Background

Edwin Hubble was an American astronomer who is best known for the research he did that helped astronomers begin to understand galaxies and the enormity of the universe. He also produced the first evidence that the universe was expanding. He earned many degrees in the field of astronomy, including a Ph. D from the University of Chicago in 1917. After earning this honor he was given an invitation to work at the Mount Wilson Observatory in California. World War I broke out and Hubble joined the fight, he then used his ballistic expertise to aide the troops in World War II as well. He did not do any significant amount of work during this time period, but after the wars were over he returned to Mount Wilson and began his study of nebulas.

Finding the Distance to a Star

There are fuzzy patches of light in the sky that were known as nebulas. Scientists now apply the term nebula to clouds of dust and gas within galaxies. At the time that Hubble began his research, scientists could not differentiate between nebulas and distant galaxies. To determine a systems distance from Earth, scientists search for Cepheid stars which are yellow stars which vary in brightness regularly. The time it takes for it to go through one complete brightness cycle tells astronomer how bright the star actually is. They compare its actual brightness with its brightness from Earth and this gives them the approximate distance to the star.

Cepheid Stars

Hubble searched for Cepheid stars in these nebulas in order to better understand them. He focused his study on the nebulas that were thought to be outside of our own Milky Way galaxy. He discovered a Cepheid star in the Andromeda nebula in 1923, which is now known as the Great Andromeda Spiral Galaxy. Within a year Hubble had discovered 12 other Cepheid stars within this galaxy and went to work on determining their distance from Earth. He was surprised when his calculations revealed that these stars were over 900,000 light years from Earth. This was even more astonishing because our Milky Way Galaxy has a diameter of only 100,000 light years, which meant that the Andromeda nebula was far from the outermost boundaries of our galaxy.

Island Universe Theory

Hubble went on to discover many more stars in other nebulas located outside the Milky Way. While study these other galaxies he found that they contained many similar features of our own Milky Way Galaxy. He found they contained stars called novas that suddenly flair brightly and globular cluster which are round, compact groups of stars. Hubble finally proposed his theory that nebulas where actually other galaxies much like our own. This theory became known as the "Island Universe Theory". He went on to classify these galaxies by shape and composition into irregular and regular forms. Approximately 97% of the galaxies were considered regular and then subdivided into elliptical and spiral shapes. The spiral galaxies where also divided into barred spiral and normal spiral galaxies.The irregular galaxies seem to have no regular shape and made up the remaining 3% of all the galaxies that Hubble observed. To see the difference between these galaxy shapes see the table below.

Hubble's Law and Universal Expansion

Hubble used his knowledge about galaxies to plot the distances to some of the closer galaxies. He plotted the velocity at which the galaxies where moving away from the Earth on one axis and distance to the galaxies on the other. When he was finished there was a clear line of best fit. These data points made a diagonal straight line and which indicated to Hubble that the galaxies are moving faster, the farther away they are from our galaxy. This discovery became known as "Hubble's Law" and provided convincing evidence that the universe was expanding.