Henrietta Swann Leavitt by our guest blogger John Faulkner

Henrietta Swann Leavitt (1868 - 1921) made one the great astronomical discoveries of the 20th Century. Her work enabled the existence of galaxies to be proved and astronomers realise the scale of our Universe.

Henrietta graduated from Radcliffe College, USA leaving her with a passion for astronomy, but a long illness had left her deaf. She recovered to work at Harvard College Observatory. With no chance of actual theoretical work (only open to men) she still rose to become head of photographic photometry.

Stars which changed in brightness were of particular interest to astronomers. By comparing the astronomer's photographic plates, Henrietta catalogued 2,400 of these stars. It was a huge painstaking manual effort comparing star fields on thousands of photographic plates to find ones that 'blinked'.  In 1908 she observed a type of star, called a Cephied Variable, had a link between blinks and luminosity. By timing the period she could calculate overall brightness. Henrietta had discovered a powerful tool for astronomers - the standard candle. 

The known brightness of a star indicates its distance. In a similar way that the dimmer a street light is, the further away it is likely to be. If a Cepheid with a blink of 5 days was fainter than another Cepheid with a blink of 5 days it was further away. The scale of the universe could now be measured by using these 'standard candle' stars. In 1923 after finding a Cepheid Variable (V1) in the Andromeda 'spiral nebula' (M31) Edwin Hubble made the astonishing discovery that it was a separate galaxy; 2 million light years away. The first indication of the true scale of the universe. 

 Henrietta was never allowed to follow her own lines of study but was remembered as 'possessing the best mind at the observatory' and later 'the most brilliant woman at Harvard'. She had been considered for the 1926 Nobel Prize for physics but unknown to her nominee, she had died 4 years earlier from cancer and did not live to see Hubble's amazing discovery using her work. A crater on the moon is named after her, in honour of the deaf men and women who have worked as astronomers and she has her own nominated Asteroid - 5383 Leavitt.