Wide-field images obtained with the 3.6 meter Canada-France-Hawaii Telescope are used to investigate the spatial distribution and photometric properties of the brightest stars in the disk of M81 ( NGC 3031 ) .
With the exception of the central \sim 2 kpc of the galaxy and gaps between CCDs , the survey is spatially complete for stars with i ^ { \prime } < 24 and major axis distances of 18 kpc .
A more modest near-infrared survey detects stars with K < 20 over roughly one third of the disk .
Bright main sequence ( MS ) stars and RSGs are traced out to galactocentric distances of at least 18 kpc .
The color of the red supergiant ( RSG ) locus suggests that Z = 0.008 when R _ { GC } > 6 kpc , and such a radially uniform RSG metallicity is consistent with [ O/H ] measurements from HII regions .
The density of bright MS stars and RSGs drops when R _ { GC } < 4 kpc , suggesting that star formation in the inner disk was curtailed within the past \sim 100 Myr .
The spatial distribution of bright MS stars tracks emission at far-ultraviolet , mid- and far-infrared wavelengths , although tidal features contain bright MS stars but have little or no infrared flux .
The specific frequency of bright MS stars and RSGs , normalized to K - band integrated brightness , increases with radius , indicating that during the past \sim 30 Myr the specific star formation rate ( SSFR ) has increased with increasing radius .
Still , the SSFR of the M81 disk at intermediate radii is consistent with that expected for an isolated galaxy as massive as M81 , indicating that the star formation rate in the disk of M81 has not been markedly elevated during the past few tens of millions of years .
The stellar content of the M81 disk undergoes a distinct change near R _ { GC } \sim 14 kpc ; the K - band light profile , which is dominated by old and intermediate age stars , breaks downward at this radius , whereas the density profile of young stars flattens , but does not break downwards .
Thus , the luminosity-weighted mean age decreases with increasing radius in the outer regions of the M81 disk .