What We Cannot Know: Explorations at the Edge of Knowledge

by Marcus du Sautoy

A Cepheid star twinkles, and in 1912 American astronomer Henrietta Leavitt discovered how to use these twinkling stars to navigate the universe. She was employed at the time not as an astronomer but as a ‘computer’ at the Harvard College Observatory, extracting data from the photographic plates for 30 cents an hour. Women weren’t allowed to operate the telescopes. She’d been assigned the task of analysing stars that grew brighter and dimmer over a period of time. Curious to know if there was any pattern to the pulse of these stars, Leavitt focussed on a batch of stars that were located in the Small Magellanic Cloud and were therefore believed to be at similar distances from the Earth. When she plotted luminosity against the period of pulsation, she discovered a very clear pattern. The time it takes a Cepheid star to pulsate is directly correlated with its luminosity: the longer the period of pulsation, the brighter the star is shining. So to know how bright a Cepheid star really is, all you need to measure is the period of pulsation, something that is much easier to do than measuring missing light frequencies. These stars were perfect for measuring distances.