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Stellar Spectral Classification Article Stars in the Cellar: StarsCLASSES in LOST the AND Cellar: FOUND BY JAMES B. KALER Hβ 38 September 2000 Sky & Telescope For more than a century, the familiar spectral sequence OBAFGKM has stood firm. It is now being extended, testimony to both modern technology and the sequence’s amazing adaptability to the discovery of new stellar types. the students thundered. Early Days I was teaching one of The modern tale of the stellar sequence my favorite subjects, begins back in the quiet time of 1890s “stellarM spectra,!” to a rather large upper-level Harvard, but it had a long prelude. The introductory astronomy class. The task birth of the sequence took place literally was to get them to learn the spectral se- in a ray of sunshine. quence — the basic stellar categories — Break sunlight with a prism or rain- without resorting to tired mnemonics drop and it splashes into an array of col- about kissing fine girls. In a fit of creativity ors from red through violet. Stretch this (or so it seemed), I burst into the class- spectrum carefully, with great color sep- room, and throwing my arms in the air aration, and out pop myriad fine (and like a college cheerleader, I called out: some not-so-fine) gaps in the colors. “Give me an O!” A dribble of “O”s fol- These “spectrum lines,” discovered in 1802 lowed. “Give me a B!” produced somewhat by British chemist William H. Wollaston, better results. As we worked our way are superimposed on the solar spectrum through A, F, G, and K the students got by the actions of atoms, their electron- ROBERT L. HURT into the act and called out louder. By the stripped ions, and by simple molecules The same sunlight that brightens our days end, M, we could have been in the stadi- in the outer layers of the Sun’s bright and warms our landscapes spreads into a um cheering the Illini on to victory. Spec- surface. Each element, ion, or molecule rainbow of colors called a spectrum. Send tral-type M stars, I told the students, lie at has its own unique pattern of light and that light through a fine prism or slit and the end of the sequence, populating the dark lines that form when the electrons grating and lines and gaps appear, creat- bottom of the temperature-pile of stars. bound to atoms absorb radiation at par- ing a sort of solar “fingerprint” (below) in- For a hundred years they’ve been consid- ticular wavelengths. Chemists create these dicating the elements in the Sun’s atmos- ered the coolest stars known. patterns in the lab by igniting pure sub- phere that absorb portions of the When something has endured for a cen- stances and recording the resulting pat- sunlight. Spectroscopy — the study of the tury, you assume it will never change — terns through spectroscopes. Comparison spectra of light-emitting and -reflecting much like a historic building. But if the ed- of laboratory spectra with the spectrum objects in the universe — reveals an ob- ifice is strong, you can surely add to it, and of sunlight allowed solar lines to be iden- ject’s temperature, composition, and ve- — remarkably — that is what has hap- tified. They turned out to be the lines for locity. Astron-omers routinely take spec- pened to the famed spectral sequence. No the common elements of Earth: hydro- tra in all wavelengths of light, including longer is it OBAFGKM (with side branch- gen, carbon, oxygen, iron, and so on. the infrared, where dwarf stars are most es R, N, and S). Now it is OBAFGKM From an absorption line’s strength readily detectable. Above: Cool, low-mass with newcomers L and T. Really! (the amount of energy it extracts from dwarfs range from true stars to Jupiter- What happened? Technology happened. the spectrum) and an application of like objects, and each has between 80 and New observational and analytical tools atomic theory, we can determine the 10 Jupiter masses. This artist’s conception came into use and, as a result, new class- composition of the Sun’s light-emitting of the T dwarf Gliese 570D shows a giant es were found. The story of the expand- surface. Measured by the number of storm, which may cause subtle variations ing spectral sequence is one of fresh dis- atoms, it turns out to be 92 percent hy- in its spectrum as the spot rotates in and covery,with profound implications for drogen. The rest is mostly helium and a out of view. JERRY SCHAD understanding the galaxy, the stars, and tiny smattering of everything else, led by even the planets. oxygen, carbon, neon, and nitrogen. Take away the light stuff, the hydrogen and helium, and the “everything else” is ap- portioned about as it is in the Earth’s crust, powerful evidence that we and the Sun were both born at about the same time from the same dusty cloud of inter- stellar gas. Our familiar landscapes are Na Hα made from the solar distillate. MAURICE GAVIN Sky & Telescope September 2000 39 Now go to the stars. What a surprise they gave to the pioneers in spec- troscopy. The first stellar spectra, ob- served in 1817 by the German physicist Joseph Fraunhofer, did not look any- thing like that of the Sun. By the 1860s, stellar rainbows were being studied in bulk by William Huggins in England and Angelo Secchi of the Roman College Ob- servatory. In spite of Huggins’s discovery that the Earth, Sun, and stars contain the same chemicals, the observational evi- dence stood firm: a great many stars did not have Sun-like patterns. Although the Sun is mostly hydrogen, its spectrum is actually dominated by absorptions from sodium and ionized calcium. However, hydrogen absorptions quite overwhelm the spectra of Vega, Altair, and Sirius. In P. SABINO MAFFEO/VATICAN OBSERVATORY other stars — Betelgeuse and its reddish Above: Under the direction of Edward C. Pick- kin, for example — hydrogen is effective- ering and Williamina Fleming (standing), a ly absent, while we see complex bands team of 15 women worked at Harvard Col- produced by such molecules as titanium lege Observatory in the late 1880s classifying oxide. Why the differences? Although all stars according to their spectra. They pored stars are made of the same basic stuff, over the spectra of star fields (like the one of their spectra make it seem as if their ac- Eta Carinae shown here), sorting and classify- tual chemical compositions vary all over ing the stars. Their work formed the basis for the place. How best to understand what’s the Henry Draper Catalogue as well as the sys- going on? tem of star classification in use today. The first step, as in any science, is to classify. But how? In 1863 Secchi invent- Huggins for the honor of making the ed a system that would act as the proto- first permanent recording. (Huggins was type for future developments. It divided the first to photograph a nebula’s spec- stars into five groups based on similar HARVARD COLLEGE OBSERVATORY trum.) Upon Draper’s death, his widow line patterns and colors. Roman numer- gave his recorded spectra, his telescope, als I through V identified the following and a memorial fund to Harvard. Picker- classes, respectively: blue-white stars with ing took up Draper’s work and built a simple hydrogen spectra (Vega, Sirius), spectrographic telescope of novel design. stars with more complex spectra like the Draper and Huggins had used a dispers- Sun (Aldebaran and Arcturus), orange- ing prism at the telescope’s focus to view red stars with more complex bands of one spectrum at a time. Pickering, how- lines (Betelgeuse), red stars with different ever, placed the prism in front of the kinds of complex bands (such as 19 Pis- lens, so he could image spectra of all the cium), and finally those containing both stars in a field at once, allowing rapid emissions (bright lines) and absorptions classification by his assistant, Williamina (Gamma Cassiopeiae and Beta Lyrae). Fleming. With all those spectra available, a sim- Classifying at Harvard ple scheme that could handle all the de- Secchi’s scheme was oversimplified, since tail was needed. Beginning in 1890 Pick- stars within any one class could be quite HARVARD COLLEGE OBSERVATORY ering and Fleming expanded Secchi’s different from one another. Help came Top: Angelo Secchi, a Jesuit priest, was also a groups with Roman letters A through O, when Henry Draper, a physician and am- spectroscopist. His early stellar-spectrum based primarily on the strengths of the ateur astronomer in New York, and Ed- classification scheme was used throughout observed hydrogen lines (P and Q were ward C. Pickering, professor of astrono- the late 19th century and was the basis for used for those that did not fit). Further my at Harvard University, focused their the expanded stellar classes developed by observation showed that some classes were attention on the problem of sorting New York astronomer Henry Draper (center) erroneously assigned, unneeded, or could stars. In 1872 Draper photographed and Edward C. Pickering (bottom), who was be merged with others. Two other assis- Vega’s stellar spectrum, just beating out director of the Harvard College Observatory. tants, Antonia Maury and Annie Jump 40 September 2000 Sky & Telescope Left: An objective-prism spectrogram of the Eta Carinae region of the southern Milky Way. This 140-minute exposure was made on May 13, 1893, in Arequipa, Peru, using Harvard College Observatory’s 8-inch Bache refractor (below) and is the first plate ever classified by Annie Jump Cannon.
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