Every student in introductory astronomy learns the mnemonic “Oh Be a Fine Girl/Guy, Kiss Me”, which is a way to remember the spectral classification of stars. Here’s how it works: the color of a star is determined by its temperature, as I outlined previously.
- Oh: O class stars are the hottest, with peak temperatures in the ultraviolet part of the spectrum. They have a blue tinge to our eyes; a prime example of an O star is Mintaka, one of the stars in the “belt” of Orion.
- Be: B class stars come next, still appearing bluish to us. Rigel, the brightest star in Orion, is a B class star — one you can see and identify easily.
- A: A class stars are white-hot; Sirius the Dog Star, the brightest star in the night sky, falls into this category. (Harry Potter fans who are astronomy buffs were unsurprised to learn that Sirius Black could turn into a black dog for that reason.)
- Fine: F stars are also white-hot, though with a yellow tinge. Polaris, the North Star, is F class.
- Guy/Girl (whichever your preference): G stars are like our Sun: white, but with a yellower tinge than F stars.
- Kiss: K stars like Arcturus in the wonderfully-named constellation Boötes are fading towards the red part of the spectrum, and finally
- Me: M stars such as Betelgeuse are very red in appearance. Most stars in the universe are M-type.
We teach this mnemonic (or one like it) because the letters are obviously not in alphabetical order. As with many things, the scheme is historical: originally the spectral classification was based on the absorption of light by atoms in the star’s atmosphere. Class A stars have the strongest hydrogen absorption, class B stars come next, etc. Harvard astronomer Annie Jump Cannon (who has one of my favorite names in astronomy) clarified and refined the system in the early 20th century.
Anything warm produces light; stars are special because their heat comes from nuclear fusion deep below the surface. Brown dwarfs are star-like objects that aren’t massive enough to generate nuclear fusion, but are larger and intrinsically warmer than gas giant planets like Jupiter. (I say “intrinsically”, because many exoplanets fall into the category of “hot Jupiters”: Jupiter-like worlds that orbit very close to their host star and as a result get really heated up. Jupiter’s cloud tops are quite cold, though its interior is hot.) Some brown dwarfs are warm enough to be class M, but others produce too little visible light and fall into additional categories L and T, so that I often refer to them as MLT or “mutton, lettuce, and tomato” objects. With L and T added, the mnemonic gets extended to “Oh, Be A Fine Guy/Girl, Kiss Me, Less Tongue” (or talk, if your professor is more squeamish).
Yesterday, NASA astronomers working on the Wide-field Infrared Survey Explorer (WISE) announced the discovery of six brown dwarfs that are even less cooler than T class. These brown dwarfs have room-temperature surfaces — still warmer than Jupiter, but by far the coldest star-like objects ever observed. Humans have warmer skins than these objects, although because they are a lot bigger than us (insert a “yo momma” joke here), there’s a lot more surface area to make the light. As you can imagine, they aren’t that easy to detect, which is why it takes a highly sensitive instrument like WISE to find them, even if they are relatively close to our Solar System.
Being cooler than T class, the new brown dwarfs have been given a new spectral class Y, which astronomy teachers will have to tack on the end of the mnemonic. I don’t think the title to this post will catch on, so leave your own suggestions in the comments!