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Henrietta Swan Leavitt, the astronomer who made it possible to measure the Universe

Photo of Henrietta Swan Leavitt taken in 1921.
Credit: AAVSO
Photo d’Henrietta Swan Leavitt prise en 1921.
  • Photo d’Henrietta Swan Leavitt prise en 1921.
  • Henrietta Swan Leavitt à son bureau de l’observatoire de l’université de Harvard.
  • Astronomes travaillant pour Edward Pickering à l’observatoire de l’université Harvard. Henrietta Swan Leavitt est la troisième en partant de la gauche.
Henrietta Swan Leavitt, the astronomer who made it possible to measure the Universe

The story of the astronomer Henrietta Swan Leavitt is unfamiliar to the general public. Nonetheless, this great lady made a fundamental discovery in the early 20th century that allowed us to measure the distances of objects in the near regions of our Universe. 

A passion for astronomy

Henrietta Swan Leavitt was born in Massachusetts, in the United States, in 1868. She studied at Radcliffe College, where she earned her degree in 1892. In her last year at university she took her first classes in astronomy, and developed a passion for that science.

After her studies, she was taken seriously ill, possibly with meningitis, which left her almost deaf after a convalescence lasting two years. This did not however prevent her from pursuing a great career in astronomy.

In 1895, Henrietta was hired as a volunteer assistant at the Harvard College Observatory. There she worked on developing a catalogue of stars measured on the basis of photographic plates – a new procedure at the time – and demonstrated great knowhow in measuring stars’ brightness.

After travels in Europe in the late 1890s, Henrietta took up her work again at the Harvard College Observatory in 1902, this time as a regular member.

Detecting variable stars

Henrietta at this point began taking an interest in variable stars, which are stars whose brightness changes over time. In 1908 she published an article identifying 1,777 variable stars observed in the Magellanic Clouds, satellite galaxies of our Milky Way. Her talent for spotting variable stars was so great that she would discover more than 2,400 in the course of her career – over half of those known in her time!

One category of variable stars, called Cepheids, piqued Henrietta’s curiosity especially. These giant stars expand and contract in a regular way, and this change in size produces a change in the star’s actual brightness. 

Henrietta focused her attention on Cepheids located in the Small Magellanic Cloud. She conjectured that all these stars are located the same distance from the Earth. The variation in brightness of these Cepheids therefore had to be a property of the star, and not an effect of distance.

Discovering the Cepheids’ secret

This allowed Henrietta to demonstrate in 1912 that the greater the period of a Cepheid’s variation, the greater will its brightness be as well.

Thus, in measuring the variation period of a Cepheid, we can calculate the star’s distance. If the Cepheid is located in another galaxy, we also find this galaxy’s distance.

Application of the period-luminosity relation

This property of Cepheids would be used some years later by the astronomer Harlow Shapley in measuring the size of the Milky Way and showing that the Sun is located on the galaxy’s periphery.

In the mid-1920s, Edwin Hubble would use the period-luminosity relation of the Cepheids to demonstrate that galactic nebulae like Andromeda are in fact galaxies outside the Milky Way.

This relation is still used today, and allows us to measure the distance of an astronomical object up to 100 million light years.

Recognizing the contribution of Henrietta Swan Leavitt

Despite the fundamental applications for astronomy of the period-luminosity relation of the Cepheids, the work of Henrietta Swan Leavitt remains little known to this day. Incidentally, she passed away in 1921 of stomach cancer, in anonymity.

Let’s hope that one day the relation discovered by Henrietta Swan Leavitt will officially bear her name and that she takes her rightful place in the pantheon of modern science.

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