The Milky Way is a luminous band spanning the starry sky. It intersects with the Celestial Equator in the constellation of Aquila, in the northern celestial hemisphere.

On the celestial sphere, it traces a belt of varying width known as the Milky Way band, ranging from as wide as 30 degrees to as narrow as 4 to 5 degrees, with an average of about 20 degrees. However, this is only a part of the Milky Way galaxy.

The Milky Way, visible to all, is the most splendid sight in the night sky. The European Space Agency's Gaia satellite has recently depicted an image of our line of sight within the Milky Way galaxy based on observations of over 1.8 billion stars. While the disk and central region of the galaxy are easily recognizable, what the Milky Way looks like from the outside remains a mystery. Let's explore the Milky Way, exploring its past, present, and future research.

Thousands of years ago, various speculations about the Milky Way abounded. In ancient Greek mythology, it was believed to be formed by the milk of the goddess Hera. It wasn't until Galileo first pointed his telescope toward the Milky Way that it was discovered to be composed of countless stars.

Yet, despite being within the Milky Way galaxy, humanity still lacks a clear understanding of its structure and the relationship between the galaxy and the Sun. British scientist Wright and German philosopher Kant made the first correct inference about the shape of the Milky Way galaxy. Wright believed the Milky Way to be flat, with the Sun situated within it. Building upon Wright's ideas, Kant further speculated:

"The Milky Way is a vast, flat, star-studded celestial system held together by gravity, rotating around some center, akin to the solar system."

The first quantitative estimation of the shape of the Milky Way galaxy was made by the late 18th-century British astronomer William Herschel. Herschel, known for his adeptness, counted stars in the night sky using telescopes he crafted, thereby providing the first observational evidence that the Milky Way galaxy has a flattened shape.

During Herschel's time, astronomers had begun to realize that the Sun was just one ordinary star among countless others in the Milky Way galaxy. However, to some extent, the Sun still held a special status—it was thought to be located at the center of the Milky Way galaxy.

In 1907, American astronomer Harlow Shapley discovered a very bright, particular celestial object within the Milky Way galaxy—a globular cluster of millions of stars. These clusters did not orbit around the Sun. Based on this phenomenon, Shapley correctly inferred that the center of these round cluster systems is the true center of the Milky Way galaxy, not the Sun.

In essence, it was only a little over a century ago that astronomers fully recognized that the Sun is not only an ordinary star within the Milky Way galaxy in terms of its physical properties but also holds no particular position in terms of its location.

Today, astronomers have a rough consensus on the structure of the Milky Way galaxy: it appears as a disc overall.

The bright part of this disc, called the Galactic disc, has a diameter of about 100,000 light-years and a thickness of approximately 10,000 light-years. The disc hosts around 300 billion stars, with the Sun situated about 26,000 light-years away from the center of the Milky Way galaxy.

The Sun is located relatively towards the outskirts of the Galactic disc. If the Milky Way galaxy were likened to a city, the Sun would be positioned in the suburbs.

Viewed along the plane of the Galactic disc, the disc appears as a band, while the central bulge exhibits a certain degree of protrusion. This further illustrates the difference in the flat structure of the bulge and the disc, which is more spherical in distribution.

Further observations are eagerly awaited to expand our understanding of the Milky Way galaxy.