Hubble's recognition of galaxies other that our own Milky Way was not the end of his discoveries. When he looked at the light from nearby galaxies, he noticed that the distinctive colors emitted by different elements seemed to be shifted toward the red (long-wavelength) end of the spectrum, compared to light emitted by atoms on earth. Hubble interpreted this redshift as an example of the Doppler effect, the same phenomenon that causes the sound of a car whizzing past to change its pitch. Hubble's observation meant that distant galaxies are moving away from the Earth. Furthermore, Hubble noticed that the more distant galaxy, the faster it moves away from us.
On the basis of measurements of a few dozen nearby galaxies, Hubble suggested that a simple relationship exits between the distance of an object from the Earth, and the object speed away from the Earth. Comparing two galaxies --one twice as far away from the Earth as the other--the farther galaxy moves away from us twice as fast. This statement,which has been amply confirmed by measurements in the subsequent half-century, is now called Hubble's law. Hubble Law states “The farther away a galaxy is, the faster it recedes.”
Hubble's law tells us that we can determine the distance to galaxies by measuring the redshift of the light we receive, whether or not we can make out individual starts in them. Astronomers continue to debate the exact value of Hubble's constant. However, most experts agree the value is between 50 and 100 kilometers per second per megaparsec. A megaparsec (Mpc) is a million parces or 3.3 million light years. The constant provides a rough estimate of the time that universal expansion has been going on and the age of the universe. If you use 50/Mpc then the universe is about 16 billion years old. 100/Mpc puts the age at about 8 billion years.
