The Joint in the Cosmic Ruler

kw: book reviews, stellar parallax, cepheids

The 2005 offering of the Great Discoveries Series of W.W. Norton & Co. is Miss Leavitt's Stars: The Untold Story of the Woman Who Discovered How to Measure the Universe, by George Johnson, a science reporter at the New York Times.

Miss Leavitt was a computer. From the late 1800s through the mid 1940s, a computer was a clerk, usually a woman, who computed primarily astronomical measures using pen and paper. Some observatories employed a dozen or more computers. There were perhaps six at Harvard in the early 1900s. Henrietta Swan Leavitt was the most prolific and exacting, and truly deserves the title Scientist in her own right. She it was who noticed the period-luminosity relationship for Cepheid variables in the Small Magellanic galaxy, and she who published it.

The book is thin, partly by design, but partly because there is so little to go on. We have the scientific data, of course, but very little personal information, and much of that concerns Miss Leavitt's illnesses. Born July 4, 1868; employed 1893 as a computer at Harvard—first as a volunteer; published "1,777 Variables in the Magellanic Clouds in 1908; died of cancer December 12, 1921.

At the time of her discovery, trigonometric parallaxes were known only for a few hundreds of stars, all closer than 50 light years. Over a generation, this was extended to 100, but other methods were needed to find the distance to the nearest Cepheids. Statistical parallax, developed by Shapley beginning in 1914, could provide good estimates within a thousand light years or so. It depended on accurate measurements of the proper motion of many stars in an open cluster.

Within Miss Leavitt's lifetime, no extragalactic measurements were made. It was in 1923 that the first Cepheid variable in the Andromeda galaxy was found by Hubble. Some decades were consumed with the discovery that old Population II variables were one-fourth the brightness of younger Population I variables. Once this was ironed out, the distance to Andromeda was determined to be about 2 million light years. Since then, Cepheid variables have established the distances of numerous within 40 million light years.

The book is not a biography, as I have said. It suffers from an insufficiently biographical approach. It is rather hard to pin down sequences, for the narrative is many-threaded, giving us capsule biographies of the many scientists who employed Henrietta Leavitt, or the results of her work. Nonetheless, it limns the career of a strong-minded, persistent, creative woman who filled in a crucial gap in the chain of measurement we use to determine distances to the edge of the universe.