In the 1920s he played a key role  in establishing what galaxies are.  In 1924 he measured the distance to a nebula named Andromeda which was a faint patch of light that had the same diameter as the moon.  He was only able to measure a handful of other galaxies and he realized that he could measure them by their brightness in the sky.  He made all of his discoveries on the best telescope in the world at that time.  Today the Hubble space telescope is named after him.  

This is a picture of Edwin Hubble.                 These are pictures taken from the Hubble space telescope.