The solar system is on the disk of the Milky Way, and we can only see the Milky Way on the “side”. If you want to get the “front” image of the Milky Way, you also need an important piece of information: distance. In other words, the side observation only gives us the positions X and Y of the celestial bodies, and we also need the distance Z to construct a three-dimensional picture of the Milky Way.
In addition to the analogy with the extragalactic galaxy, we have gained an overall understanding of the Milky Way, thanks more to the observations of “atomic hydrogen” and “molecular hydrogen”.
According to the introduction of “Weeds” from the answer of Zhihu, even though there are many ways to determine the distance to a star, the starlight cannot be blocked by the interstellar dust, and only a small range of stars around the solar system can be seen. However, there is a kind of wave that can penetrate interstellar dust. It is the light-radio wave emitted by the interstellar medium of the “giant molecular cloud” and “atomic cloud” in the Milky Way.
Most radio telescopes are equipped with a spectrum analyzer, and like all the information that depends on the “spectrum”, it brings us such an important observation—speed. This Velocity is called the “local stationary velocity”, which corresponds to the distance between celestial bodies-the gas rotates around the center of the Milky Way, and in the direction of the line of sight, different distances will naturally show different speed characteristics.
At the earliest, the “atomic hydrogen” survey obtained the distribution map and velocity-position map of the atomic gas of the Milky Way, and made the “rotation curve” of the Milky Way accordingly, showing that the shape of the Milky Way is: a spiral galaxy with a spiral arm structure.
But spiral galaxies can also look different. As a result, people became interested in the number and position of the spiral arms of the Milky Way, and then observed the molecular gas of the Milky Way—different from the atomic gas that is everywhere in the Milky Way, the molecular gas is mainly concentrated on the silver disk.
Through the observation of molecular gas (such as molecular hydrogen, that is, hydrogen gas), people have discovered that the Milky Way has multiple spiral arms, and the center of the galaxy has a “rod”-like structure. After synthesizing all the pictures, an image of the structure of the Milky Way can be drawn.
Next, a larger telescope (Qinghai Delingha 13.7m millimeter wave telescope) observed the far part of the Milky Way, and in 2015 saw the newly extended spiral arms. The solar system is not on the main spiral arm, it is on the local spiral arm between the Perseus spiral arm and the Sagittarius spiral arm. Between the local spiral arm and the sagittarius spiral arm, there is a “thorn”-like structure, like an inclined bridge connecting the two spiral arms.