NOTES from OBSERVATORIES 353 from the Main Sequence, but Also

NOTES FROM OBSERVATORIES 353

from the main sequence, but also during the process of formation by gravitational contraction, which eventually carries the star to the main sequence. For a star as massive as ζ Persei probably

is, however, the Helmholtz-Kelvin time scale rc is only of the 5 order of 10 years. While a rigorous calculation of tc for large masses has not been carried out, for stellar models with electron

scattering as main source of opacity, χ0 should vary nearly in- versely proportional to the mass. Adopting with Henyey, Le- 6 6 Levier, and Levée the value τ0 =: 3 X 10 years for m = 3 mQ,

it follows that for ζ Persei tc, as given before, is much shorter than the expansion age of the association. We may thus conclude that the hypothesis which makes the formation of ζ Per sei simul- taneous with the beginning of the expansion of the association is not corroborated by the estimate of the lifetime of the cBl stars at high galactic latitude.

1 W. W. Morgan, A. D. Code, and A. E. Whitford, Ap. J. Supplements, 2,41,1955 (No. 14). 2 P. C. Keenan and W. W. Morgan, "Classification of Stellar Spectra," Astrophysics, J. A. Hynek, ed. (New York: McGraw-Hill Book Co., 1951), p. 12. 3 R. E. Wilson, General Catalogue of Stellar Radial Velocities (Wash- ington : Carnegie Institution of Washington Pub. No. 601, 1953). 4J. H. Oort and A. J. J. van Woerkom, B.A.N., 9, 185, 1941 (No. 338). s J. H. Oort, B.A.N., 12,177,1954 (No. 455). 6 L. G. Henyey, R. LeLevier, and R. D. Levée, Pub. AS.P., 67, 154, 1955.

THE SOURCE OF ILLUMINATION OF NGC 1579

George H. Herbig Lick Observatory, University of California

The diffuse nebula NGC 15791 is an irregular, mottled mass of rather bright nebulosity lying in a dark lane, in which are also found a number of nebulous stars. Direct photographs do not show any bright near-by star that can be convincingly identified as the source of the illumination of NGC 1579, although Hubble described two near-by stars (for which he gave types AO and B5, and photovisual magnitudes 12.0 and 12.2) as "involved/'2 Presumably these two are to be identified with the nebulous

stars northeast and north of the nebula, the second of which is HD 279899, Harvard type B3. A slit spectrogram of NGC 1579, taken with the nebular spec- trograph of the Crossley reflector, shows, however, that neither star can be responsible for the illumination of the NGC nebulosity. The nebular spectrum consists of bright hydrogen lines, having a steep Balmer decrement, on a very strong continuous background, and is quite unlike that of an ordinary emission nebula because of the absence of bright lines of [On] and [O in]. On the other hand, a slitless spectrogram of the field shows that neither the stars mentioned by Hubble nor any other bright stars in the vicinity have the strong emission at Ha that is required to explain the bright-line spectrum of NGC 1579 as due to reflection. The slitless plate does, however, reveal what appears to be the source of the illumination of NGC 1579. The nebulosity is di- vided into two unequal portions by a dark channel, and in this lane, only about 5" south of the sharp edge of the brightest part of the nebula, is a faint star of about photographic magnitude 17 that was mentioned by Pease in his description of NGC 1579.3 The slitless spectrogram shows that this star has Ha in emission, with a remarkably high intensity with respect to the continuous spectrum. This star has been given the designation LkHa-101.* A slit spectrogram of this star, covering the photographic region, shows a fairly strong Hß emission line and a weak Ηγ, on a continuum whose intensity falls off quite rapidly to shortward. If this very faint star is responsible for the illumination of NGC 1579, a great violation of Hubble's relation between the apparent magnitude of exciting star and dimensions of nebula4 is indicated. The discrepancy amounts to about 9 magnitudes. The explanation must be that LkHa-101 is very heavily ob- scured by dark material in the dark channel upon which it ap-

*In earlier lists of bright-Ha stars found in NGC 2264 (Ap. /., 119, 483, 1954) and in IC 34β (Pub. AS.P., 66, 19, 1954), the prefix "LHa" was used. Subsequently, it was learned that the same designation is being used by Dr. Karl G. Henize for the emission-line stars discovered in his Lamont-Hussey Observatory survey. Accordingly, to avoid confusion, it has been decided to continue the numbering system of the earlier lists but to change the prefix to "LkHa."

pears to be süperimposed. This view is supported by a comparison of direct photographs exposed in the blue and in the ultraviolet with an infrared exposure (see Plate I) : LkHa-101 brightens by several magnitudes with respect to neighboring stars (themselves probably rather red) in passing from the short- to the long-wavelength photograph. The one available slit spectrogram of LkHa-101 is unwidened, and the continuum falls off so quickly in intensity to shortward that no absorption features can be seen. The energy distribution in the spectrum of the brightest part of the nebulosity is more favorable, and the nebula shows the H and Κ lines in absorption. Their relative strength, together with the weakness or absence of the G band, indicates that LkHa-101—assumed to be the only source of illumination of NGC 1579—must be about spectral type F. In the absence of accurate magnitude and color data, only the roughest estimate can be made of the absolute magnitude of LkHa-101. The dark material in the region is apparently part of the Taurus clouds, for which a true distance modulus of 5 to 7 magnitudes is appropriate. If the Hubble relation is to be obeyed

Plate I Above : A photograph, taken with the Crossley reflector on the Kodak 103a-O emulsion (no filter). The wavelength region covered is about λλ 3300-5000. The dimensions of the area shown are 24' by 24". North is above and east to the left. LkHa-101 is the faint star in the dark lane just below the brightest part of the nebula. Lower left : NGC 1579 photographed in the infrared with a Kodak I-N plate and a Schott RG-8 filter. The wavelength region is about λλ 700O- 8800. Lower right: NGC 1579 in the ultraviolet. The emulsion used was 103a-O behind a Schott UG-1 filter, a combination that responds to the wavelength region between about λ 3300 and λ 3800. The relative expo- sures of the two lower photographs are such that a faint star of color index +0.5 would produce images of about the same density on the two ex- posures. The relative brightness of LkHa-101 in the infrared as compared to the ultraviolet is well shown, as is the strength of the nebulosity in the ultraviolet as opposed to its faintness in the infrared.

Direct Photographs of NGC 1579 (See facing page for description)

by LkHa-101, then its apparent photographic magnitude freed from local absorption would be about 8, which corresponds to

MVg =: -[-1 to _[_3. This is roughly the absolute magnitude of an early F-type dwarf, a result that is in approximate accord with the spectral type. Although no variation in the light of LkHa-101 has been ob- served, its absolute magnitude, its association with a bright reflection nebula, the steep decrement in its hydrogen emission lines, the presence of H and Κ in absorption, and the lack of other strong emission lines are all reminiscent of R Monocerotis, the nucleus of Hubble's Variable Nebula, at some phases of the variation of that remarkable object.

ι α = 4h23m7, δ = +35° 04' (1900). 2 Ε. Hubble, Αρ. /., 56, 162, 1922. 3 F. G. Pease, Αρ. /., 46, 24, 1917. 4 S. Cederblad, Lund M edd., Ser. II, No. 119, 1946, figure 6.

SPECTROSCOPIC CLASSIFICATION OF RED STARS

Paul W. Merrill Mount Wilson and Palomar Observatories Carnegie Institution of Washington California Institute of Τechnology

The cooler red giant stars fall into three main groups called types Μ, N, and S, whose spectra differ markedly. The ranges of temperature covered by the three types are so nearly the same that, even with allowance for possible differences in density, the characteristic spectroscopic differences cannot be ascribed to differences in excitation of gases in the reversing layers. The pres- ent consensus, therefore, is that the atmospheres of Μ-, N-, and S-type stars differ in actual chemical composition.* Hence for a descriptive classification of the spectra of red stars, one or more chemical parameters will be needed. For many years it has been believed that the differentiation between Ν-types

* We should, however, not assume that the whole interiors of the stars necessarily differ in the same way ; this may or may not be true.