Henrietta Leavitt
Henrietta Leavitt: Measuring Distance in the UniverseBy Cynthia Stokes BrownHenrietta Leavitt discovered the relationship between the intrinsic brightness of a variable star and the time it took to vary in brightness, making it possible for others to estimate the distance of these faraway stars, conclude that additional galaxies exist, and begin mapping the Universe. 1300LLearn more at www.oerproject.com
2Henrietta Leavitt: Measuring Distance in the UniverseBy Cynthia Stokes BrownEarly life and educationHenrietta Swan Leavitt was a minister’s daughter whose family moved frequently. When she was about 14, the family moved to Cleveland, Ohio, and in 1885 Leavitt enrolled in Oberlin College to prepare for the strict entrance requirements of the college she really wanted to attend—the Society for Collegiate Instruction of Women, later known as Radcliffe College (now part of Harvard University), in Cambridge, Massachusetts – a dream she achieved at age 20. She discovered her calling in her senior year when she took a course in astronomy.At the Harvard College ObservatoryLeavitt liked astronomy so much that after graduation she became a volunteer at the Harvard College Observatory as a “computer.” This was the name used for women who examined tiny dots on time-exposed photographs of the night sky and then measured, calculated, and recorded their observations in ledger books. Eventually, in 1902, Leavitt was hired at 30 cents an hour; she continued to work at the observatory for the remaining 19 years of her life.Leavitt took a special interest in the Magellanic Clouds, a pair of luminous hazes now known to be irregular galaxies, the nearest ones to our Milky Way. At the time, no one knew what the clouds were. Since the Magellanic Clouds are only visible in the southern hemisphere, Leavitt could not see them directly. She could merely look at photographic plates taken at Harvard’s auxiliary observatory, in Arequipa, Peru, and sent to Cambridge by ship around the tip of South America.Using Cepheid variablesOne of Leavitt’s jobs was to examine the variable stars, which, unlike most stars, vary in brightness because of fluctuations within themselves. In the course of her work, Leavitt discovered 2,400 new variable stars, half the known ones in her day. A certain group of variable stars, later called Cepheid variables, fluctuate in brightness (luminosity) in a regular pattern called their “period.” This period ranges from about one day to nearly four months.By comparing thousands of photographic plates, Leavitt discovered a direct correlation between the time it takes for a Cepheid variable to go from bright to dim and back to bright, and how bright the star actually is (its “intrinsic brightness”). The longer the period of fluctuation, the brighter the star. This meant that even though a star might appear extremely dim, if it had a long period it must actually be extremely large; it appeared dim only because it was extremely far away. By calculating how bright it appeared from Earth and comparing this to its intrinsic brightness, one could estimate how much of the star’s light had been lost while reaching Earth, and how far away the star actually was.BornJuly 4, 1868Lancaster, MassachusettsDiedDecember 12, 1921Cambridge, Massachusetts
3Henrietta Leavitt: Measuring Distance in the UniverseBy Cynthia Stokes BrownLeavitt published her first paper on the period-luminosity correlation in 1908. Four years after that, she published a table of the periods of 25 Cepheid variables. Nine years later, in 1921, she died of cancer at age 53 in Cambridge.Leavitt’s LegacyBefore Leavitt established the period-luminosity relationship, astronomers could determine cosmic distances out only about 100 light years. Using her insights, astronomers were able to estimate the Magellanic Clouds to be in the range of 100,000 light years from Earth—much further than anyone had imagined—meaning they could not be within the Milky Way galaxy.The largest telescope then in existence opened in 1904 at Mount Wilson, near Los Angeles, California. In 1919, the astronomer Edwin Hubble took a job there, after finishing his PhD in astronomy at the University of Chicago. Using the Mount Wilson telescope and building on Leavitt’s work, Hubble located Cepheid variables so far away that they conclusively established the presence of other galaxies. By 1925, most astronomers agreed that our galaxy is one among a multitude—a small outpost in a Universe full of galaxies.Leavitt initially worked under a director of the Harvard College Observatory who did not encourage theorizing but preferred only to accumulate data. A later director even tried to take some of the credit for her work after her death. Now, however, Leavitt is recognized as a key contributor to our understanding of the size of the Universe. Photographs of Computers Henrietta Leavitt, center, and Williamina Fleming, top left, hang in the Plate Stacks office at the Harvard Smithsonian Center for Astrophysics in Cambridge, MA on Aug. 1, 2017. © Getty Images)A modest lifeLeavitt never married. She gradually became deaf, starting with an illness when she was a young adult. She was buried in Cambridge in the family plot, near the graves of Henry and William James. Her total estate was appraised at $314.91. In her obituary, a senior colleague wrote: “[She] was possessed of a nature so full of sunshine that, to her, all of life became beautified and full of meaning.”
4Henrietta Leavitt: Measuring Distance in the UniverseBy Cynthia Stokes Brown
5Henrietta Leavitt: Measuring Distance in the UniverseBy Cynthia Stokes BrownCynthia Stokes BrownCynthia Stokes-Brown was an American educator-historian. Stokes Brown wrote Big History: From the Big Bang to the Present. Using the term big history, coined by David Christian at Macquarie University in Sydney, Australia, Stokes Brown told the whole story from the Big Bang to the present in simple, non-academic language to convey our common humanity and our connection to every other part of the natural world.Image credits This work is licensed under CC BY 4.0 except for the following: Cover image: Henrietta Leavitt © Photo Researchers Henrietta Swan Leavitt (July 4, 1868 – December 12, 1921). Public domain. https://en.wikipedia.org/wiki/Henrietta_Swan_Leavitt#/media/File:Leavitt_aavso.jpgPhotographs of Computers Henrietta Leavitt, center, and Williamina Fleming, top left, hang in the Plate Stacks office at the Harvard Smithsonian Center for Astrophysics in Cambridge, MA on Aug. 1, 2017. © Craig F. Walker/The Boston Globe via Getty Images.Articles leveled by Newsela have been adjusted along several dimensions of text complexity including sentence structure, vocabulary and organization. The number followed by L indicates the Lexile measure of the article. For more information on Lexile measures and how they correspond to grade levels: www.lexile.com/educators/understanding-lexile-measures/To learn more about Newsela, visit www.newsela.comThe Lexile® Framework for ReadingThe Lexile® Framework for Reading evaluates reading ability and text complexity on the same developmental scale. Unlike other measurement systems, the Lexile Framework determines reading ability based on actual assessments, rather than generalized age or grade levels. Recognized as the standard for matching readers with texts, tens of millions of students worldwide receive a Lexile measure that helps them find targeted readings from the more than 100 million articles, books and websites that have been measured. Lexile measures connect learners of all ages with resources at the right level of challenge and monitors their progress toward state and national proficiency standards. More information about the Lexile® Framework can be found at www.Lexile.com.OER Project aims to empower teachers by offering free and fully supported history courses for middle- and high-school students. Your account is the key to accessing our standards-aligned courses that are designed with built-in supports like leveled readings, audio recordings of texts, video transcripts, and more. Offerings include a variety of materials, from full-year, standards-based courses to shorter course extensions, all of which build upon foundational historical thinking skills in preparation for AP, college, and beyond. To learn more about The OER Project, visit www.oerproject.com