Henrietta Swan Leavitt: The Woman Who Measured the Universe

A Childhood Marked by a Passion for Stars

Born on July 4, 1868 in Lancaster, Massachusetts, Henrietta Swan Leavitt (1868-1921) developed an early interest in astronomy. After studying at Oberlin College and Radcliffe College (then called the Society for the Collegiate Instruction of Women), she graduated in 1892.

Academic and professional journey:

• 1893: Began working as a volunteer assistant at Harvard College Observatory
• 1902: Became a permanent staff member in the "Harvard Computers" group
• 1907: Appointed head of the stellar photometry department

Despite her modest salary ($0.30 per hour) and status as a woman in a male-dominated field, she devoted her life to studying variable stars.

The Revolutionary Discovery of Cepheids

1. The Study of Variable Stars

Leavitt focused on studying variable stars in the Magellanic Clouds, where she discovered:

• More than 2,400 variable stars (half of all those known at the time)
• 1,777 variable stars in the Small Magellanic Cloud alone
• A clear relationship between the period and luminosity of Cepheids

2. The Period-Luminosity Law (1908-1912)

In 1908, she published her first observations, then in 1912, she clearly formulated the relationship that would later bear her name:

• Brighter Cepheids have longer periods
• This relationship allows calculating cosmic distances with unprecedented precision
• The mathematical formula: \( M = -2.81 \log P - 1.43 \) (where M is absolute magnitude and P is period in days)

This discovery, although published under her supervisor Edward Pickering's name, would become the basis for measuring distances in the Universe.

Impact on Modern Astronomy

1. Measuring Cosmic Distances

Leavitt's law enabled:

• Harlow Shapley to determine the size of the Milky Way (1918)
• Edwin Hubble to prove the existence of other galaxies (1924) and measure their distance
• The calculation of the Hubble constant and the expansion of the Universe

2. Contemporary Applications

Even today, the period-luminosity relationship is used to:

• Measure distances up to 100 million light-years
• Calibrate other distance indicators like Type Ia supernovae
• Study the large-scale structure of the Universe

Late Recognition and Legacy

Despite the importance of her discoveries, Leavitt received little recognition during her lifetime:

• Her salary remained modest (less than that of men in equivalent positions)
• She was never nominated for a Nobel Prize (though her discoveries deserved several)
• Her name did not appear in the first publications using her law

However, her legacy is now recognized:

• Asteroid (5383) Leavitt is named after her
• Lunar crater Leavitt honors her
• The Leavitt Prize from the American Astronomical Society rewards women in astronomy
• Google dedicated a Doodle to her for her 148th birthday (July 4, 2016)

Personal Life and End of Career

Despite fragile health (she lost her hearing during her career), Leavitt continued her work until her death:

• 1921: Died of cancer at age 53
• Had cataloged more than 3,000 variable stars by the time of her death
• Her last article, published posthumously, concerned variable stars in the globular cluster Ω Centauri

In a letter to her nephew, she wrote: "I have spent my life trying to understand the stars, and I believe I have managed to wrest some of their secrets from them."

Sources: Harvard-Smithsonian Center for Astrophysics, Original 1912 publication (ADS), American Association of Variable Star Observers, American Museum of Natural History, Women in Astronomy (AAS).