The Milky Way galaxy may be a different shape than we thought

The shape of our galaxy may reveal a history of collisions with other galaxies or even galactic clusters.

By Robert Lea

16 May 2023

milky way illustrat.jpeg
(Classic) Illustration of our galaxy, the Milky Way. (Image credit: Mark Garlick/Science Photo Library/Getty Images)

New measurements suggest that the Milky Way galaxy may have a different shape than we thought.

Over the past few years, astronomers have increasingly discovered that galaxiesseem to come in three main shapes: Elliptical, irregular and spiral. The majority of known galaxies that fit in this last category seem to have two prominent "arms" that branch out and split into lesser arms.

But the traditional portrayal of the Milky Way is that of a galaxy with four major spiral arms extending out from a thick centralized bulge of stars. This makes our spiral galaxy stand out as an extremely rare outlier with an odd shape that must have some very unique properties to grant it four major arms.

That portrayal could be wrong, however. A team of astronomers has published new research that suggests we have been wrong about the shape of the Milky Way for decades, with our galaxy instead having two main arms just like its contemporary spiral galaxies.

The revelation that could reshape our understanding of the Milky Way came about when space scientists with the Chinese Academy of Sciences based at the Purple Mountain and National Astronomical Observatories analyzed multiple sources of astronomical data to get a better understanding of our galaxy's true shape.

"In spite of much work, the overall spiral structure morphology of the Milky Way remains somewhat uncertain," the astronomers wrote in a paper(opens in new tab) describing their research and conclusions. "In the last two decades, accurate distance measurements have provided us with an opportunity to solve this issue."

The team assessed data from a new generation of space instruments that can better measure the distance to individual stars which allowed them to measure the distances to around 200 stars and start putting together a map of the Milky Way. They then added data from the European Space Agency (ESA)'s Gaia space telescope which precisely observes the movement of stars and their location in relation to Earth.

In particular, the astronomers honed in on hot and massive stars called OB stars in the Gaia data. Because these stars are short-lived they move very little during their main-sequence hydrogen-burning lifetime which makes them useful for mapping purposes. Data collected from 24,000 OB stars was added to the map as were Gaia observations of over 1,000 open galactic clusters.

This led the astronomers to suggest that the Milky Way is a barred spiral galaxy with just two main arms extending from this dense central bar.

two arms milky way.jpeg
NGC 1300, a barred spiral galaxy with two arms, photographed by the Hubble Space Telescope. (Image credit: NASA, ESA, and The Hubble Heritage Team STScI/AURA)

"Using the precise locations of very young objects, for the first time, we propose that our galaxy has a multiple-arm morphology that consists of two-arm symmetry," they wrote. "The Norma and Perseus Arms are likely the two symmetric arms in the inner Milky Way. As they extend from the inner galaxy to the outer parts, they bifurcate, and connect to the Centaurus and SagittariusArms, respectively."

At the outskirts of the Milky Way, the astronomers write, are distant and fragmented irregular arms that are not connected to the central bulge of the galaxy where the majority of its stars are located. The fragmentation of spiral arms may have been caused by our galaxy colliding with other galaxies or even galactic clusters in its ancient history.

The team of astronomers concluded that this new model of the Milky Way's shape could provide an alternative basis for future studies of galactic structure. They add that more details should be revealed by further observations of nearby radio sources taken by multiple telescopes that would allow their distances from Earth to be calculated, and by improved data from the Gaia spacecraft. Gaia launched in 2013 and is expected to observe the universe for at least another two years until 2025.
The team's research is published(opens in new tab) in the Astrophysical Journal.


Here are some elements of this paper from The Astrophysical Journal:

What Does the Milky Way Look Like?

Y. Xu1,2, C. J. Hao1,2, D. J. Liu1,2, Z. H. Lin1,2, S. B. Bian1,2, L. G. Hou3, J. J. Li1,2, and Y. J. Li1

Published 2023 April 20 • © 2023. The Author(s). Published by the American Astronomical Society.

The Astrophysical Journal, Volume 947, Number 2

In spite of much work, the overall spiral structure morphology of the Milky Way remains somewhat uncertain. In the last two decades, accurate distance measurements have provided us with an opportunity to solve this issue. Using the precise locations of very young objects, for the first time, we propose that our galaxy has a multiple-arm morphology that consists of two-arm symmetry (the Perseus and Norma Arms) in the inner parts and that extends to the outer parts, where there are several long, irregular arms (the Centaurus, Sagittarius, Carina, Outer, and Local Arms).

Determining the detailed spiral structure of the Milky Way (MW) has long been a difficult issue in astronomy. Since we are deeply embedded in the Galactic disk, there are always multiple structural features superimposed along the observational line of sight. For a spiral galaxy, there may be two different components of spiral arms (Dobbs & Baba 2014 and references therein). One is composed of a spiral pattern indicated by the distribution of the older stellar population and the other is a spiral picture traced by diffuse or dense interstellar gas and young objects, e.g., high-mass star-forming regions (HMSFRs), massive OB stars, H ii regions, young open clusters (YOCs), etc.

Pioneering work was done by Morgan and colleagues in the 1950s. They found three short spiral-arm segments in the solar neighborhood using spectroscopic parallaxes of high-mass stars (Morgan et al. 1952, 1953). Soon after, larger-scale spiral structure, extending almost across the entire Galactic disk, was mapped with H i survey data using kinematic methods (e.g., van de Hulst et al. 1954; Kerr et al. 1957; Bok 1959; Burton & Shane 1970). Later, however, the distances of H i clouds, derived from kinematic methods, were found to have large uncertainties due to noncircular (peculiar) motions. Thus, the H i results in these early studies are not very reliable. Photometric methods are much more accurate than kinematic methods, although they can only be used to determine objects at distances up to ∼2 kpc, i.e., much smaller than the size of the MW. For this reason, kinematic methods are still widely used to study the entire MW. By analyzing a number of H ii regions with photometric and/or improved kinematic distances, a paradigmatic map of the Galaxy's spiral arms was made by Georgelin & Georgelin (1976). They first proposed that the MW has four major arms. This picture was then frequently updated by using other spiral tracers, such as molecular clouds (e.g., Burton & Gordon 1978; Dame et al. 1987, 2001; Sun et al. 2015; Du et al. 2016), star-forming complexes (Russeil 2003), a larger sample of H ii regions (e.g., Paladini et al. 2004; Hou & Han 2014), H i gas (e.g., Levine et al. 2006), etc., most of which relied on kinematic methods.

Although kinematic methods are being improved all the time, they sometimes cause significant uncertainties in the locations of sources. Therefore, the debate continues over such basic facts as the existence of some spiral arms, the number of arms, and the size of the MW. Measuring distances as accurately as possible using spiral tracers is key to settling these disputes and correctly uncovering the Galaxy's spiral structure. Recently, substantial progress in tracing the Galactic structure using young objects has been achieved. On the one hand, very long baseline interferometry (VLBI) can yield trigonometric parallax accuracies down to a few μas (e.g., Hachisuka et al. 2006; Xu et al. 2006; Sanna et al. 2017), allowing precise distance measurements toward masers associated with HMSFRs throughout the Galaxy. Focusing on mapping the MW, the Bar and Spiral Structure Legacy (BeSSeL) Survey (Brunthaler et al. 2011) and the VLBI Exploration of Radio Astrometry (VERA) array (VERA Collaboration et al. 2020) have measured accurate distances for up to ∼200 masers. These measurements indicate that the MW is a four-arm spiral, with some extra arm segments and spurs (Reid et al. 2019, hereafter R19). On the other hand, a large amount of young stars provided by the Gaia mission has densified the spiral-arm segments traced by masers, which also helps determine the Galactic spiral structure in the solar neighborhood (Xu et al. 2018a, 2018b, 2021a; Hao et al. 2021; Hou 2021; Poggio et al. 2021).

External spiral galaxies can also act as a mirror to help us understand Galactic morphology better as the MW is one of trillions of galaxies in the observable universe. Pictures of external spiral galaxies show that there are largely three distinct types of morphology. In two extreme cases, grand-design spiral galaxies are highly symmetric, characterized by clear, long, and symmetric spiral arms, whereas flocculent ones are fragmented, consisting of many short, irregular, and patchy segments. An intermediate type lies in between, so-called multiple-arm spiral galaxies whose main characteristic is an inner two-arm symmetry and several irregular arms in the outer parts (Elmegreen & Elmegreen 2014). Different from flocculent galaxies, both grand-design and multiple-arm galaxies have two prominent, symmetric arms in their inner regions, and almost no external galaxies present four spirals extending from their centers to their outer regions (Elmegreen & Elmegreen 1982, 1987, 1995). Today, it is widely accepted that the MW galaxy has four continuous spiral arms extending outward from the inner Galaxy to distant outer regions (e.g., Georgelin & Georgelin 1976; Reid et al. 2019; Minniti et al. 2021). If that is the case, the MW may be an atypical galaxy in the universe.

In the past few years, the number of young objects in the MW with precisely measured distances have increased significantly. It would be constructive to combine these high-quality data together to provide a better understanding the MW's spiral structure. In this contribution, largely following the previously developed spiral-arm-fitting approach presented by R19, we have synthesized the available data set of spiral tracers with precisely measured distances, including the parallax measurements of HMSFR masers from VLBI observations, massive OB2-type stars, and YOCs from the Gaia mission, aiming to uncover the real image of our archetypal Galaxy.

Currently, VLBI maser parallax measurements are mainly carried out in the northern hemisphere, with uncertainties of typically ∼20 μas, while a number of observations have uncertainties down to ∼10 μas or better, allowing reliable distances to be determined for objects located at the Galactic center (GC) and beyond (Zhang et al. 2013; Sanna et al. 2017; Reid et al. 2019; Xu et al. 2021b). On the other hand, Gaia Data Release 3 (DR3) data have uncertainties of order 20–30 μas, which enable us to reveal the spiral structure within ∼5 kpc of the Sun. In addition to using these distances to trace the spiral structure directly, ascertaining arm tangencies is also a good way to determine the locations of spiral arms, which are identified from the distributions of stars, interstellar gas, dust, and star formation sites in the Galactic plane (Hou & Han 2015).


Scientists have now discovered, through the study of MASERs*, OB2 type** stars, and Young Open Clusters (YOC)***, that the Milky Way's elegant spiral structure is dominated by just two arms wrapping off the ends of a central bar of stars. Previously, our galaxy was thought to possess four major arms.

*MASER: or M icrowave A mplification by S timulation E mission of R adiation. By monitoring the position of a maser bearing star in the sky with respect to a back-ground extragalactic source, it is possible to estimate its proper motion and parallax (e.g., Honma et al., 2008). Maserastrometric measurements of high-mass star forming regions using VLBI have recently made substantial progress on our understanding of the Milky and its shape.


** OB2 stars: Stars come in different sizes and masses. Our Sun is an average-sized starthat will have a lifespan of some 10 billion years. More massive stars, like those found in Cygnus OB2, only last a few million years. During their lifetimes, they blast large amounts of high-energy winds into their surroundings.


*** Young Open Clusters (YOC):
Open clusters are strongly concentrated toward the Milky Way. They form a flattened disklike system 2,000 light-years thick, with a diameter of about 30,000 light-years. The younger clusters serve to trace the spiral arms of the Galaxy, since they are found invariably to lie in them. Very distant clusters are hard to detect against the rich Milky Way background. A classification based on central concentration and richness is used and has been extended to nearly 1,000 open clusters. Probably about half the known open clusters contain fewer than 100 stars, but the richest have 1,000 or more. The largest have apparent diameters of several degrees, the diameter of the Taurus cluster being 400 arc minutes (nearly seven arc degrees) and that of the Perseuscluster being 240 arc minutes. The linear diameters of young open clusters range from the largest, 75 light-years, down to some 5 light-years.