Harvard Computers

Young Astronomers Blog, Volume 28, Number 7.

Stars come in all sizes, colors and temperatures. They also have a spectrum consisting of dark (absorption) lines, which identify the elements in the star’s atmosphere. March is Women’s History Month. So, let’s recognize some of the prominent women astronomers of the early 20^th century who played a crucial role in classify the stars of the night sky.

Henry Draper was a doctor and prominent amateur astronomer during the 19^th century. He is best known for producing some of the first astronomical photographs including those of stellar spectra. He married Anna Mary Palmer in 1867, who carried on (and funded) his legacy after Draper’s early death in 1882.

By the end of the 19^th century, the Harvard Observatory had collected a significant number of glass photographic plates of the night sky. Edward Pickering, the director of the Harvard Observatory, set out to develop a catalog of the stars utilizing these plates. The work was funded by Henry Draper’s widow. Several volumes of The Henry Draper Catalog were published between 1918 and 1924.

For this project, he recruited around eighty women who became known as the “Harvard Computers”. They did the tedious work of viewing the glass plates and assigning a spectrum to each of the stars.

After becoming dissatisfied with the work of one of his male employees, Pickering is said to have commented that his Scottish maid could do better. Williamina Fleming was Pickering’s housekeeper and Pickering hired her in 1881. She eventually became the manager of the “computers” and, in 1899, curator of the astronomical photographs. Fleming along with Antonia Maury developed a methodology to classify the spectrum of stars using twenty-two letters of the alphabet.

In 1896, Pickering hired Annie Jump Cannon. Beginning in 1911, Cannon cataloged thousands of stars based on their spectral lines. She became one, and possibly the most proficient, of the “computers”, classifying a few thousand stars a month. Cannon simplified Fleming’s classification system relating a star’s spectrum to one of only seven letters. The classification scheme was later related to a star’s surface temperature by Cecilia Payne, who also discovered that stars are made of hydrogen and helium. The classifications ranged from hot large blue stars (type O) to Sun like stars (type G) to small red dwarfs (type M). This approach became known as the “Harvard Spectral Classification”.

The Harvard Classification was extended in 1943 and 1953 by William Morgan, Philip Keenan and others. It is now known as the Yerkes or Morgan-Keenan classification and is still in use today. Stars are assigned to one of the O, B, A, F, G, K, M classifications. Our sun is a type G star, the blue giant Rigel is a type B star, and the red giant Betelgeuse as well as small red dwarf stars are type M. To remember the Harvard/Morgan-Keenan classifications (O B A F G K M) use this mnemonic; “Oh Be A Fine Guy/Girl Kiss Me”. To provide more precision, each letter is further divided into numbers. And to make things even more complicated, there is a Roman numeral often added indicating a star’s luminosity class. So, the Sun is really a class G2V star.

Henrietta Swan Leavitt began working as a “Harvard Computer” in 1893, left and returned in 1903. Leavitt was asked to help measure the brightness of variable stars (Cepheids) and she noticed that some of these stars in the Magellanic Clouds followed a pattern. She published this “period-luminosity relationship” in 1908 and again in 1912. Leavitt eventually became head of stellar photometry at the Harvard Observatory, but regrettably passed away soon after.

Later in 1924, Edwin Hubble was able to find the same type of stars in the Andromeda nebula and he used Leavitt’s relationship to discover the true size of the Universe. He proved that Andromeda was a distant galaxy and that the Universe was huge containing many galaxies other than just our Milky Way.