Earth’s Home Galaxy
Although recognized since antiquity, philosophers and scientists could only guess at what it represented until fairly recently. (See “How the Milky Way Galaxy got its name, Astronomy, December 2015, p. 33). With the invention of the telescope, it became clear that the Milky Way was the collective glow of stars too faint to be seen by the naked eye. More than a century later, English astronomer, Thomas Wright, suggested that this glowing band was precisely what one would expect to see if the Sun were embedded in a flat disk of stars.
We now know that the Milky Way is the primary structure of our galaxy see edgewise. Additional detail and especially the physical scale of the galaxy took another two centuries to work out. The process continues today as astronomers wrestle with conflicting evidence and make new discoveries. Much like mapping a fogbound city from a single intersection, scientists must decipher the galaxy’s structure while viewing it from inside a disk where dust clouds dim and block starlight.
The true scale of the Milky Way Galaxy—and indeed, the universe as a whole—became dramatically clearer in the 1920s. That’s when a new generation of large telescopes coupled with photography revealed that “spiral nebulae” were actually entire galaxies like our own—“island universes” in the evocative parlance of the time. Surveys showed that most disk-shaped galaxies possessed winding spiral arms where young stars, gas, and dust were concentrated. Astronomers assumed our galaxy was a spiral too. In the 1950sd, radio telescopes produced the first crude maps of the Milky Way’s spiral arms by tracking how gas clouds moved around the galaxy.
Fast Fact: The Sun’s distanced from the center of the Milky Way is 27,200 ly.
 From Astronomy (December 2015).