…continuedThe Spectral Types of Stars
Odds and Ends
Spectra can reveal many other things about stars. Accordingly, lowercase letters are sometimes added to the end of a spectral type to indicate peculiarities. Here is a partial list:
|Some Spectral Peculiarity Codes|
two spectral types are blended, indicating that|
the star is an unresolved binary
|e||Emission lines are present (usually hydrogen)|
|m||Abnormally strong "metals"
(elements other than hydrogen and helium)|
for a star of a given spectral type; usually applied to A stars
|n||Broad ("nebulous") absorption lines due to fast rotation|
|nn||Very broad lines due to very fast rotation|
|neb||A nebula's spectrum is mixed with the star's|
|p||Unspecified peculiarity, except when used with type A, where it denotes abnormally strong lines of "metals" (related to Am stars)|
|s||Very narrow ("sharp") lines|
|sh||Shell star (B to F main-sequence star with emission lines from a shell of gas)|
|var||Varying spectral type|
|wl||Weak lines (suggesting an ancient, "metal"-poor star)|
Notations can also be added for elements showing abnormally strong lines. For example, Epsilon Ursae Majoris in the Big Dipper is type A0p IV:(CrEu), indicating strong lines of chromium and europium. The colon implies uncertainty in the IV luminosity class.
Certain spectral subtleties are not widely known among amateurs. Some visual observers pride themselves on being able to nail a star's spectral type to the nearest letter by its color in the eyepiece. Color is indeed a close indicator of spectral type for stars earlier (hotter) than about K5, at least when no interstellar reddening is present. But the relationship often breaks down among the later K and M stars. Compare the tint of Betelgeuse, type M2 Iab, to that of Aldebaran, K5 III. Most people can't see any difference.
In addition, dwarf G, K, and M stars are not as red as giants and supergiants of the same types. The color difference is equivalent to about one-half to one whole letter class.
Lastly, differences between spectra are far greater than differences in the actual chemical compositions of stars. An A star might seem to be almost pure hydrogen, while a K star shows only trace evidence of hydrogen in a spectrum packed with lines of "metals" (the astronomer's term for all elements other than hydrogen and helium). But A and K stars are in fact made of the same stuff. Different atoms and ions merely display their spectral lines at different temperatures. Even carbon stars are made mostly of hydrogen and helium. The true "abundances" of elements can indeed be measured in a star. But it's a tough job of comparing precise line strengths in a high-quality spectrum with the strengths predicted by atomic theory or measured in the lab.
For much of the 20th century, the study of visible-light spectra practically was astronomy. In recent decades the opening of nonvisible wavelengths and other exciting advances have distracted attention from this field. Nevertheless it remains the bedrock on which modern astronomy rests.