So cool, and some are still stars.

"Oh, Be a Fine Girl (Guy), Kiss Me."

That line may get your face slapped in a singles bar, but it's an old standby in astronomy textbooks.

In the stellar realm, those words of wisdom are a mnemonic device for the sequence of letters OBAFGKM. Each letter stands for one of the seven standard classes of stars, in order of descending temperature. At birth, O stars rank as the most massive and the hottest, while those in the M class have been thought to be the least massive and the coolest.

After surveying 1 percent of the sky at the near-infrared wavelength of 2 micrometers, researchers report that they have discovered a class of heavenly bodies cooler than M stars. The 20 newly identified bodies, dubbed L dwarfs, have less than one-third the temperature of the sun and less than one-tenth its mass, note J. Davy Kirkpatrick of the California Institute of Technology in Pasadena and his colleagues.

Spectra of the L dwarfs, taken with the Keck II Telescope on Hawaii's Mauna Kea, reveal that six are failed stars known as brown dwarfs. Another four may also be brown dwarfs. The other 10 are presumably bona fide stars.

Astronomers knew that such low-mass stars existed but hadn't classified them as a distinct type. The spectra reveal, however, that L dwarfs have atmospheres rich in iron hydride and chromium hydride. In contrast, M stars are rich in metal oxides. The L dwarfs therefore constitute a new class.

"This is the first new spectral class in more than 50 years and will mean that ... astronomy textbooks will have to change," notes Gibor S. Basri of the University of California, Berkeley. The number of observed L dwarfs within an estimated few tens of light-years of the sun suggests that "there are gobs of them out there," says Kirkpatrick. Some may lie nearer the sun than does Proxima Centauri, the closest known star.

To include the L dwarfs, Kirkpatrick suggests a new mnemonic: "Oh Be A Fine Girl (Guy), Kiss My Lips."