Final answer:
Astronomers progressed in understanding the Milky Way's structure following the discovery of the 21-cm line from hydrogen, which enables observations through interstellar dust. Edwin Hubble's observations of galaxies furthered this understanding, and recent surveys have provided more data on the structure of galaxies.
Step-by-step explanation:
Astronomers were able to make tremendous progress in mapping the spiral structure of the Milky Way after the discovery of the 21-cm line that comes from cool hydrogen. The significance of this discovery lies in the ability of the 21-cm radio waves to penetrate the obscuring effect of interstellar dust, which prevents us from seeing stars at large distances in the disk at visible wavelengths. As a result, astronomers have been able to detect hydrogen atoms throughout the Galaxy, allowing them to map its spiral structure despite being inside it, akin to being located in the outskirts of New York City trying to map the city without an overhead view.
In 1785, William Herschel's star counting method using his large reflecting telescope laid the foundational work for understanding the Milky Way's structure, revealing a flattened system encircling the sky. Edwin Hubble's later work on galaxies in the 20th century contributed crucial findings for understanding the evolution of the universe. Recently, surveys of infrared emission from stars and data from the Hubble Space Telescope have provided additional clarity on our Galaxy's structure and the structure of distant galaxies.