Hidden within the subtle hues of the stars are the keys to their temperatures and compositions. Get acquainted with the classic OBAFGKM spectral sequence through real stars you can see on a spring night.
Color has always been a source of wonder for me. I recall the shimmery, refractive colors of my grandmother's rhinestone jewelry when I was a child. To this day, cut glass catching the sun still makes me ooh and aah. At night, stars substitute for rhinestones. Their colors may be more subtle, but they flare with the same clarity and fire.
When it comes to stars, color contains valuable information about temperature and composition. English chemist William Wollaston passed the Sun's light through a prism in 1802 and discovered it was crossed by fine, dark lines. He assumed they were natural boundaries between the colors. German optician Joseph Fraunhofer measured and cataloged 574 such lines in the early 1800s still known to this day as "Fraunhofer lines".
Not until the early 1860s, when English astronomer William Huggins matched some of the dark "absorption" lines in the Sun's spectrum with those in terrestrial substances, did astronomers come to understand that stars are composed of familiar materials, not some exotic fifth element, or quintessence, as Aristotle had thought.
Hot, dense objects like the tungsten filament in a light bulb or an electric stove burner give off every color of light and emit a line-free, continuous spectrum that looks like a rainbow. A star is likewise hot and dense and produces a continuous spectrum. But before that light reaches your eyes, it must pass through cooler, less dense gas in the star's outer layers. There, atoms in the gas absorb specific colors of light, leaving narrow gaps or lines in the spectrum.
Atoms and molecules in a star's outer envelope reveal their identity by the patterns of lines they produce. Every single element and compound produces its own unique set. Since nearly every star displays absorption lines, astronomers could use the patterns they saw in solar and stellar spectra as "fingerprints" to probe their composition.
Different stars show different spectra. Some have only faint lines, while others are missing chunks of color. By the late 19th century, astronomers were developing schemes to classify spectra The most prolific classifier, Annie Jump Cannon, worked as an assistant to Harvard College Observatory director, Edward C. Pickering, and created a catalog of 325,300 stellar spectra by the early 1920s. Simplifying earlier, more complicated schemes, Cannon divided stars into seven categories, each of which was assigned a letter.
Although not fully understood at the time, her prescient ordering classified stars according to their surface temperature, from the hottest blue-white supergiants to the coolest red dwarfs using this letter sequence: O B A F G K M.
To help us remember the order of this unpronounceable acronym, someone, possibly Cannon herself, proposed the mnemonic "Oh, be a fine girl, kiss me!", which has since morphed into the more egalitarian "Oh, be a fine girl/guy, kiss me!" If that doesn't suit you, consider these others found while trawling the Web:
Only bad astronomers feel good knowing mnemonics
Only boys accepting feminism get kissed meaningfully
Oh boy, another Ferengi getting Klingon money
Odysseus begged Athena for guidance killing many lusty Trojans (this one includes the new L and T dwarf classifications)
One brutal astronomer fought gnarly karate monsters
I could go on but ...
Another way to remember and appreciate the colors of the spectral classes is to get acquainted with the real stars behind the letters. The early spring sky offers up naked-eye stars of all seven spectral types. I've included two maps to help you find them. One shows all seven within the bounds of a single constellation, Orion. The other highlights only the brightest representatives across many constellations.
Temperatures across the classes range from 50,000K (90,000°F) for massive O stars like Alnitak in Orion's Belt to 5,780K (10,000°F) for the sun and down to 2,500K (4,000°F) for the coolest supergiants. As far as staying chill, Betelgeuse, with a surface temperature of around 3,200K (5,300°F), is the coolest, easily-visible star on early April evenings.
Each class is subdivided into 10 subclasses, numbered from 0 to 9 (our Sun is a G2 star with characteristics part-way between G and F). Further, temperature defines which atoms and molecules show up in a star's spectrum. In blazing O stars, searing heat ionizes helium, a very tough thing to do, since the gas holds its electrons tightly and is loathe to part with them. Spectra of cooler M stars are blanketed by familiar atomic absorption lines of "metals" (elements other than hydrogen and helium) and molecular lines from titanium oxide, carbon molecules, and even water.
All this you can see with your own eyes using a visual spectroscope. I have a Rainbow Optics model that does a great job showing the classic hydrogen Balmer line absorptions in Sirius and Vega (A-stars) and the lush banding in Betelgeuse. When you realize you're seeing the internal workings of atoms hundreds of light years away, it almost feels like touching a star.
Rhinestones. The wonder of color goes deeper than we ever imagined.