CHAPTER VELEMENTS AND COMPOUNDS IN STELLAR ATMOSPHERES
THE identification of stellar lines and bands with those observed in the laboratory has furnished a rich source of data for astrophysics. About 25 per cent of the observed solar lines are assigned to elements in Rowland’s Table of Solar Spectrum Wave-lengths. The majority of the solar lines which are still unidentified are faint. Notwithstanding practical difficulties of identification caused by blending, and the consequent uncertainty of wave-length, most of the observed lines, at least in the cooler stars, have been satisfactorily accounted for. There remain some important strong lines and bands of unknown origin, which have been usefully summarized by Baxandall.[125]
The present chapter contains a summary of the stellar occurrence and astrophysical behavior of the chief spectrum lines which are of known origin and series relations. A few other lines, such as those of C++, N++, and O+ are included, as their series relations will probably be forthcoming in the near future. The observed chemical elements are arranged in order of atomic number. At the conclusion of the chapter the elements which have not been detected in stellar spectra are enumerated. The series notation employed follows the system advocated by Russell and Saunders,[126]which appears to meet, more fully than any other, the practical needs of modern spectroscopy.
HYDROGEN (1)
Hydrogen is represented in stellar spectra by the Balmer series (); the ultimate lines are those of the Lyman series ()in the far ultra-violet, and cannot therefore betraced in the stellar spectrum. The occurrence of the secondary spectrum of hydrogen, ascribed to the hydrogen molecule H₂, has been suspected,[127]but not definitely established. Only one of the lines has been recorded, and this should almost certainly be attributed[128]to N++. The familiar Balmer series appear as emission lines in the Wolf-Rayet stars, but normally they are absorption lines in all succeeding classes.
The intensity of the hydrogen lines is at a maximum[129]in the neighborhood of Class.They vary greatly in width, however, within a given spectral class,[130]and it is difficult to find a method of photometry applicable to the comparison of lines of very different widths. The maximum of the Balmer lines has been placed by Menzel[131]at.The writer is inclined to believe that no significant maximum can in fact be derived for the Balmer lines; beyond,however, their intensity falls off rapidly.
It is peculiar to the Balmer series to appear in every class of the normal stellar sequence, and its lines at maximum exceed in strength the lines of every other element which appears in stellar spectra, excepting those of ionized calcium.
Although hydrogen is presumably unable to give rise to an “enhanced” spectrum, as the atom only possesses one extra-nuclear electron, the lines of the Balmer series share with those of neutral helium the peculiarity of behaving like the lines of an ionized atom.[132]They are weakened in dwarfstars, and greatly strengthened in the cooler super-giants, such asOrionis. The peculiarity of the astrophysical behavior of the hydrogen atom also appears in the impossibly high value that is assigned by ionization theory to the relative abundance of this element.[133]An explanation, in terms of metastability, has been suggested by Russell and Compton,[134]but although the hypothesis appears very satisfactory in the caseof hydrogen, it is not applicable to the similar problem of helium. Russell[135]has remarked that “there seems to be a real tendency for lines, for which both the ionization and excitation potentials are large, to be much stronger than the elementary theory would indicate.”
The hydrogen lines are often conspicuously winged. Measures of the width and intensity-distribution of the wings are discussed elsewhere.[136]Wings are probably not peculiar to the hydrogen lines, but the hydrogen wings can be studied because of their strength. The feature is also seen in helium, calcium and iron lines, and wings of greater or less strength are probably universal.
The width of the hydrogen lines instars has been correlated with absolute magnitude, and used for the estimation of luminosities.[137]It appears, however, that the line width may not furnish an accurate measure of absolute magnitude, although it serves to discriminate stars having the c-character from those of smaller luminosity.[138]The occurrence of wings seems, moreover, to be independent of line width and of absolute magnitude.[139]These questions are connected with the problem of classifying thestars, and are discussed in a later chapter.[140]
The continuous spectrum of hydrogen, beyond the limit of the Balmer series, corresponding to the continuous radiation observed in the laboratory for sodium by Wood,[141]and for helium by Lyman,[142]was first noted in stellar spectra by Sir William Huggins.[143]The beginning of the band appears just to the red of the last Balmer line observed.[144]It appears, from work in progress at the Harvard Observatory,[145]that the limit is nearer to the violet, the higher the luminosity, and in a nebular spectrum quoted by Hubble,[146]it almost coincides with the theoretical limit of the series.
The largest number of hydrogen lines recorded is thirty-five, measured by Mitchell[147]in the flash spectrum. Thirty-three were observed in emission by Evershed[148]in the solar chromosphere, and Deslandres[149]traced twenty-nine in the spectrum of a bright solar prominence. Twenty-seven Balmer lines have been observed by Curtiss[150]in the spectrum ofTauri—the greatest number recorded for the spectrum of a star. The number of Balmer lines observed is related inChapter IIIto the pressure in the reversing layer.
HELIUM (2)
Helium is represented in stellar spectra by the,,,,and possibly theseries. Lines associated with these series appear almost simultaneously as we progress through thestar sequence, attain a maximum[151]at,and have disappeared[152]in normalstars. The ultimate lines are theseries,[153]in the far ultra-violet, and cannot be traced in the stars.
The helium lines vary much in width and definition and are often winged. Their intensity does not certainly appear to vary with absolute magnitude within a given spectral class, and they cannot therefore be used in the estimation of spectroscopic parallaxes.[154]The question of absolute magnitude effects cannot be usefully pursued in the absence of more reliable parallaxes, for thestars, than are at present available.
Although the lines of helium do not appear in the normalstar, they are observed in the spectrum of the super-giantCygni, where the pressure is presumably exceedingly low. Thelines also appear in the flash spectrum.[155]
IONIZED HELIUM
The lines of ionized helium appear only in the hottest stars, being peculiar to thesequence. Thelines (the “Pickering,” or “Puppis” series) are well marked in the hotterstars, although all the lines usually available are probably blended.[156]The alternate Pickering lines are practically superposed on the Balmer lines, and the components were separated for several stars of Classby H. H. Plaskett.[157]The “4686” series ()appears in absorption in all the so-called “absorptionstars,” and is even faintly seen in somestars. The line at 4686 appears very readily as an emission line, and the wide bright “band” at this wave-length, which is a conspicuous feature of the Wolf-Rayet stars, of gaseous nebulae, and of certain stages of a nova, is also presumably due to ionized helium.
LITHIUM (3)
The element lithium is represented in the sunspot spectrum by the(ultimate) doublet at 6707, which is not, however, strong enough to be detected in stellar spectra. Russell[158]has called attention to the fact that this line is fainter, in the sun, than would be anticipated from the terrestrial abundance of the element. Compton[159]has suggested that the faintness may be ascribed to low atomic weight, and the consequent blurring of the line by a Doppler effect, owing to the high velocity of thermal agitation.
CARBON (6)
There is no evidence of the presence of neutral carbon in stellar atmospheres. The apparent absence of the element is partly due to the fact that the ultimate[160]line is at 2478, too far in the ultra-violet to be detected. The spectrum of neutral carbon is as yet unclassified, and other lines cannot, therefore, be sought for in the stellar spectrum. The temperature at which the element vaporizes isgiven by Kohn and Guckel[161]as 4000°, and by Violle[162]as 3800°. The heat of vaporization has been evaluated by de Forcrand.[163]At stellar temperatures, the carbon present is probably vaporized, but possibly it is largely in combination as cyanogen or as an oxide, since spectra associated with these compounds appear in low-temperature stars.
IONIZED CARBON
Ionized carbon[164]is represented in the stellar spectrum by the fundamental doublet (), at 4267, and by the principal doublet ()at 6580. The occurrence of these lines is of great interest. The line at 4267 is found in thestars,[165]reaches a maximum at,and is last seen[166]at.It also occurs in the spectra of some gaseous nebulae.[167]In the stellar spectra in which it occurs, the line is sharp and clear, and, apart from appearing as an emission line in certain stars of Class,it has no abnormal stellar behavior.
The principal doublet at 6580 has been said to occur in the Wolf-Rayet spectrum,[168][169][170]and to be much stronger than the fundamental doublet. The identification has been discussed by Wright,[171]and does not appear to be very probable. A knowledge of the behavior of the line at 6580 in the lateand earlystars is greatly to be desired.
DOUBLY IONIZED CARBON
Merton[172]has described a spectrum, produced under conditions of high excitation, which shows several correspondences with the emission bands of the Wolf-Rayet stars. His spectrum contains thefundamental and principal doublets of C+, as well as a number of other lines, which have not as yet been assigned to series. Some of these lines are probably to be referred[173]to the atom of C++, and the writer[174]considers it unnecessary to assume the occurrence of a higher degree of excitation for the Wolf-Rayet spectrum. Some of the lines which are bright in the spectra of emission-line stars have been attributed to C+++ on astrophysical grounds,[175]and also from a discussion of frequency differences.[176]The four strongest groups in Merton’s spectrum, however, consist of triplets, and this points more probably to C++, as does also the ionization potential deduced astrophysically[177]from the behavior of the only group accessible in ordinary stellar spectra. When the doublets due to C+, and the triplets already mentioned, are accounted for in Merton’s spectrum, there remain only two lines at 5696 and 5592. A line[178]with the latter wave-length is attributed by Fowler and Brooksbank[179]to O++. The evidence for stellar C+++ appears, therefore, to be inconclusive.
COMPOUNDS OF CARBON
Cyanogen. The bands headed at 3885, 4215, have been attributed[180]to the CN or the C₂N₂ radical, or to the molecule of nitrogen. The assignment to a particular atom is essentially a question for the terrestrial physicist, and to discuss it here would be out of place.[181]The bands are universally known as the “cyanogen bands,” and this designation will therefore be adopted.
The 3885 and 4215 bands are conspicuous inandstars of low density,[182]and furnish a valuable method for the measurementof absolute magnitude—a method which has been used both at Mount Wilson[183]and at Harvard.[184]The band at 3885 is largely responsible for cutting off the ultra-violet light of the cooler stars.[185]
Cyanogen absorption has been reported as early[186]as,and according to Lindblad[187]it reaches a maximum at.The cyanogen bands reach great intensity in thestars, and are indeed the most conspicuous feature of these spectra. Shane[188]places the maximum in Class,and is doubtless correct in so doing. The maximum given by Lindblad refers to the series,and thestars are notoriously not members of that sequence.[189]Cyanogen is also a typical constituent of the comet-head spectrum.[190][191][192]
Carbon Oxides. The band spectrum attributed to the CO molecule,[193][194]is a strong feature of the spectra ofandstars.[195][196]It is also the chief component of the spectrum of the comet tail, which has been reproduced in the laboratory, at very low pressures, by A. Fowler.[197]
Swan Spectrum. The bands of the Swan spectrum are clearly to be assigned to some compound of carbon,[198][199]but the source is not as yet certainly established. They are characteristic of the comet head.[200]Another band, presumably to be associated with the Swan spectrum, was identified in the heads of nine comets by Baldet.[201]The ordinary Swan bands are also identified in the comet tail.[202]
Hydrocarbon. The identity of the “” band with the 4314 hydrocarbon group was pointed out by Newall, Baxandall and Butler.[203]The strength of the band is increased, in the stellar spectrum, by the superposition of thelines of calcium, and by thelines of titanium, as well as other metallic lines, but the presence of the hydrocarbon band is certain, and is of the highest interest. The “” band is first seen in some spectra[204]of Class,and it attains a maximum ator.
The number of carbon compounds which occur lends plausibility to the suggestion that much of the stellar carbon is in combination at temperatures below 5000°.
NITROGEN (7)
The spectrum of neutral nitrogen has not as yet been satisfactorily analyzed into series.[205]It is quite possible that the first ionization that takes place is the ionization of the molecule,[206]which is accompanied by the production of the well known band spectrum. This spectrum has not been observed in the stars; presumably it would appear at lower temperatures than those involved in the coolest spectral classes. It is, however, stated to be a conspicuous feature of the spectrum of the aurora,[207][208]and it is found in the spectrum of the comet head.[209]These occurrences seem to point to very low temperature and pressure at the source. It is possible that much of the nitrogen present in cooler stars is in combination with carbon.[210]
The green Aurora line was thought by Vegard[211]to coincidewith a line emitted in the laboratory by solid nitrogen. The conclusion was questioned by McLennan and Shrum,[212]who failed to produce the line under similar conditions, and subsequently found a line, of the same wave-length as the aurora line, in the spectrum of a mixture of oxygen and helium.[213]Various previous attempts to identify the aurora line with a line produced in the laboratory had failed conspicuously.[214]
IONIZED NITROGEN
The spectrum of ionized nitrogen has recently been analyzed by A. Fowler.[215]The line which is most conspicuous in stellar spectra is the one at 3995 (), which appears[216]ator earlier, reaches maximum at,and is last seen at.Many of the fainter lines[217]are not observed.
DOUBLY IONIZED NITROGEN
The lines of doubly ionized nitrogen were singled out by Lockyer[218]as showing “abnormal behavior”—they do not appear in the same classes as the N+ line. The early work on the subject is discussed by Baxandall.[219]The most conspicuous lines are those at 4097, 4103, and they attain great intensity in thestars;[220]they are, for example, very conspicuous in29Canis Majoris. H. H. Plaskett[221]places the maximum of the N++ lines in the Victoria class.They are last seen in somestars.
The occurrence and behavior of the N+ and more especially the N++ lines in the Nova spectrum has been the subject of numerous investigations.[222][223][224][225][226][227]
OXYGEN (8)
The ultimate lines of neutral oxygen occur[228]at a wave-length of about 1300, and accordingly cannot be observed in the spectra of stars. It was long supposed that neutral oxygen was entirely absent, but thetriplet at 7700 is observed in the solar spectrum,[229]is strengthened in sunspots, and is strong in the high level chromosphere.[230]The ionization and excitation potentials corresponding to the production of these lines are of the same order as those for the Balmer series of hydrogen, and the astrophysical behavior of the triplet should therefore be similar to that of the hydrogen lines, with a maximum at or near.Special work in the red is, however, required to trace the behavior of the series. The second member, the triplet at 3947, is not certainly present in the solar spectrum, and is not recorded for any star of Class.In the laboratory, the second triplet is about as powerful as the first,[231]and its apparent weakness at the theoretical maximum is difficult to explain.
IONIZED OXYGEN
The spectrum of ionized oxygen should consist of pairs, and numerous lines have been tabulated as belonging to this atom.[232]The lines are found instars, as seems first to have been noticed by Lunt.[233]According to the present writer,[234]they are first seen at,although H. H. Plaskett, working with slit spectra, records[235]some O+ lines in Class.The maximum of the O+ lines falls betweenand,and their disappearance is mentioned[236]as a criterion of Class.
The lines at 4069, 4072, 4076, appear to form a triplet, but are more probably two pairs with two of the lines coalesced. Some stronger lines (page 207) persist in Class.
DOUBLY IONIZED OXYGEN
The spectrum of O++ has been tabulated by A. Fowler and Brooksbank,[237]but not analyzed into series. The lines of this atom are certainly present[238][239]in the stars of Class.The astrophysical behavior of the lines of doubly ionized oxygen has led to the estimation of an ionization potential,[240][241]of 45 volts for the corresponding atom.
COMPOUNDS OF OXYGEN
Oxides.—Numerous oxides, such as carbon monoxide CO, titanium oxide TiO₂, zirconium oxide ZrO₂, and water H₂O, are present in the cooler stars. The metallic oxides are discussed under the corresponding metallic element. The occurrence of steam in the spectrum of the sunspot was announced by Cortie,[242]who supported his argument, originally based upon the widening, over sunspots, of telluric water vapor bands, by the observation that the presence of water vapor is essential, in the laboratory, to the production of the spectrum of magnesium hydride, which also occurs in the sunspot spectrum.[243]It is possible that the formation of oxides may account for the weakness of the spectrum of neutral oxygen in the cooler stars, but this explanation can hardly account for the absence of the second member of theseries from the spectra of thestars, where the lines should have their maximum intensity.
Ozone.—The ozone bands which appear in solar and stellar spectra have been shown by Fowler and Strutt[244]to be of telluric origin. The maximum thermal formation of ozone occurs[245]atand 3500°, and thus its presence in giantandstars might possibly be anticipated.
SODIUM (11)
The ultimate lines ()of the neutral atom of sodium are thelines, which lie at 5889, 5895. These are the only sodium lines which are certainly identified in stellar spectra.[246]They are first seen in the laterclasses, and appear to be strengthened in cool stars, in accordance with theory.
Stationary sodium lines are observed[247]inScorpii,Orionis, and other Classstars.[248]
Thelines are said to show an absolute magnitude effect, being strengthened in giant stars.[249]
No lines of ionized sodium are found in stellar spectra, presumably because they all lie in the far ultra-violet.
MAGNESIUM (12)
The neutral atom of magnesium is represented in the solar spectrum by the,the,and theseries, and the first triplet of the latter series constitutes the conspicuous “b” group in the green. The “b” group and the second member of theseries, the triplet near 3800, are first seen[250]at,have a maximum nearor,and are still strong in the coolest stars examined. Theseries, represented in the solar spectrum by a line at 4571, are faint ultimate lines; the strong ultimate lines[251]are thelines beginning at 2852, and are therefore outside the range of observed solar and stellar spectra.
IONIZED MAGNESIUM
The ionized magnesium atom gives rise to the important combination doublet at 4481 (). These lines appear in thesequence,[252]reach maximum[253]at(not at,as stated by several investigators), and are lost in the increasing strength ofthe iron line at the same wave-length, at about.The doublet varies with absolute magnitude, and may be found to furnish a useful criterion of that quantity. It has been used by H. H. Plaskett[254]in the estimation of the temperatures of some of the stars of Class.
COMPOUNDS OF MAGNESIUM
Magnesium hydride.—The compound magnesium hydride, MgH₂, which has been studied in the laboratory by Brooks[255]and Fowler,[256]was detected by the latter in the sunspot spectrum.[257]It is perhaps significant that the only other hydride reported in celestial spectra is that of calcium, the next heavier alkaline earth after magnesium.
ALUMINUM (13)
Neutral aluminum is represented in the solar spectrum by thelines, the series that constitutes the ultimate lines in the third column elements of the periodic table.[258]The two conspicuous lines of the series in aluminum are those at 3944, 3957, and they are strengthened in cool stars,[259]in accordance with theory. They are especially mentioned as being strong in the spectrum[260]of61Cygni, as might be expected for a dwarf star. Theseries is also traced in the solar spectrum, but is too far in the ultra-violet to be studied effectively in the stars.
The series lines[261]of Al+ and Al++, although they might be expected in thestars, apparently have not yet been traced in stellar spectra.
SILICON (14)
Four stages of the silicon atom are observed in stellar spectra. The line at 3905 is found in the coolest stars, has an observedmaximum[262]at,and disappears at about.This line is regarded by A. Fowler[263]as the ultimate line of the neutral atom, and on this basis an ionization potential of 10.6 volts was assigned to silicon. In view of the fact that the line appears to have a maximum within the stellar sequence, and is of temperature class II, according to King,[264]while the true ultimate line[265]of silicon is at 2881, it seems possible that 3905 is actually a subordinate line.
IONIZED SILICON
Ionized silicon is represented by the lines 4128, 4131, which appear at,attain maximum[266]at,and disappear at.These lines are of especial interest, as they form the characteristic feature of the “silicon stars” which occur in the earlyclasses. The silicon stars are specially discussed[267]inChapter XII.
DOUBLY IONIZED SILICON
The lines associated with the atom Si++ which appear in stellar spectra are the three at the wave-lengths 4552, 4568, 4574. These lines are first seen at,have a maximum[268]betweenand,and disappear at.Fowler[269]regards these lines as constituting a principal triplet; it might be expected, however, that principal lines would show a more persistent maximum.
TRIPLY IONIZED SILICON
The atom of silicon which has lost three electrons is the most highly ionized atom of which we have certain evidence in stellar spectra. The lines at 4089, 4096, and 4116 are strong[270]among the coolerstars, and are last seen at Class.The hotterstars, such as H.D. 165052, do not display the lines of Si+++, and probably the intensity of the lines has fallen, owing to the temperature, which is above that required for the maximum of these lines.
SULPHUR (16)
The spectrum of neutral sulphur which has hitherto been analyzed is chiefly in the far ultra-violet,[271]and is therefore not traceable in the sun or stars.
Two sets of sulphur lines, differing in astrophysical behavior, were noted by Lockyer[272]at 4163, 4174, 4815, and at 4253, 4285, 4295. These lines have been attributed by the writer,[273]and by Fowler and Milne,[274]to S+ and S++ respectively. The S+ lines appear to be in pairs, and the S++ lines suggest a triplet, although one of the three lines is extremely faint in stellar spectra, and it would be expected that the once and twice ionized spectra of sulphur would display even and odd multiplicities respectively. The two series have maxima atand at,but the stellar intensities of the lines are small. An amplification of our knowledge of stellar sulphur is greatly to be desired.
POTASSIUM (19)
The ultimate lines[275]of potassium ()are at 7664 and 7699, and have been traced in the solar spectrum, although they are very faint. They appear to be absent from the flash spectrum.[276]Russell[277]expresses the opinion that they persist, with rising temperature, as far asin the stellar sequence.
CALCIUM (20)
The element calcium is extensively represented in stellar spectra. The ultimate line of the neutral atom is at 4227 ()and appears at.The line increases in strength in all cooler stars, in accordance with theory, and has a distinct variation withabsolute magnitude. The,andmultiplets[278]are satisfactorily identified in the solar spectrum and can be traced with certainty in the spectra of stars cooler than.The,,,andlines appear to be present with the appropriate intensities in the sun, but are too faint to be seen with small dispersion. Thus all the classified lines of calcium which are strong in laboratory spectra have been traced in the spectra of the sun and stars.
IONIZED CALCIUM
Theandlines of ionized calcium are seen throughout the stellar sequence, and reach a maximum within thetype,[279]where their intensity is greater than that attained by any other line in any class. They vary with absolute magnitude.[280]In the sun the lines are doubly reversed, and they are probably singly reversed[281]in61Cygni.
Stationary calcium lines have long been known to occur in the spectra of certain spectroscopic binaries, having first been noticed by Hartmann[282]forOrionis. Various “calcium cloud” hypotheses have been advanced to account for the phenomenon. It appears, from several considerations, notably the apparent small oscillation of the calcium lines with the same period as the star, that there is some physical connection between the two. Lee[283]discussed the idea that the system of9Camelopardalis was surrounded by a cloud of calcium vapor, which, as he showed, could be made to account for the behavior of the lines of ionized calcium. The same idea was discussed by J. S. Plaskett, who suggested that we might “assume that the absorbing material is near to or envelopes the stars, which is probable from its wide distribution, and in this form it combines the two original hypotheses of interstellar and surrounding clouds.”[284]Thelines of sodium[285]and possibly the hydrogen lines[286]have been added to the list of stationary lines, and Plaskett[287]has suggested that the ultimate lines of the ionized atoms of strontium and barium should also show the effect, which has not yet, however, been observed.
SCANDIUM (21)
The element scandium[288]is represented in the solar spectrum by faint lines corresponding to the multiplets,,.The multipletmay possibly be present, but the lines are very weak. The element is not recorded in the spectra of stars; most of the lines are unsuitably placed in the green.
IONIZED SCANDIUM
Six multiplets of ionized scandium, out of the eight tabulated by Meggers, Kiess, and Walters[289]appear in the solar spectrum, and all the corresponding lines have been traced in Rowland’s tables. The intensity of two of the lines is great enough for their behavior to be traced through the stellar sequence, and they are greatly enhanced in the spectra of the c-stars. The ultimate lines are near 3600, but in the solar spectrum they are less powerful than the lines near 3500.
Table XIon page 73 contains, in successive columns, the series relations, the wave-length as determined in the laboratory, the intensity, the temperature class, and the attribution, solar intensity, and wave-length given by Rowland, for the six multiplets which lie within the observed range of the solar spectrum. Ultimate lines are designated by an asterisk.
TITANIUM (22)
The spectrum of titanium is so rich in lines, and is so largely represented in stellar spectra, that a tabulation would occupy an undue amount of space.