Donna Weistrop, a student of Maarten Schmidt’s, has found evidence for an unexpectedly high space density of very faint M-dwarfs. This was confirmed by Murray and Sanduleak. The space density inferred from these investigations is comparable to, or possibly higher than, the local gas density. Because there is evidence that the stars concerned are young this appears to present a problem.

As the investigation by Murray and Sanduleak is the simpler and more direct of the two I confine my discussion to this. The authors determined proper motions for 21 M dwarfs down to 17m found in an objective-prism survey at the Warner and Swasey Observatory in regions close to the North Galactic Pole. The distance of the stars can be inferred from the reflection of the solar motion. Assuming the ‘basic’ solar motion they found and a space density of 0.23 pc-3 for these stars, corresponding with a mass density of about 0.04 M⊚ pc-3. This is exceedingly high, in view of the fact that the average gas density near z = 0 is only 0.03 M⊚ pc-3. The problem is aggravated by the fact that the stars have low velocities, the dispersion in velocity in one coordinate coming out ± 10kms-1. This is considerably lower than the velocity dispersion of A-type stars, and shows that the stars are young, perhaps about 108 yr. How is it then that the gas has not been completely used up long ago?