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STARSStories

From 1896

A sequence written by looking

At Harvard a woman sits with spectra until the letters mean temperature. The catalogue is hundreds of thousands of stars.

Harvard College Observatory

SCIENTIFIC FIGUREA schematic spectrum. Not Cannon’s handwritten Draper cards, and not a plate from the Henry Draper Catalogue.

The cards

Annie Jump Cannon comes to Harvard in 1896. The observatory has a factory of photographic spectra. Fleming has already lettered types. Maury has a more intricate scheme. Cannon looks until the order that survives is a temperature sequence: O, B, A, F, G, K, M. Blue-white to red. She writes it onto cards, star after star, for decades.

This is not a lone night. It is a working life in a room of women computers. Leavitt is in the same house, on variables. Payne will later give the sequence a physics. Cannon gave it a usable sky.

ASTRONOMY

The Henry Draper Catalogue and its extensions become the map spectroscopists still start from. Spectral class is not a recipe of elements. It is, once Saha and Payne have spoken, ionization and temperature. In Cannon’s years it is a practiced eye and a letter.

Question

How many stars must you letter before a zoo becomes a sequence?

Sources

Also this sky