The Stars Behind Our Stellar Classification System

The following is an excerpt from The Glass Universe: How the Ladies of the Harvard Observatory Took the Measure of the Stars, by Dava Sobel.

The monumental work of stellar classification known as the Henry Draper Catalogue and Extension, begun under Williamina Fleming in the 1880s and continued through 1940 by Annie Jump Cannon, is still in regular use. Every astronomy student learns the temperature order of the stars by memorizing Oh, Be A Fine Girl/Guy, Kiss Me. A contest to come up with a cleverer, less sexist mnemonic was held for several years in Harvard’s introductory astronomy course, but the anonymous original retains its utility and pride of place. The thousands of Henry Draper identification numbers, assigned to the stars by the female computers, remain in effect as well. Star number HD 209458, for example, a variable in the constellation Pegasus, made news when modern detection methods located a planet in orbit around it.

Antonia Maury’s classification system, with its twenty-two spectral types and several subtypes, struck her contemporaries as too complex to gain traction. Some of its distinctions proved crucial, however, in discerning the different magnitudes and ages of stars that shared the same general categories. After Ejnar Hertzsprung first complimented Miss Maury’s acumen in 1908, the Draper classification made room for one of her notations in 1922, and in 1943 the MKK innovation incorporated additional Maury-type gradations. In 1978, some twenty-five years after her death, her system won further vindication when William Morgan published the Revised MK Spectral Atlas for Stars Earlier an the Sun with new coauthors Helmut Abt and J. W. Tapscott. Morgan dedicated this volume “To Antonia C. Maury (1866–1952) Master Morphologist of Stellar Spectra.”

Henrietta Leavitt did not participate in the classification effort, but her pursuit of variable stars and her discovery of the relationship between period and brightness among the Cepheid variables has had an equal, if not greater, impact on progress in astronomy. Once calibrated and applied to the problem of measuring distances across space, Miss Leavitt’s period-luminosity relation allowed Harlow Shapley to extend the boundaries of the Milky Way. The same Cepheid stars, subjected to the same analytical techniques, enabled Edwin Hubble to appreciate the enormous distances to the spiral nebulae. Hubble used Cepheids in 1924 to show that the Milky Way was not the only galaxy in the universe, and later to demonstrate that the universe was expanding to ever vaster proportions, as evidenced by the speedy outbound flight of most external galaxies. The Cepheids, however, had still more to say about cosmic distances. During World War II, Walter Baade, a German immigrant who had been working at Mount Wilson since 1931, took advantage of dark skies made darker by area-wide blackouts. Baade’s detailed study of the stars of the Andromeda Galaxy split the Cepheids into two subgroups. He accordingly recalibrated the distance scale and arrived at an overall size of the universe that doubled Hubble’s estimate. Today, astronomers rely on the period-luminosity relation to measure the current expansion rate of the universe.

The relationship between redshift and distance that Hubble saw in the realm of the nebulae has come to be known as Hubble’s law. By the same token, some scientists argue, the relationship between period and brightness that provided the basis for Hubble’s discoveries should rightly be renamed the Leavitt law. Awareness of this proposed terminology has been spreading since January 2009, when the executive council of the American Astronomical Society unanimously passed a resolution in favor of the change. The occasion was the one-hundredth anniversary “of Henrietta Leavitt’s first presentation of the Cepheid Period-Luminosity relation, a seminal discovery in astronomy that continues to have great significance.” Although the councilors allowed that the AAS had “no authority to define astronomical nomenclature,” they said that they personally “would be very pleased” to see the designation “Leavitt Law” in wide use.

When the female computers of the Harvard College Observatory come up in present-day conversation, they are often portrayed as underpaid, undervalued victims of a factory system. Pickering stands accused of giving them scut work that no man would stoop to do, yet this is far from true. Before astronomy morphed into astrophysics around the turn of the twentieth century, both men and the few women engaged in the science were willing slaves to routine. Arthur Searle, the acting director during the interregnum between Winlock and Pickering, tried to explain this reality to a journalist intent on chronicling the excitement of observatory life. “It is only fair to warn you,” Searle admonished Thomas Kirwan of the Boston Herald, “that your proposed article cannot be at once true and entertaining. The work of an astronomer is as dull as that of a book-keeper, which it closely resembles. Even the results reached by astronomical work, although they relate to more dignified subjects than the ordinary affairs of trade, are far less interesting than the result of book-keeping, at least to the general reader, unless they are so disguised by fancy as to have little to do with science.”