In 1923 Hubble was studying novae in M31 (now known as the Andromeda Galaxy), when he realised that one of the objects he was observing was in fact a Cepheid variable. Earlier that century, a female astronomer, Henrietta Leavitt, discovered a relation between the period of Cepheid variables and their luminosity, which allowed their distances to be calculated. Hubble studied galaxy distances using Cepheids as his measuring tool, and in 1929 published his results. His plot of the velocity of galaxies vs. their distance showed that most galaxies are moving away from us, and the further away they are, the faster they are moving. This became known as Hubble's Law.

In this activity you will use Hubble's original data from 1929 to find a value for Hubble's constant and calculate the age of the Universe. You will then use more recent data to plot your own Hubble diagram, and find more accurate values for these.

• Course Leader: Sarah Roberts