Milky Way. Everything that we saw was within it. However, Edwin Hubble was able to show that what people thought were "spiral nebulae" were actually other islands of stars that lay far beyond the breadth of our own galaxy.

Besides this, Hubble was able to use the technique he had employed to prove the existence of galaxies to show that the entire universe was expanding. Read below to learn how he showed this.

In 1923, Edwin Hubble showed that the "spiral nebulae" that were presumed to be within our galaxy, the Milky Way, were actually other galaxies that lay far beyond our. The following year, he showed that the Milky Way was just one of many galaxies in the universe.

In order to accomplish this, Milton Humason and he, using the newly completed 2.5 m (98 inch) reflector telescope at Mt. Wilson, California, were able first to resolve individual stars in the Andromeda Galaxy (M31). But resolving stars was not enough, for many stars have been resolved within true nebulae that lie within the Milky Way.

To prove that an object was actually a galaxy, they had to measure its distance. To accomplish this, they used individually resolved Cepheid variable stars within M31. Cepheid variable stars are very useful in calculating distances to many nearby galaxies. This is because they vary their brightness in a very regular pattern, and the duration of the variation is thought to be a direct indicator of their absolute magnitude - true brightness. Then, by measuring how bright the star appears to be, a simple formula can be used to determine the distance, assuming you know the absolute magnitude.

Using the Cepheids that he could resolve in Andromeda, Hubble was able to show that it was over 25 times farther away than the most distant stars in the Milky Way.

In 1925, Hubble invented a simple classification system for galaxies by using their shape (spiral, elliptical, and irregular) and the degree to which their shape was pronounced (such as a nearly circular elliptical is an E0 and a very elongated elliptical is an E7).

Because galaxies are moving away so quickly, their light waves are stretched out, making them appear redder. He noticed that dimmer objects, thus objects farther away, had a larger redshift. (Picture raisins spreading farther and farther apart in a rising loaf of bread.)
Hubble showed that the high redshifts (discovered by Vesto M. Slipher) of distant galaxies showed that the galaxies are speeding away at possibly thousands of miles per second.
Hubble showed that the farther away a galaxy was from us, the greater its redshift - the more its light was spread out and thus the faster it was moving.

Thus, Hubble's Law (right) states that redshifts increase in proportion to their distance from us in a very simple relationship. Hubble invented a constant, known as Hubble's Constant, that is equal to the velocity of a receding galaxy divided by the distance to it. Unfortunately, this number is extremely difficult to measure, and is one of the goals of modern astronomy.

There are two reasons that this number is very important. First of all, it would give astronomers a valuable, and relatively easy-to-use tool to measure distances in the far-away universe. The second reason is number is important is because taking its inverse (dividing 1 by it) will tell us how old the universe is.

In the 1930's, Hubble made a mistake in claiming that the galaxies are evenly distributed in space. To "prove" his point, Hubble took a large number of photographs of small regions of the sky. And, except for an area around the Milky Way which he called a zone of avoidance, he found galaxies in roughly equal numbers everywhere. Unfortunately for Hubble, other scientists disagreed and were able to prove their point.

Harlow Shapely and Adelaide Ames took large pictures of the Northern Hemisphere's sky. They noted large discrepancies in the concentration of galaxies. Clyde Tombaugh, who had discovered Pluto in 1930, confirmed their conclusions, and in 1937 discovered that galaxies are arranged in clusters and superclusters.

Despite his one large mistake, Hubble made huge contributions to the field of astronomy, and to honor Hubble, NASA named the Hubble Space Telescope after him in 1989.