Determining the structure of the Milky Way as a spiral galaxy was not achieved through a single photograph or a "selfie" from the outside. Instead, it involved a combination of observations, measurements, and the development of models over many years. Here are the key methods used:
Observations of Other Spiral Galaxies: Astronomers studied numerous other galaxies, both nearby and distant, to understand their structures and characteristics. By examining their appearance, such as their arms and disk-like shapes, astronomers identified common features shared by many spiral galaxies. These observations provided crucial evidence that the Milky Way might also be a spiral galaxy.
Stellar Observations: Astronomers observed and cataloged the positions, motions, and properties of individual stars within the Milky Way. They measured the distances to these stars using various techniques, such as parallax and standard candles (stars with known luminosities). By mapping out the distribution and motions of stars, astronomers inferred the overall structure of the Milky Way.
Mapping of Interstellar Dust: The distribution of interstellar dust within the Milky Way can be observed through different wavelengths of light, including infrared and radio waves. By studying this dust distribution, scientists could trace the spiral arms of the galaxy. Dust tends to concentrate in certain regions, outlining the spiral pattern as it follows the density waves moving through the galaxy.
Radio Astronomy and Hydrogen Mapping: Radio telescopes allowed astronomers to map the distribution of hydrogen gas in the Milky Way. The hydrogen gas emits radio waves at a specific frequency, which can be observed and mapped. By studying the distribution and motions of the hydrogen gas, scientists were able to identify the spiral arms and overall structure of the Milky Way.
Stellar Kinematics: Observations of the motions of stars in the Milky Way helped astronomers deduce its structure. By measuring the velocities and trajectories of stars, they could identify patterns and motions indicative of a spiral structure, including the rotation of the galaxy.
By combining all these observations and data, astronomers gradually built a comprehensive understanding of the Milky Way's structure as a spiral galaxy. While we still cannot obtain a direct "selfie" of the entire galaxy from the outside, these observations and analyses have provided us with a detailed picture of our galactic home.