SPECTROSCOPIC OPTICAL OBSERVATIONS OF THE CENTRAL STAR

Abstract

Spectroscopic observations of the two stars near the center of the Crab nebula are not particularly easy. The stars are not very bright, but in the age of the image-tube spectrographs this is no longer important. They have a background of bright nebulosity. The difficulty caused by the background has been somewhat exaggerated; the stars are actually in an area of relatively low-surface brightness. Under good seeing conditions, when use of a narrow slit permits minimizing the contribution of the background, quite satisfactory spectrograms can be obtained with large telescopes. This is best shown by the fact that the absorption lines in the spectrum of the north-following star can easily be seen. There is a chance that the thin wisp (Scargle 1969a,fo, 1970) might at times be close enough to the star to contribute to the background brightness. Faint line emission in the position of the star cannot be avoided; a faint filament seems to cover the star. Interspaces between faint emission lines, particularly the double lines of an expanding nebula, can easily be misinterpreted as faint absorption lines. Great care must be taken to avoid such misinterpretations. This has recently been emphasized by Lynds (1969). The first spectrum of each star was obtained with low dispersion at the Cassegrain focus of the 100-inch telescope (Minkowski 1942). The spectrum of the north-following star was easily recognizable as F type, but the south-preceding star showed a continuous spectrum without recognizable absorption lines. Baade (1942) and I were unwilling to accept this as conclusive proof that the star is the central star for two reasons.