How Does the Milky Way Look?

Starting from 1610, when the renowned Renaissance polymath Galileo Galilei observed the night sky using his self-made telescope, astronomers gradually came to understand that our Solar System is just a small part of a vast assemblage of stars now known as the Milky Way Galaxy. By the 20th century, astronomers had gained a good understanding of its size and structure, which consists of a central bulge surrounded by an extended disk with spiral arms. However, determining the true morphology of the Milky Way has remained a challenge for astronomers.

Due to our position within the Milky Way’s disk, we are unable to see through the central region and observe what lies on the other side. Nonetheless, astronomers have been employing various methods to approximate what a “bird’s-eye” view of the galaxy would look like. For instance, a team of researchers from the Chinese Academy of Sciences (CAS) recently utilized the precise positions of very young objects within our galaxy to measure its morphology for the first time. Their findings revealed a multiple-arm structure with two symmetrical arms in the inner region and numerous irregular arms in the outer region.

The team, led by Xu Ye, a radio astronomer affiliated with the Purple Mountain Observatory in Nanjing and the School of Astronomy and Space Science (SASS) at the University of Science and Technology of China (UCAS), collaborated with multiple astronomers and astrophysicists from the Purple Mountain Observatory, SASS/UCAS, and the National Astronomical Observatories of China in Beijing. Their research, titled “What Does the Milky Way Look Like?,” was recently published in The Astrophysical Journal.

Our understanding of the Milky Way’s structure owes much to the pioneering work of William W. Morgan, who played a key role in the classification of stars and galaxies. During the 1950s, Morgan and his colleagues used spectroscopic parallax measurements of high-mass stars to map out the Milky Way and discovered three short segments of spiral arms. The large-scale structure was subsequently mapped based on distances derived from neutral hydrogen (HI) clouds using kinematic methods, revealing that these spiral arms extended nearly across the entire Galactic disk.

However, the distances of neutral hydrogen clouds were found to have significant uncertainties due to noncircular motions, rendering the results of these early studies unreliable. Photometric methods offer greater accuracy than kinematic methods but are limited to objects within distances of approximately 2 kiloparsecs (~6500 light years) – roughly 6.5% of the Milky Way’s diameter. As a result, kinematic methods continue to be widely used for studying the entire Milky Way. This led to modern maps that included four major arms: the Sagittarius, Orion, Perseus, and Cygnus Arms (from closest to farthest from the Galactic Center).

Despite improvements in kinematic and optical methods, determining the structure of the Milky Way remains a significant challenge for astronomers. Consequently, debates persist regarding fundamental questions such as the number of arms, the existence of certain spiral arms, and the overall size of the Milky Way (estimates subject to an uncertainty of ±3,590 light-years). According to Xu Ye, the best way to resolve these debates is by employing young stars as “spiral tracers,” which can provide accurate distance measurements:

“Directly mapping the spiral structure of the Milky Way has proven to be a challenging endeavor. Star-forming regions and young stars can trace the spiral arms of galaxies. Knowing the distances to star-forming regions or young stars, one can locate them in three dimensions and construct a ‘plan view’ – a view from above the disk – of the Milky Way. Astronomers can make sense of the Milky Way only if they know the distances to young objects, so methods of distance measurement are important.”

For galaxies like the Milky Way, spiral arms are composed of two main components: a spiral pattern indicated by the distribution of the older stellar population, and regions of diffuse or dense interstellar gas and young objects. These young objects include high-mass star-forming regions (HMSFRs), massive O-B stars, regions of interstellar hydrogen gas (Hii), young open clusters (YOCs), and others. Astronomers have made substantial progress in measuring the structure of the Milky Way using young objects since the turn of the century.

The emergence of Very-Long Baseline Interferometry (VLBI) has allowed for precise distance measurements (via trigonometric parallax) for star-forming regions throughout the galaxy, leading to more accurate distance and velocity measurements. The ESA’s Gaia Observatory has also provided precise data on the proper motion and distance of many young stars, helping astronomers determine the galaxy’s spiral structure in the vicinity of the Solar System. Combining these measurements has led to some interesting conclusions.

Using precise locations of very young objects provided by the BeSSeL and VERA projects (VLBI) and by ESO’s Gaia satellite, for the first time, astronomers have revealed that our galaxy has a multiple-arm morphology that consists of two-arm symmetry in the inner parts and extends to the outer parts, where there are several long and irregular arms.

According to the team’s results, the structure of the Milky Way consists of the Perseus and Norma arms in the inner region and the Centaurus, Sagittarius, Carina, Outer, and Local Arms in the outer region. These findings are supported by what has been observed with external spiral galaxies, which fall into three largely distinct categories based on their morphology: grand-design galaxies, multiple-arm, and flocculent galaxies.

Whereas grand-design galaxies have well-defined spiral arms, multi-arm and flocculent galaxies are more patchy and have discontinuous spiral arms, lending them a more “fluffy” appearance. In addition, almost no external galaxies have been found where four spiral arms were observed extending from the centers to their outer regions. These findings hint that the spiral structure of the Milky Way may not be as unique as previously thought.

These findings could have drastic implications for our understanding of the Milky Way. It is also comforting to know that in terms of morphology, the team’s results show that our galaxy is not an outlier but an example of a statistically significant astronomical object.

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