Why is it difficult to specify the dimensions of the disk and halo of the Milky Way?

Answer 1

Imagine that you were inside a large office building. Quick, what does it look like from the outside? That's basically why. While nearly all the individual stars we can see are in the Milky Way galaxy, we can only see a small fraction of them and being inside it, with dust clouds getting in the way (we can't even see, visually, the center of the galaxy, because of all the dust), it's kind of hard to tell what the overall shape is.

Now, to put things into perspective: when we say that we're "not certain" of the shape, what we really mean is fine detail. Just as if you were in the central courtyard of the Pentagon, you could be pretty sure that the building is pentagonal, the chances of the Milky Way NOT being a spiral galaxy are so close to zero as to be indistinguishable from it in any practical sense, and we're pretty sure it's a barred spiral. Exactly how many arms and how many times they wrap around ... that we can't really tell, but the gross structure is pretty nearly a dead lock.

Answer 2

Specifying the dimensions of the disk and halo of the Milky Way is difficult because we are inside the galaxy and cannot observe its structure directly. Additionally, the galaxy is filled with dust that blocks our view in certain wavelengths of light. Different observational techniques and models are used to estimate the size and shape of these components.

Answer 3

There currently is not any capability to see the entire galaxy at once. Because of this, there is no way of getting exact measurements, only approximations.

Answer 4

Because they lack sharp boundaries. They are not well defined objects.