Oh be a fine girl, kiss me. That’s a mnemonic that adolescent astronomers use to remember the seven classes of stars, in order of decreasing temperature.
O stars are the hottest of all stars, with temperatures around 40,000 degrees Celcius. B stars follow with temperatures around 20,000 degrees. A stars are around 10,000 degrees, F stars around 7,000 degrees; G stars are about 6,000 degrees, and K and M stars are about 4,000 and 3,000 degrees, respectively.
OBAFGKM — why such a strange order? It’s a fair question. During late 1800s, astronomers began a most systematic study of the stars. Using telescopes equipped with prisms to disperse the colors in rays of light from individual stars, and cameras to record these tiny stellar rainbows, astronomers were able to examine in detail the spectrum of hundreds of thousands of stars.
A white-light spectrum contains the colors red, orange, yellow, green, blue, indigo and violet (Roy G. Biv, another familiar mnemonic). And while all stars, the sun included, make basically a Roy G. Biv spectrum, astronomers noted when they began to study this in detail, that from some stars a certain pattern of colors was missing to various degrees.
These colors are absorbed by hydrogen gas in the outer atmospheres of the stars. On the basis of how strongly hydrogen gas imprints its signature on the light from a given star, astronomers sorted the stars into groups: hydrogen absorption is strongest in A stars, next strongest in B stars, and so on, in the order of the alphabet.
In the early 1900s, astronomers realized that classifying stars really amounted to sorting stars by temperature, but not in the old alphabetical order. So instead of changing the spectral class of each and every star that had been meticulously analyzed, the order of the star classes was scrambled to OBAFGKM.
On any clear night you can hunt for stars in each of the spectral classes. Here is a guide to get you started.
O stars are not too common. The constellation Orion is the best O-star hunting ground. The left-most star in Orion’s belt (Zeta Ori) and the star at the tip of Orion’s sword (Iota Ori) are prototype O stars.
B stars are more common than O stars. Spica (Alpha Vir), the brightest star in the constellation Virgo is one of the brightest B stars in the sky. Look for Spica tonight in Virgo near the gibbous moon.
A stars are pretty common too. This time of year at dusk you can find the A star Deneb (Alpha Cyg), the brightest star at the head of constellation Cygnus the Swan, soaring above the horizon in the southeast.
For an F star, look to Procyon (Alpha Cmi), the brightest (of only two) stars in the constellation Canis Minor, the Little Dog, and the seventh brightest star in the entire sky. Unfortunately, Procyon sets just after sunset this time of year, so you’ll have to wait until fall for a good look.
You don’t have to look to far to find a G star. The sun is a G star; just don’t look at it too long.
For a K star, find Arcturus (Alpha Boo) in Bootes by following the arc of the handle of the big dipper away from the dipper. Arcturus is high overhead at dusk this time of year.
It’s slim pickings for M stars. The nearest star other than the sun — Proxima Centauri — is a normal M star, but it cannot be seen without a telescope. Betelgeuse, the brightest star in Orion is also an M star, but it’s a supergiant.
