Star Counts in the Andromeda Nebula.

Counts of stars from magnitude 15.0 to magnitude 18.5 in and around the Andromeda nebula have been made on blue-sensitive plates taken with the 24-36-inch Schmidt telescope of the Case School of Applied Science. In order to calibrate the counts, magnitudes of a sequence of stars near M3 1 from 14.9 mag. to 18.3 mag. were determined by comparisons with Selected Area 20 and the North Polar Sequence. The counts, corrected for the foreground stars, show a total of 2564 stars belonging to M3 1 between the limits of absolute magnitude -3.8 and -6.3. Possible errors in the counts arising from the limited re- solving power of the plates used, the variable nebulous background of M31, and the distance correction on the Schmidt plates were investigated and found to be negligibly small. The apparent shape of M31 as outlined by the counts is roughly elliptical and agrees reasonably well with isophotal contours of the spiral. The observed luminosity distribution ofstars in M31~ is compared with previously published data for M33, NGC 6822, the Magellanic Clouds, and the Milky Way. The shape of the luminosity curi~e of the main body of M3 1 (diameter 6000 parsecs) is very similar to the corresponding curve for the solar neigh- borhood within the observed range of absolute magnitudes. The border region of M31 (6,000-11,000 parsecs in diameter) reveals a statistical absence of stars brighter than -5.3 in absolute magnitude. The thickness of M3 1 as shown by the high-luminosity stars is estimated to be of the order of 200 parsecs, a value comparab1~ with the thickness of the Milky Way as estimated by Oort. The present results indicate an absence of highly luminous stars in the outermost envelope of M31. This fact suggests that the outer, shell of the Andromeda nebula may be composed of material similar to that found by Baade in the nucleus of M31 (i.e., lacking in highly luminous 0- and B-type stars)