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 three new spectral types past M. These are L, T, and Y, 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 Y.

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.

Dwarfs and Giants

Even in stars of the same spectral type, the absorption lines don't always look alike. In some stars the lines are narrow and sharp; in others they are broadened by various different effects. Chief among these effects is atmospheric pressure, which also changes the intensity ratios of certain pressure-sensitive lines.

Astronomers quickly realized that atmospheric pressure tells a star's surface gravity and therefore suggests its size. Narrow lines indicate an immense, bloated star with a weakly compressed atmosphere far from its center of gravity. In the Henry Draper Catalogue, spectral types were prefixed with d for dwarf, sg for subgiant, g for giant, and c for supergiant.

You'll still run across these letters from time to time, but beginning in 1941 they were replaced by a more detailed scheme first published by William W. Morgan and Philip C. Keenan. With only minor changes, this "MK" system of spectral classification remains the standard today. Stars are assigned to luminosity classes by Roman numerals: I for supergiants (often subdivided into classes Ia-0, Ia, Iab, and Ib in order of decreasing luminosity), II for bright giants, III for normal giants, IV for subgiants, V for dwarfs on the main sequence (defined in the illustration below), and occasionally VI for subdwarfs.

Thus a designation such as G2V, the Sun's spectral type, tells both temperature and luminosity. When these quantities are plotted against each other on a graph, the result (shown at right) is called a Hertzsprung-Russell or H-R diagram. This has been a fundamental astrophysical tool ever since it was invented around 1911.

Most stars gather in certain narrow regions of the H-R diagram according to their masses and ages. Stars arrive on what's called the main sequence soon after they are born, and this evolutionary track is where they spend most of their lives. Massive stars blaze brightly on the hot, blue end of the main sequence. They burn up their nuclear fuel in only millions or tens of millions of years. Stars with lower masses comprise the yellow, orange, and red dwarfs on the lower-right part of the main sequence, where they remain for billions of years.

As a star begins to exhaust the hydrogen fuel in its core, it evolves away from the main sequence toward the upper right and becomes a red giant or supergiant. Stars that began with more than eight times the Sun's mass then evolve left and right through complicated loops on the H-R diagram as if in a frenzy to keep up their energy production. Then they finally explode as supernovae. Less massive giants evolve to the left and then down to become white dwarfs; this is the track the Sun will trace through the H-R diagram 8 billion years from now.