Amanda Maxwell

Feb 20th 2018

Henrietta Leavitt and the Starlight Distance Key


Chances are you’ve never heard of Henrietta Leavitt. So you may be surprised to learn that without her observations of stars in 1908, Edwin Hubble would not have spotted that the universe is expanding … or have a space telescope named after him. Leavitt is a great example of forgotten history — not unlike that recently revealed by the movie Hidden Figures — a phenomenon in which contributions to society go unrecognized, simply because they were made by the “wrong” gender.

Humans as Computers

In the early 1900s computing was mostly repetitive number crunching, done manually with pen and paper, and for this reason was considered women’s work. Henrietta Leavitt joined many other highly educated women in cataloging star photographs for the entirely male astronomers at the Harvard College Observatory. Eventually, working alongside other human computers like Annie Jump Cannon, Leavitt’s work on variable stars illuminated a path for others to measure the universe and laid foundations for modern astronomical exploration.

Born in Lancaster, Massachusetts in 1868, Henrietta Leavitt attended Radcliffe College from the age of 20 and earned her bachelor’s degree in 1892. In her final years of study, Leavitt was introduced to astronomy and was immeditaley fascinated. However, like a lot of other women graduates at the time, the only way that she could pursue her interest was as a laboratory technician. She first worked as a volunteer before joining the staff in 1902 at a rate of 25 cents per hour — roughly the wage for a servant.

Women computers at the Harvard College Observatory.

Laboratory head Edward Pickering set Leavitt to work cataloging the vast photographic library of telescope images from the Magellanic Clouds. These clouds, visible from the southern hemisphere without a telescope, are giant galaxies that orbit our own Milky Way about 200,000 light years away from Earth. They hold vast pockets of gas that are constantly collapsing to form new stars, and as such, hold valuable clues about the creation of the universe.

Leavitt’s Law

Leavitt’s task lay in cataloging Cepheids within the Magellanic Clouds. These are variable stars, so named because they periodically blink brighter or pulsate at regular intervals. Examination of plates taken over the years from the telescope images eventually revealed the periodicity. As Leavitt pored over image after image, she noticed that there was a direct relationship between the brightness of each star and its period; stars with greater luminosity took longer to blink from bright to dim and back again.

In 1908, Leavitt described this discovery and then expanded on it in a paper published in 1912. In explaining that the brighter stars had longer periods, she found that “a straight line can readily be drawn among each of the two series of points corresponding to maxima and minima, thus showing that there is a simple relation between the brightness of the variables and their periods.” In other words, there is a linear relationship between a star’s true brightness or intrinsic luminosity and the logarithm of its period.

Variable Stars Unlock the Distance Key

Leavitt’s Law meant that scientists finally had a better way to measure distances across space, but it took a few more steps to unlock the key.

Since a star’s apparent brightness steadily decreases the further it is away from our planet, there is a way to estimate distance through comparison with its calculated intrinsic luminosity using the Inverse Square Law. Henrietta Leavitt knew that if she could figure out the distance from the Cepheids to Earth, she could calibrate her data into a workable formula.

Once Danish astronomer Ejnar Hertzsprung was able to establish the distance between the Earth and the Cepheids within the Magellanic Cloud, and Pickering’s successor Harlow Shapley had further verified the findings, Leavitt’s Law helped establish the first ‘standard candle’ for modern astronomy, pushing reliable distance estimation from 10,000 to 10,000,000 light years away.

Per ardua ad astra

The women at the Harvard College Observatory worked on tedious and repetitive data analysis; to reach her conclusions, Leavitt studied the periodicity and luminosity of around 1,777 variable stars. She also analyzed 299 plates from 13 telescopes to construct the Harvard Standard, a logarithmic scale for cataloging stars by photographic brightness.

Fellow computer Annie Jump Cannon improved on a spectral class system for classifying stars developed by her predecessors, Antonia Maury and Willemina Fleming

Cecilia Payne, recruited by Shapley as the first beneficiary of award funds set up by Cannon, discovered in her Ph.D. studies that the sun is largely comprised from hydrogen. This finding ran contrary to current theories at the time, but when proven showed that the metallicity of stars could be used to gauge their age and development — important concepts in the study of the cosmos.

Thanks to their meticulous attention to detail, the human computers like Henrietta Leavitt pushed the boundaries of space exploration. Years before a famous telescope would bear his name, Hubble used Leavitt’s Law to show that other galaxies lay beyond the Milky Way and prove that our galaxy is not at the center of the universe. With this information, he showed that the universe itself is expanding, giving rise to the Big Bang theory as an explanation for the start of the universe.

Barely recognized for her contributions, Henrietta Leavitt died of cancer in 1921; a proposed nomination for the Nobel Prize in Physics only arrived four years later.