Stars 101: Oh Be A Fine Gal, Kiss Me!

Previous Stars 101 Posts:
Introduction
The Fundamentals
The HR Diagram

There is something in human nature that loves to sort things. Several months ago I watched my 4 year old niece, Keira (in all her cuteness to the right), sort her fruit snacks into piles of gummy strawberries, grapes, lemons, and oranges. After she had four neat little piles, she ate the strawberries, grapes, and oranges, but she gave the lemons to her mom - apparently lemons don’t make the cut.

As much as we in the ivory tower like to think we are different than a 4 year old with fruit snacks, when it comes to sorting things, we have not changed much. Almost every academic discipline spends a good chunk of time and energy classifying things. Biologists sort species into kingdoms and families, chemists make periodic tables, and astronomers sort stars into spectral types.

In the late 1800’s astronomers started to do spectroscopy and before long they started to notice patterns. In the 1890‘s a Harvard professor named Edward Charles Pickering began observing and photographing the spectra of large number of stars. Stellar spectra can have hundreds of “lines” or colors which are brighter or dimmer than average and sorting out how many lines are present and looking for patterns was a laborious and time consuming process. To give you an idea, here is about 10% of the spectra observed by looking at the sun (each dark band is a spectral line):
To help with the monumental task of measuring and recording all these lines, Pickering hired large numbers of assistants (generally male undergrads) to help. However these undergrads generally only worked for Pickering for a year or two, meaning he had to spend large amounts of time hiring and training them. One day after some poor fellow had made one too many mistakes, Pickering was quoted as yelling “my maid could do a better job”.

Well it turns out she did. Pickering hired his maid, Williamina Paton Stevens Fleming. Before long, Fleming had devised a simple scheme for classifying stars based on the strength of hydrogen lines in their spectra. Her classification went from A to N, where A type stars had the strongest hydrogen lines and N had the weakest. She later added O, P, and Q to the list to deal with very bright stars, so-called planetary nebulae, and hard to classify stars, respectively.

Back then women weren’t allowed to be professors at Harvard, so Williamina Fleming never became a professor. She did eventually become the first American woman to become a member of the Royal Astronomical Society and was appointed an honorary fellow of the all-women’s Wellesley College. Today, with some minor modifications, all astronomers use her classification system. Yesterday I had a discussion centered on F-type stars.

Advances in our understanding of why certain stars do or don’t have strong hydrogen lines has alter how we use Fleming’s system. If we sort stars from most massive (and therefore brightest and hottest) and largest to least massive and smallest, the types are O, B, A, F, G, K, and M, as shown in the roughly-to-scale snapshot below.
The types can be memorized by the helpful mnemonic “Oh Be A Fine Gal, Kiss Me!” For those of you who don’t wish to kiss gals, guy can be substituted in quite nicely.

So why are astronomers still using a modified system that requires a mnemonic to remember? Take it away Tevye!