We present V and I photometry of the resolved stars in the cometary dwarf irregular galaxy NGC 2366 , using Wide Field Planetary Camera 2 images obtained with the Hubble Space Telescope .
The resulting color-magnitude diagram reaches down to I \sim 26.0 mag .
It reveals not only a young population of blue main-sequence stars ( age \lesssim 30 Myr ) but also an intermediate-age population of blue and red supergiants ( 20 Myr \lesssim age \lesssim 100 Myr ) , and an older evolved populations of asymptotic giant branch ( AGB ) stars ( age \gtrsim 100 Myr ) and red giant branch ( RGB ) stars ( age \gtrsim 1 Gyr ) .
The measured magnitude I = 23.65 \pm 0.10 mag of the RGB tip results in a distance modulus m - M = 27.67 \pm 0.10 , which corresponds to a distance of 3.42 \pm 0.15 Mpc , in agreement with previous distance determinations .
The youngest stars are associated with the bright complex of H ii regions NGC 2363 \equiv Mrk 71 in the southwest extremity of the galaxy .
As a consequence of the diffusion and relaxation processes of stellar ensembles , the older the stellar population is , the smoother and more extended is its spatial distribution .
An underlying population of older stars is found throughout the body of NGC 2366 .
The most notable feature of this older population is the presence of numerous relatively bright AGB stars .
The number ratio of AGB to RGB stars and the average absolute brightness of AGB stars in NGC 2366 are appreciably higher than in the BCD VII Zw 403 , indicating a younger age of the AGB stars in NGC 2366 .
In addition to the present burst of age \lesssim 100 Myr , there has been strong star formation activity in the past of NGC 2366 , from \sim 100 Myr to \lesssim 3 Gyr ago .