At around this time there was a lot of work being done on stars, and Hertzsprung and Russell discovered the relation between types of star and their brightness - immortalised in their famous Herzsprung-Russell (H-R) diagram, which is a central part of any astronomy course. In 1923 Edwin Hubble proves that there are galaxies beyond the Milky Way, and in 1929 he discovers a relationship between the distance of a galaxy from Earth and its velocity. This was evidence that galaxies are moving away from each other.

Throughout the mid 20th century, more and more discoveries were made of celestial objects. In 1967 Jocelyn Bell Burnell discovered pulsars. Read her remarkable story ... Her story is part of the Open University Astronomy and Planetary Science course (S281).