Spiral galaxy NGC 4921 as seen by the Hubble Space Telescope. The image was used to identify Cepheid variable stars. Credit: NASA/ESA/K. Cook (LLNL)

Today is Ada Lovelace day, which is all about celebrating the achievements of women in science, technology, engineering and maths. I want to use it as an excuse to highlight the work of Henrietta Leavitt.

Leavitt was a pioneering woman astronomer at a time when women were mainly employed in observatories to be nothing more than ‘human computers’. Their job was to pore over images, record data and do basic calculations. Basically, they got to do all of the tedious stuff and none of the fun stuff.

In 1893 Leavitt began work as a computer at the Harvard College Observatory, under astronomer Edward Pickering. She worked on variable stars, which are exactly what they sound like: stars whose brightness varies over time.

She studied hundreds upon hundreds of astronomical images (or plates) and discovered almost two and a half thousand variable stars. In 1912 she was able to show that there existed a relationship between the brightness of a type of star called a Cepheid variable and the time period over which its brightness changed. The brighter the star, the longer the transition from bright to dim.

That means that we can learn what the intrinsic brightness of a Cepheid variable is by how long it takes to brighten and dim. This relationship makes Cepheid variables useful as standard candles, objects of known brightness that can be used to measure distances.

A video from the Royal Observatory, Greenwhich, offers a neat explanation of how we measure cosmic distances, and how Cepheid variables fit in.

Measuring the Universe from Royal Observatory Greenwich on Vimeo.

Cepheid variables are the first step on the cosmic ladder astronomers use to measure distances in the universe. Remember the recent story about the newest measurement of the expansion of the universe? That was calculated using Cepheid variables. And it all started with Henrietta Leavitt.