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And is also a question of the topology it seems. Depending on the way our universe is 'folded', open or closed. You can close it differently, and in some circumstances the light we see as from 'two objects' coming from opposite sides may be one objects light reflected two ways (think inside a football). Very confusing
In that piccy of the milky way you can see there are some very bright bits, but also some very dark bits. The bright bits are concentrations of stars that we can see (which is why they're bright) but the dark bits doesn't mean that there aren't any stars in that direction. Those dark patches are where vast clouds of dust block the light from the stars in that direction so while we can be pretty sure that there are stars behind the dust we can't actually see and count them (although looking at these areas in longer wavelengths does allow some degree of penetration through the clouds, just like radar in an aircraft can still see the ground even when it's cloudy). Longer wavelengths though, while penetrating the dust, give a lower resolution so although we can see that there are stars behind the dust the pictures may not be sharp enough to count individual stars, and although these longer wavelengths penetrate a lot of the dust they can't penetrate all of it.There's also an issue with separating closely aligned stars, which may be many light years apart, but just happen to be in a direct line from our direction, which just compounds the issues with using longer observational wavelengths.Surprisingly enough, it's not just the number of stars in our galaxy that's uncertain; we don't even know for sure how many globular clusters our galaxy has, and these are pretty mucking big collections of stars. The same reason applies though; although we're pretty sure that they're there (because it would be a bit strange if there weren't any in the particular directions that we can't see), we just can't actually see them through the dust and the masses of stars that are in front of them.The number of stars in our galaxy is estimated by a combination of what we can actually see and what we can see in other galaxies. Although we can't see through our galactic plane (which is what the milky way is) because of the dust, we can look at other galaxies from different angles (Andromeda is a good example) where the dust doesn't obscure our view and where we can see the entire galaxy. What we do then, is to count the number of stars in different types of regions i.e. the arms, the interstitial spaces between the arms, the core, the globular clusters etc. of other similar galaxies, estimate how many stars there is in that galaxy, and then scale the number by the comparative size difference between the observed galaxy and our own.The final answer is uncertain because although we think our galaxy is very similar to a number of other galaxies, Andromeda for example, and we think we know how much bigger or smaller it is compared to others, we can't be absolutely sure, neither in size nor in the overall density of stars; some galaxies are relatively 'fat' and dense with stars whilst other are relatively 'skinny' and sparsely populated, so until we develop FTL travel and take a look at it from 'outside' we'll never know for certain.