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Hubble's Law

In the 1920's, the America astronomer Edwin Hubble, made one of the most important discoveries in astronomy - after studing Cepheid variable stars (stars whose brighntess changes periodically over time) in M31 (the Andromeda Galaxy), he used the relation between the star's period and luminosity to find the distance to the Cepheids in M31 and proved that it was outside our own Galaxy. This ended 'The Great Debate' between astronomers who had argued over the nature of the spiral nebulae identified in the sky. Some astronomers had believed that these objects were clouds of gas in our Milky Way, but Hubble's distance measurement proved this was not the case - the spiral nebulae were distant galaxies like our own.

Hubble continued his study of galaxy distances using Cepheids as his measuring tool, before publishing his results in 1929. In his paper, Hubble plotted a graph of the velocity of galaxies (obtained from determining the redshift of the spectra of these galaxies) against their distances (from Cepheid variabes). This plot showed that most galaxies are moving away (receding) from us, but also that the speed at which they are moving away (the recessional velocity) is proportional to their distance – distant galaxies recede faster than nearby ones. This became known as Hubble's Law.

In this activity you will use Hubble's original plot from 1929 to find a value for the Hubble constant and the age of the Universe. You will then plot your own diagram using more accurate values for the distances to galaxies using data from Type Ia Supernovae, and calculate Hubble's constant from your plot. This will give you a more refined value for the age of the Universe.


Hubble's Law - Activity Sheet (.pdf)

Hubble's Law - Answer Sheet (.pdf)

Image of M31 (Andromeda Galaxy) courtesy of John Lanoue (