Neil Shubin describes The Universe Within as a "timeline" covering great events and processes of the history of the cosmos, the planet and life on earth. But his is also a timeline of scientists and scientific discoveries that enlarged our understanding of the world. One scientist who stood out for me was Henrietta Leavitt (1868-1921).
Leavitt became interested in astronomy while a student at Radcliffe College. After graduation, an illness robbed her of her hearing and she became a researcher at the Harvard College Observatory where she cared for telescopes and directed the photographic photometry department. Edward Charles Pickering was director of the observatory and hired his maid, Williamina Fleming, Leavitt and other women to do research. As Shubin explains, "What Pickering could never have planned was that from this team grew some of the greatest astronomers of the time, or any time for that matter. These women collectively became known as the Harvard Computers: they sat with the raw data of astronomy - pictures of the heavens - and made sense of them."
Leavitt catalogued photographic plates of stars and discovered how to measure the true magnitude of a star: "Leavitt became fascinated by one type of star that changed regularly from bright to dim over the course of days or months. Mapping seventeen hundred stars, she charted every property she could measure: how bright they were, where they sat in the sky, and how rapidly these variable stars went from bright to dim. With all of these data, Leavitt uncovered an important regular-cycle from bright to dim and their real brightness. Leavitt's idea seems awfully esoteric, but it is utterly profound. Starting with the principle that light travels at a constant speed, and knowing how bright the star actually was and how bright it appeared, meant that the distance of the star from Earth could be estimated. With this insight, Henrietta Leavitt gave us a ruler with which to measure distances in deep space."
Stars can appear dim or bright from earth. It's hard to tell when looking at a star if it's bright because it's close to the earth or because it really is bright. By studying the Cepheid variable stars in the Milky Way, Leavitt figured out how to find the real brightness of stars despite differences in distance from the earth. Her findings, published in 1912, plotted the periods (the period of a star is the time it takes to complete one cycle of brightness) of 25 stars and correlated them to their apparent brightness. Using this, astronomers only need to know the period of a Cepheid star to figure out how bright, and therefore how far away it was.
This way of measuring stars helped Edwin Hubble confirm that the universe was much larger than previously thought. Hubble's work with one particular Cepheid star in the Andromeda Nebulae led him, through Leavitt's distance calculation work, to figure out that the Andromeda was much further away than our own Milky Way.
Leavitt, through her groundbreaking work, paved the way for many advances in modern astronomy.
This article was originally published in January 2013, and has been updated for the
October 2013 paperback release.
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