The Universe Revealed by Leavitt’s Law
In Leavitt’s day, most astronomers believed that our Milky Way galaxy was the entire universe. Then, in 1923-1924, Edwin Hubble and Milton Humason made a startling discovery. Peering through the new 100-inch Hooker Telescope on Mount Wilson, they found Cepheid variable stars in a faint spiral-shaped nebula called Andromeda (M31).
They determined the distance to Andromeda using Leavitt’s method, and to their surprise, it was some 2.5 million light-years away. The Milky Way galaxy is only about 100,000 light-years across, so Andromeda could not be inside our Milky Way galaxy – it had to be a separate galaxy.
Hubble and Humason found Cepheids in several other nebulae, indicating that they, too, were separate galaxies. Our Milky Way, they concluded, is just one of a number of galaxies in the cosmos! (We now know there are over 200 billion galaxies in our visible universe.)
In 1927, Belgian physicist Georges Lemaître used the Hubble/Humason galaxy distance measurements, known redshift data, and general relativity to make an even wilder prediction – that our universe is expanding. (Hubble independently reported the galaxy distance/redshift relationship two years later.) This led Lemaître to propose what we now call the big bang in 1931. Henrietta Swan Leavitt’s breakthrough had begun the 20th century cosmology revolution.
H. Leavitt: Posthumous Honors
Leavitt published some 25 technical papers while at Harvard College Observatory. Despite this, and her Cepheid period-luminosity discovery, Pickering continued to treat her as lab assistant. Only when Harlow Shapley became Observatory director in 1921 was she appointed to head the photographic photometry department. Unfortunately, Leavitt succumbed to cancer later that same year. She was fifty-three-years old.
Three years later, the Swedish Academy of Science wrote to the Observatory to begin paperwork for a Nobel Prize nomination for Leavitt, only to learn that she had passed away. (The award is not given posthumously).
Today, the Moon crater Leavitt in the southern hemisphere on the far side of the Moon is named in her honor, as is Asteroid 5383 Leavitt. In addition, a play based on the life of Henrietta Swan Leavitt, Silent Sky, by Lauren Gunderson, had its world premier this month at the South Coast Repertory theatre in Costa Mesa, California. Perhaps, finally, this unsung pioneer of astronomy will get the credit she deserves for her discoveries.
PBS Online. Henrietta Leavitt. Accessed April 15, 2012.
Yellow Magpie. Henrietta Swan Leavitt: She Changed The World But Paid The Price. Accessed April 15, 2012.
Leavitt, H. S. & Pickering, E. C., Periods of 25 Variable Stars in the Small Magellanic Cloud. (1912). Harvard Observatory Circular 173. Accessed April 15, 2012.
NASA Goddard Space Flight Center. Cepheid Variables as Cosmic Yardsticks. Accessed April 15, 2012.
Ouellette, J. Henrietta Swan Leavitt Comes to Life on Stage. (2012). DiscoveryNews. Accessed April 15, 2012.
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