We have models for how much light a star of a given mass, age, chemical composition etc. puts out as a function of wavelength. We can then take mixtures of stars and predict the total light output of a stellar population in different bandpasses. So from observations of a galaxy in these different bandpasses, we can use these models to determine things like the age of the stars in the galaxy—from how old the stars in the best fit stellar population are—as well as the mass (or number) of stars—from how many are needed to match the observed brightness of the galaxy.

That makes sense. Thanks!

no, due to gravitational lensing, you can see stars. For a straightforward break down, see Dr Becky, https://www.youtube.com/watch?v=VChgsXbIgdw

Oh grief, I've tried getting into her channel a few times because the content and analysis is fantastic, but the 'effervescent' delivery style is excruciating.

What’s your email address? I’ll send you my toenail moon images.

This galaxy is not being gravitationally lensed though.