APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

[guenthert]

[..]
You are very wrong indeed if you think it is impossible for the Hubble Space Telescope to resolve individual stars in the Andromeda Galaxy.
[..]

Yes, indeed. Sleeping over it helped, but thanks for setting me straight here. Hopefully others might find this educational as well.

My mistake was that I failed to put into consideration that the density of stars in the galaxies is very in-homogeneous, with most stars in the 'bulge'. In the outskirts (spiral arms) there will be region, where average distance between stars will be e.g. 4ly or larger. At an apparent size of 30" and an estimated diameter of 40,000ly this would be about 0.1" between stars in such a lower density region. The 2.5m telescope at Mount Wilson has a resolving power of 0.05", so should be able to resolve those (barely, but there will be regions with even lower density).

[Chris Peterson]

[..]
You are very wrong indeed if you think it is impossible for the Hubble Space Telescope to resolve individual stars in the Andromeda Galaxy.
[..]

Yes, indeed. Sleeping over it helped, but thanks for setting me straight here. Hopefully others might find this educational as well.

My mistake was that I failed to put into consideration that the density of stars in the galaxies is very in-homogeneous, with most stars in the 'bulge'. In the outskirts (spiral arms) there will be region, where average distance between stars will be e.g. 4ly or larger. At an apparent size of 30" and an estimated diameter of 40,000ly this would be about 0.1" between stars in such a lower density region. The 2.5m telescope at Mount Wilson has a resolving power of 0.05", so should be able to resolve those (barely, but there will be regions with even lower density).

You can also do photometry on a star that isn't optically separated from a few other stars.