No, Black Sky Affair hasn’t lost its mind. Not yet. The above is a mnemonic device for remembering the seven types of main-sequence stars in descending order from hottest to coolest according to the Morgan-Keenan-Kellman spectral classification.

In 1835 Auguste Comte, French philosopher and originator of the school of positivism, notoriously predicted humankind would forever remain ignorant of the temperatures and chemical compositions of stars. Famous last words – a mere three decades later, scientists began erasing that ignorance through the combination of spectroscopy and photography. Astrophysics was born.

click to enlarge Black Sky Affair: Oh, Be A Fine Guy (or Gal), Kiss Me
Morgan-Keenan spectral classification system

As a general rule, the brighter a star, the hotter and more massive. Classes M and K are coolest, commonly known as red dwarfs. Proxima Centauri, our closest stellar neighbor, is one of these; they emit a fraction of the light and heat our sun produces. They do, however, live long. The less massive a star, the less quickly it exhausts its nuclear fusion fuel, extending its lifetime. The vast majority of stars in the Milky Way are red dwarfs, at least 100 billion worth and often existing in binary or trinary with other stars, though these red dwarfs are too dim to be seen from Earth with the naked eye.

Class G stars are typically referred to as yellow dwarfs. Our sun, officially designated G2V65, is of this family. While not as common as red dwarfs, G-types comprise about 20 percent of our galaxy’s stars; Sol is rather average for its type, even somewhat unremarkable. 4.5 billion years old, it is expected to burn steadily for another five billion until, having consumed its intrinsic supply of hydrogen, its outer layers expand into a red giant phase engulfing all the rocky inner planets, including Earth.

Similar to G-types are Class F, though more massive, hotter, and brighter, shortening their lifespans in terms of our own sun and giving off anywhere from two to seven times the ultraviolet light Sol does. Life would still be theoretically possible in the habitable zone of such a star, provided such life is shielded from UV radiation by an atmospheric ozone layer such as our own; even without, life may yet persist underwater or underground.

Classes A, B, and O bring us into the realm of the blue-white stars. A-types shine brightly and aren’t too awfully different from ours in terms of mass, though they burn far hotter and more quickly. Those we observe in the sky, like stellar neighbor Vega, are only as old as Earth’s late Ordovician period, and some as young as our late Jurassic.

B-types are up to 16 times more massive than our sun, extremely luminous, and blaze blue. Class O blue-white giants, supergiants, and hypergiants are – while rare – frankly insane. Leading brief cosmological lives, these celestial titans color the arms of spiral galaxies and frequently die in violent supernovae, leaving behind black holes and neutron star remnants.

The next BSA will feature star types rather more atypical. Until then, here’s to variety, and the spice of life.

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