Lets consider the Earth first. Ancient Babylonians knew the Earth was a sphere 5,000 years before the first satellite pictures of the Earth from space. You might attempt the same kind of puzzle. The next step was to examine the position of very luminous objects called globular clusters. They mapped the position of the clusters and found that most of the clusters are located to one side of the Sun indicating that we are not at the center of the disk.
White light is a combination of all the different colors.
A prism can separate white light into a rainbow of colors. In the same way, when we see the light from stars of all different colors the combination of their light appears white. The Andromeda galaxy is the closest large galaxy to the Milky Way. It is very similar in size to the Milky Way. Thus, the Andromeda galaxy contains about 10 billion stars.