What is a Cepheid Variable Star?

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RS Puppis

The Cepheid variable star RS Puppis, in an image captured by the Hubble Space Telescope, is one of the brightest in the Milky Way Galaxy.


In the latest episode of the web series Shelf Life, How To Time Travel To A Star, Museum astrophysicist Ashley Pagnotta introduces one of her areas of interest: Cepheid variable stars. In 1912 Henrietta Leavitt discovered a way for these stars to be used as distance markers in the cosmos while identifying and studying variable stars at the Harvard Observatory.

Cepheid variables can be used to map objects in space because they pulsate between dim and bright phases over a regular span of time, usually between one and 70 days. The period between the star’s consecutive brightest points can be used to determine the star’s luminosity, or brightness compared to the Sun. The brighter the star is, the longer it takes to swing between its brightest and dimmest points. With this information in hand, astronomers can reliably calculate how far from the Earth the star is.

Leavitt’s work was a breakthrough: it meant that to estimate how near or far any star was from Earth, astronomers just needed to find a nearby Cepheid variable. Soon, major discoveries followed. A decade after Leavitt first observed the relationship between brightness, period, and distance in Cepheid variable stars by studying variable stars in the Small Magellanic Cloud, Edwin Hubble used Cepheid variables to demonstrate that a spiral nebula lay one million light years away— and so outside the Milky Way Galaxy—suggesting for the first time the vastness of the universe and the many galaxies that make it up.

Harvard Computers

Some of the biggest names in 20th-century astronomy worked as Harvard Computers.

Wikimedia Commons/Harvard College Observatory

Leavitt discovered the relationship between the pulsation period and the brightness of Cepheid variable stars while working as one of the so-called Harvard Computers, a group of dozens of women who worked in the Harvard College Observatory in the early 20th century sorting huge amounts of astronomical data. Initially, the Computers were hired by observatory director Edward Pickering because he could pay women less than men to do the same work. 

Throughout the years, many pioneers in the science of astronomy worked as Computers, including Williamina Fleming, the discoverer of the Horsehead Nebula; Annie Jump Cannon, who co-created the first major scheme for classifying stars alongside Pickering and for whom one of the most prestigious awards in the field is named; and Cecilia Payne-Gaposchkin, who discovered that the Sun—and by extension, all stars—was composed primarily of hydrogen.

Watch the latest episode of Shelf Life.