By Mike P. Usher
Classes in Collier County will soon come to an end in a few days when school lets out for the summer, but stars also have classes which they are stuck in for most of their lives. Classes are assigned to stars based on a very detailed analysis of their light; once you know the class a star belongs in it can tell you much of its past history, distant future, temperature, chemical composition and color. As far as a casual sky gazer is concerned class really only means color which in turn is based upon temperature.
Classes are given letter designations A, B and so on and used to be in alphabetical order; but as the decades wore on they were shuffled around, dropped old ones and added new ones because of original misinterpretations and the sequence wound up as something of a mess. Nowadays the order goes, from hottest to coolest, O, B, A, F, G, K, M plus a handful of specialty classes which do not concern us here.
Astronomers have developed a mnemonic to remember the sequence, which after all is basic to their work. “Oh Be A Fine Girl, Kiss Me!” Women astronomers have their own mnemonic as well: “Only Boys Accepting Feminism Get Kissed Meaningfully!”
O stars have surface temperatures above 33,000 degrees and are very blue in color. They are also extremely rare, the easiest found examples are the outer two belt stars in Orion.
B stars have surface temperatures 10,000 to 33,000 degrees and are blue. Rigel, also in Orion, is a good example.
A stars are a bit cooler at 7,500 to 10,000 degrees and are white to blue white in color. Sirius is a great example.
F stars have 6,000 to 7,500 degree surface temperatures and are pure white in color. Polaris or Canopus are good examples.
G stars are 5,200 to 6,000 degrees on the surface and are commonly called yellow although the eye usually perceives them as white. The Sun is the classic example.
K stars are 3,700 to 5,200 degrees and are yellowish orange in color. Arcturus and Aldebaran are wonderful examples.
M stars are the coolest, below 3,700 degrees and are distinctly orange red in color. Antares and Betelgeuse are examples but are not typical representatives of the class.
M stars are by far the most common star in the galaxy, over three quarters of all stars are in the M classification but most are dwarfs and none of these are visible to the naked eye and but few are visible even in binoculars. Antares, shown in the chart tonight, and Betelgeuse are the only two M class red supergiants visible to the naked eye.
See you next time!
Mr. Usher is President of the Everglades Astronomical Society which meets every second Tuesday at 7:00PM at the Norris Center, Cambier Park, Naples.