Stellar Classification
When classifying stars, the Morgan–Keenan system is the one most commonly used, ranging from O stars, which are the hottest, to M stars, which are the coolest. The starts are further broken down by numbers, with 0 as hottest and 9 as coldest. So a O0 star would be the hottest, and a Y9 would be the coldest. The letters from hottest to coldest are: OBAFGKM. More recently it was expanded to include the letters: WLTYCSD.
So why are those particular letters used? And why are they arranged in that particular order?
Well, that has to do with some of the older star classification systems, and how they were remixed as time went on. The first classification system to use characters from the alphabet was the Draper classification system. It followed the pretty standard order of A, B, C, D, etc... with the letters divided up to follow the even older Secchi classification. Class I in Secchi corresponded to the letters A through D in the Draper system, and Class I tended to be the hot white and blue stars with dominant hydrogen lines on the spectrographs.
There was a time when the classification followed the standard order of the alphabet. So what happened? Well, in the late 1890s there was a debate going on about how to classify the stars, despite the older classification systems already in place. Some wanted a more complex classification system that took into account the size of stars (and later lead to the dwarf and giant classifications) and some wanted a simpler system. Into this debate stepped Annie Jump Cannon with a third solution.
She took some of the letters from the Draper classification and arranged them based on Balmer absorption lines which measure visible light emissions from hydrogen atoms. The letters she kept are the ones which make up the base of the Morgan–Keenan system: OBAFGKM. Where originally O was off in a separate class because it was so bright, it turns out it was the leader of this new system. It had already been discovered by Antonia Maury that B class stars should be listed before A class stars, and so the rest fell into place. And what about WLTYCSD? That will a topic of its own!
So why are those particular letters used? And why are they arranged in that particular order?
Well, that has to do with some of the older star classification systems, and how they were remixed as time went on. The first classification system to use characters from the alphabet was the Draper classification system. It followed the pretty standard order of A, B, C, D, etc... with the letters divided up to follow the even older Secchi classification. Class I in Secchi corresponded to the letters A through D in the Draper system, and Class I tended to be the hot white and blue stars with dominant hydrogen lines on the spectrographs.
There was a time when the classification followed the standard order of the alphabet. So what happened? Well, in the late 1890s there was a debate going on about how to classify the stars, despite the older classification systems already in place. Some wanted a more complex classification system that took into account the size of stars (and later lead to the dwarf and giant classifications) and some wanted a simpler system. Into this debate stepped Annie Jump Cannon with a third solution.
She took some of the letters from the Draper classification and arranged them based on Balmer absorption lines which measure visible light emissions from hydrogen atoms. The letters she kept are the ones which make up the base of the Morgan–Keenan system: OBAFGKM. Where originally O was off in a separate class because it was so bright, it turns out it was the leader of this new system. It had already been discovered by Antonia Maury that B class stars should be listed before A class stars, and so the rest fell into place. And what about WLTYCSD? That will a topic of its own!
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Enduring Ephemera
I collect random historical facts and obscure bits of information like a dragon collects gold. Here is where I dispense those nuggets of wisdom.
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