Maximum distance at which the star would still be visible using the same

[Chris X]

telescope with different data? A main sequence star at a distance of 20 pc is barely visible through a certain telescope. The star subsequently ascends the giant branch, during which time its temperature drops by a factor of 3 and its radius increases 100-fold. what is the new maximum distance at which the star would still be visible using the same telescope?
Does it have anything to do with angular resolution or diffraction-limited resolution? I don't know which equation I should use...