Question

How do we know our galaxy is disk shaped?

Answer 1

From massive data on luminosity and distance of stars far and beyond, the separation of Milky Way (MW) from neighbors is mapped. The shape of MW is a by product in the mapping.

Explanation:

A MON AVIS: It appears that, in gravitational systems, there is a center. The constituents of the system are held by the central force, in orbits.

This is easy to realize in a star system, like the Sun-system. It can be extended to systems of stars, up to nearer galaxies and farther galaxies and star clusters. The ultimate center is the center of our observable universe.

Also, the density of the space objects is high in the neighborhood of the equatorial plane of the system. This the a primary reason for the mapping of the systems within a frame of a disc..