The spectral classes of stars

Al,.. . . The spectral class O stars show higher T than the class B, up to well above 50000 K. A part of these (few, but spectacular) stars show emission lines, and are the strongest light-emitters (stable on a centennial time-scale) with absolute luminosities (the absolute magnitudes in literature refer to a distanee of 10 parsec = 32.6 light-years) 2000 times that of Sirius. A related, early (i.e. high 7 class is W, Wolf-Rayet stars which seem to be rather short-lived precursors... 

In the classical view of star formation [21,26], young proto-stars are surrounded by an accretion disk, also called a circumstellar disk, which contributes to the feeding of the proto-star before it reaches its final mass. The so called class 0 proto-stars are deeply embedded and radiate mostly in the far infrared and sub-millimetre spectral range. At this stage, proto-stars are actively accreting, as testified by the presence of jets and molecular outflows. These outflows contribute to the release of energy and... 

The spectral features observed by astronomers have led to the classification of stars into seven broad classes outlined in Table 4.1, together with their surface temperatures. The highest-temperature class, class O, contains may ionised atoms in the spectrum whereas the older stars in class M have a much lower temperature and many more elements present in the spectrum of the star. Observation of a large number of the stars has lead to extensive stellar catalogues, recently extended by the increased sensitivity of the Hubble Space Telescope. Making sense of this vast quantity of information is difficult but in the early 19th century two astronomers... 

W Ursae Majoris stars can be understood as contact binary stars with a common envelope (Lucy 1968). They subdivide into two types The A-type are earlier in spectral class than about F5, are believed to have radiative envelopes, and associate primary (deeper) eclipse minimum with transit eclipse. The W-type have spectral classes later than F5, are believed to have convective envelopes, and associate primary minimum with occultation eclipse. Controversy has surrounded the explanation of W-type light curves. 

Hopefully Bob has impressed Miss Muxdroozol with his barrage of facts. He decides to continue. The less massive the star, the longer it lives. The average star in the Milky Way is about half the Sun s mass and lives about 50 billion years. Bob speaks into his flexscreen, Brunhilde, show spectral class, and the following table is displayed. 

Yes. And let me impress you with my knowledge of a few other classes. You don t need to know these minutiae for the rest of our lessons, but there are some less common classes, such as WC or WN class stars (Wolf-Rayet stars— white stars that are hot like the O class stars), C class stars (dim red stars probably too cool to support life), and S class stars, which are cool like the M-class stars and reddish-brown in color. N stars are also not part of the standard spectral sequence. They differ from the M stars not by temperature but by composition, having a carbon-to-oxygen ratio reversed from M stars. R stars are warmer carbon-rich versions and have the same temperatures of K and G stars. For now, don t let all these letters confuse you, just focus on the stars in the table. Miss Muxdroozol nods. The Wolf-Rayet stars sound interesting. What do we know about them ... 

Mr. Plex looks at the table of stellar spectral classes. But Sir, you said that stars are mostly hydrogen and helium. Why such different spectra if stars have mostly the same elements ... 

The flat disk consists of a complex spiral pattern, rather than the two graceful amis found in srane galaxies. In addition to the spiral arm tracers menticned previously, the disk and spiral arms contain young stars of all spectral classes, galactic clusters composed of several hundred young stars, and interstellar clouds of gas, mol-... 

Cowley has done much to dissipate the belief that the Sr, Cr, Eu stars constitute a fairly homogeneous class (with the exception of the three maverick stars discussed above). The number of detectable spectral lines, and their intensities, of yttrium, barium and lanthanides [40] in 29 Ap and A , stars were carefully compared. In most of these stars, yttrium and lanthanum are unexpectedly scarce. The relative solar abundances of 12 lanthanides (altogether... 

In this sense, the solar data in Table 1 represent the modem normality. However, there a few, but highly spectacular, deviant stars. Red stars [112] of spectral class S (T close to 3000 K) show absorption bands of the diatomic molecule ZrO (the far more frequent class M at the same T shows TiO absorption bands) and may also show spectral lines of atoms of teehnetium (Z = 43), of which no isotope is known to have t yj above 2.6 million years, and a quite conspicuous [46, 58] group is Ap stars (peculiar stars of class A), constituting about 5 percent of stars with T in a narrow interval close to 10000 K. The first one to be discovered was the star of third magnitude (with the ancient name Cor Caroli after the British king Charles II, a benefactor of the Greenwich Observatory) Canum Venaticorum, where Baxandall in 1913 detected strong Eu Fraunhofer lines of europium (We are not aware of any reaction to this from the 81-year old Crookes). 

Draper originally devised a classification system in which stars were placed into lettered groups A, B, C, D, and so on. Over time, that system was changed and refined. Today, only seven color groups, or spectral classes, remain (from hottest to coolest) O, B, A, F, G, K, and M. While the variety of stars is such that more complex classification schemes have been developed to accurately describe them all, the chart on page 52 provides basic information on each of the seven spectral types. 

According to [158], the spectra of O(C) planetary nebula central stars are dominated by an absorption line spectrum of C. During the course of the PG survey, a small group of eight stars showed spectra dominated by helium, but distinguished by a broad absorption from He II 4686, C IV 4658 and possible other C and N contributors [95]. The O(C) and PG1159 spectral classes are essentially equivalent, and it is evident that stars of similar spectral type are to be found both with and without planetary nebulae. Stars of both O(He) and O(C) classes appear with and without nebulae in roughly the ratio 1 1. 

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