Certain issues that make it difficult for us to know precisely how many stars are in our galaxy also make it challenging to untangle some of its other mysteries and even resolve some enigmas of the Universe.
Our Milky Way Galaxy is currently thought to contain 100 to 400 billion stars. This is a vast spread of estimates. About 73% of suns are very faint red dwarfs, and their masses start from about 0.08 times the mass of the Sun. It's difficult to spot these and other tiny types of stars like white dwarfs and brown dwarfs and count them from considerable distances. The Milky Way is 100 000 light-years across. Many stars are part of multi-star systems that are very close together, and from a significant distance, it's difficult to tell if a dot in the sky is a single star or a binary or more.
Besides that, our galaxy contains many gas clouds and many denser regions, like clusters of stars, and many are pretty big. The central bulge of the galaxy is the most prominent such feature. We are located in a spiral disk about 1000 light-years thick, and when we look edge on from where we are, like in the image above of another galaxy, because of all these other features that block our view, we cannot see what is behind. Astronomers try to estimate the number of suns in our galaxy by counting their number near us and extending the figure that we get from our region to the galaxy as a whole. Still, our location in the spiral disk is not that typical. The density of stars in the spiral varies.
The same issues that make it difficult to estimate the number of stars also make it hard to know our galaxy's exact structure and shape. It took a lot of effort to discover that we have a bar in the middle of it and that our galaxy is a barred spiral. We also eventually figured out that we are in a mini arm called the Orion Spur, smaller than the two primary arms, the Perseus and Scutum-Centaurus. Even more recently, we determined that our galaxy's spiral disk is not flat but slightly twisted. These features would be easily visible if we could view our galaxy from a distance of millions of light-years.
Furthermore, our location within also makes it hard to know what is outside of our galaxy in the direction of the bulge and the plane of the spiral disk. Our Local Group of galaxies, including the Milky Way, Andromeda, and their satellites, are moving towards the point in the sky obscured by the central bulge. Gravitational anomaly millions of times more massive than our galaxy was identified there and named the Great Attractor. Because it's so difficult to survey this part of the sky, it was discovered recently that this is not even the whole story. There is another even bigger overdensity of matter even further in this direction called Shapley Attractor, to which the Great Attractor and we are moving.
The question was: For a long time,e the Milky Way Galaxy had 100 billion stars, but the figure was changed to 400 billion stars. Why was the number of stars revised? Is there a rationale for the change?
Our galaxy has two primary arms, the Perseus and Scutum-Centaurus, but we are in a smaller Orion Spur arm.
It has recently been discovered that the disk of our galaxy is not flat but slightly twisted.
Our galaxy is being pulled by the Great Attractor and an even more massive Shapley Attractor behind it that happen to be in the part of the sky obscured by the bright bulge and plane of our galaxy.