Ordering Stellar Spectra

The first great classifier of stellar spectra was Angelo Secchi, a Jesuit priest in Rome. In the 1860s he examined the spectra of hundreds of stars visually in a telescope and classed them into five main types, mostly named for bright examples. "Sirian" stars, for instance, showed spectra like Sirius's, dominated by absorption lines of hydrogen atoms.

Today's classification scheme was born at Harvard College Observatory. Starting in 1886 under Edward C. Pickering, the observatory staff photographed and classified thousands of stellar spectra. They assigned them letters from A through Q, generally in alphabetical order from the simplest-looking to the most complex. But soon a more natural system became clear. By rearranging and merging classifications, Antonia C. Maury and Annie J. Cannon found that they could fit nearly all stars' spectra into one smooth, continuous sequence. The sequence matched the stars' color temperatures, from the hottest, blue-white stars at one end to relatively cool, orange-red ones at the other.

But it was too late to reassign the letters. When the dust cleared, the rearranged sequence ran O B A F G K M from hot to cool. Spectral types on the blue end were called "early" and those on the red end "late." These terms are still used today, though the incorrect idea they embody — that stars simply cool with age — has been obsolete for generations.

The sequence could be cut even more finely. Cannon subdivided each letter with numbers from 0 to 9, so that a spectrum whose appearance placed it halfway between standard G0 and K0 stars was called G5.

Using this scheme, Cannon led the classification at Harvard of 325,300 spectra recorded on wide-field photographs. The resulting Henry Draper Catalogue (HD) and Henry Draper Extension (HDE), published beginning in 1918, remain standard references today.

The time-honored mnemonic for remembering the spectral sequence, invented by Henry Norris Russell when astronomy's leadership was all male, is "Oh Be A Fine Girl Kiss Me." In 1995 Mercury magazine published a student's rejoinder: "Only Boys Accepting Feminism Get Kissed Meaningfully."

The discovery in recent years of very dim, very red objects — the smallest, coolest red dwarf stars and barely glowing "brown dwarfs" — has led to the creation of two new spectral types past M. These are L and T, chosen from among the few remaining letters that might not have some other, confusing meaning. So now the full spectral sequence runs O B A F G K M L T.

A few other spectral types don't fit the sequence but instead parallel it. Type W or Wolf-Rayet stars are as hot and blue as the hottest O stars but show strong emission lines, either of nitrogen (WN), carbon and oxygen (WC), or neither (WR). Emission lines indicate an especially large, thick shroud of hot gas surrounding these stars. The W stars appear to have blown off their original outer layers of hydrogen, exposing other materials beneath.

Some giant stars at the cool end of the spectrum have an excess of carbon. These were originally called R and N but have been merged to form type C. "Carbon stars" can often be spotted at a glance in a telescope by their deep red color. A bright example in the autumn sky is 19 Piscium (TX Piscium) in the Circlet of Pisces, spectral type C5. Their distinctive absorption bands (masses of overlapping spectral lines) due to the carbon compounds C₂, CN, and CH darken or "blanket" the blue end of the spectrum. In other words, a carbon star's atmosphere is a red filter. When seen in emission instead of absorption, these same spectral bands glow blue; the same compounds that redden a carbon star in absorption give comets their blue-green tint in emission.

The rare type-S stars are also red giants. They parallel type M but show strong bands of zirconium oxide and lanthanum oxide instead of an M star's titanium oxide. We can imagine that planets of S stars, bathed in chemically peculiar stellar winds, might be encrusted with gems of cubic zirconia.