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Moon and Antares

September 20, 2012

Oh, be a fine guy — kiss me!

No, that’s not flirtation, it’s science — a way to remember how astronomers classify stars — with the letters O, B, A, F, G, K, and M.

A star at the “me” end of the scale is in good view this evening. Antares, the heart of Scorpius, is to the lower left of the Moon at nightfall.

The classification system was devised about 150 years ago. It grouped the stars based on features in their spectra — the rainbow of colors that’s produced when you break starlight into its individual wavelengths. Later, the system was revised to classify the stars by letters of the alphabet, starting with “A” and working up.

In the early 20th century, Annie Jump Cannon reworked the system, also based on spectra. She dropped most of the letters, and rearranged the ones that were left.

The new arrangement also classified stars by their temperature. “O” stars are the hottest, with surface temperatures of more than 60,000 degrees Fahrenheit, so the stars are blue-white. “M” stars are the coolest, with temperatures as low as about 5,000 degrees, so they shine orange or red.

Most M stars are of a type that’s far cooler and fainter than the Sun, so not a single one is visible to the unaided eye.

But some M stars are much brighter giants or supergiants. Antares fits into the “super” category. It’s one of the biggest, heaviest stars in the galaxy — an impressive feat for a star at the bottom of the scale.

More about Antares tomorrow.


Script by Damond Benningfield, Copyright 2012

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