Stellar Spectra

Spectroscopic Parallaxes.

Since the nature of the spectrum shown by a star is conditioned both by the temperature and pres sure of the reversing layer, a complete classification of spectra should be two-dimensional. Ordinarily we recognize only a linear series—the Draper sequence of types considered above; but by attention to detail we can recognize also a classification trans verse to this. Taking the Draper sequence to correspond roughly to temperature, the transverse classification will correspond to pressure. Since low pressure favours ionization, it will be marked in the spectrum by great strength of the enhanced lines relatively to the arc lines. The giant and dwarf stars of the same spectral type can easily be distinguished from one another by this method; the reversing-layer pressure is low in the giant stars owing to the small value of gravity at the surface.

It was early recognized by E. Hertzsprung that those spectra marked by Miss Maury as having the "c-characteristic" belong exclusively to the giant stars. This characteristic is an unusual fineness of the lines—a feature which also results from low pres sure. More precise criteria were found by Adams and Kohl schutter in 1914; and the method has been developed by Adams into a means, not only of distinguishing giant and dwarf stars, but of determining quantitatively the absolute luminosities of stars. At present the procedure is empirical; the curve connecting absolute magnitude with differential intensity of certain selected pairs of spectral lines is deduced from and tested by stars of known trigonometrical parallax; it is then applied to find the absolute magnitudes and hence the distances of other stars. Paral laxes of some thousands of stars determined in this way have been announced, and are known as spectroscopic parallaxes. The underlying principle of the method may be summarized as fol lows: Stars may differ in mass, and stars of the same mass may differ in density ; but, when the mass and density are given, no other important difference can occur, and the luminosity, surface temperature and pressure in the reversing layer should be uniquely determined; hence the spectrum should be determinate. Con versely, a sufficiently close examination of the spectrum (its posi tion both in the Draper sequence and in the classification trans verse to it) should fix the mass and density of the star, and hence its other physical properties, including luminosity. Ideally the deduction of luminosity might be made theoretically, but in any case the connection between luminosity and spectrum can be observed experimentally and formulated as an empirical law.

Calcium Cloud in Space.

In certain spectroscopic double stars the phenomenon of "fixed calcium lines" is observed. Whereas the other spectral lines shift to and fro as the star ap proaches and recedes in its orbit, the H and K lines of ionized calcium remain stationary. It is clear that somewhere between us and the star's photosphere there must be an absorbing cloud of calcium vapour which does not follow the star in its orbit.

The phenomenon was first pointed out by Hartmann in 1904 for the star 6 Orionis. Later Miss Heger discovered that the D lines of sodium also remain fixed in 6 Orionis. The same behaviour has now been observed in a large number of stars, but no other "fixed" spectral lines have been found. The important question to decide is whether the cloud belongs to the double star or whether it is a continuous cloud filling interstellar space. J. S. Plaskett has shown that the latter alternative is correct ; the motion of the calcium cloud is often different • from that of the centre of mass of the star. After removing the solar motion the velocity of the cloud relative to the mean of the stars is found to be small. Just as there are lines in the solar spectrum which do not share in the sun's rotation and are accordingly to be attributed to absorption during the passage of the light through the earth's atmosphere, so we have fixed lines of calcium and sodium which do not share in the orbital or individual motion of the star and are to be attributed to absorption in an interstellar "atmosphere." The fixed lines only appear in the spectra of the hottest stars, but that is perhaps due to the fact that cooler stars have strong H and K lines of their own, masking the lines of the cloud, or because the cooler stars, being less luminous, are not observed at a sufficient distance to give the cloud absorption a chance. It has been suggested that the presence of the hot star is necessary in order to ionize the calcium vapour and render it capable of absorbing H and K light, so that although the ab sorption is performed by the interstellar cloud only the parts of the cloud stimulated by the star are effective. But this explana tion would not hold good for the D lines of sodium which are absorbed by unionized unexcited atoms. There can be little doubt that the absorption occurs along the whole track of the light through space, and the intensity of the lines should be an indi cation of the length of track, that is of the distance of the star. This appears to be confirmed by the researches of 0. Struve, who has found that the strength of the fixed lines increases steadily with the average distance of the objects observed.

In order to avoid a huge mass of the stellar system inconsistent with the observed velocities of the stars, it must be postulated that this interstellar cloud is of very low density. About one atom per cubic centimetre is the maximum that can be allowed. It is calculated that matter so diffuse as this would take up a high temperature not much lower than the photospheric tem peratures of the stars; although a black body in interstellar space would sink to a temperature of 3° absolute. The density is too small to give any appreciable scattering or absorption of light in space other than the special line-absorption of calcium and sodium light.

See also NOVA; STAR CLUSTER ; ALGOL ; SIRIUS ; STELLAR Evo LUTION ; COSMOGONY ; CONSTELLATION.