Dance of the Stars: The Arms of the Milky Way

The Spiral Architecture of Our Galaxy

A barred spiral galaxy

Our galaxy, the Milky Way, is a barred spiral galaxy, meaning it is composed of a central bulge of stars, a bar of stars crossing this bulge, and a flat disk surrounding the whole.

The disk and the galactic arms

It is in this disk that the galactic arms are found, those luminous spiral structures we observe in other galaxies. These arms are not rigid structures, but rather zones of higher density where stars, gas, and dust accumulate. Our solar system is located in one of these arms, the Orion Arm (or Local Arm), between the Perseus Arm and the Sagittarius Arm.

Introduction: The Mystery of Galactic Spiral Arms

Denser bands

The spiral arms of our galaxy are like denser bands where there are more stars, gas, and dust. Physically, this means that matter is not distributed uniformly, but forms concentrated regions that rotate around the galactic center.

Understanding their formation

To understand how these arms form and evolve, we must study how the galactic disk, which rotates on itself, remains stable or not, how density waves can appear, and how gas and stars are born and interact in these zones.

Morphology and Mapping: What We Observe

Observational tracers

To trace the arms, astronomers use several "indicators": atomic gas detected via a radio wave called 21 cm, molecular gas (mainly CO), regions where stars are born (\(\text{ionized H II}\) regions), and groups of young stars. The most precise tracer remains the maser associated with massive star-forming regions: its trigonometric parallax, measured by very long baseline radio interferometry (VLBI), gives a direct distance, independent of dust extinction that hinders optical observations.

Mapping the spirals

By combining precise measurements of velocity, distance, and position (for example with the Gaia satellite or the masers of the large international BeSSeL survey), we can map the shape of the arms. These often resemble logarithmic spirals, that is, curves that wind around the center in a regular way, defined by an opening angle called the "pitch angle," on the order of 10 to 15° for most of the Milky Way's arms.

Dynamic Framework of the Galactic Disk

The rotation of the disk

The galactic disk is modeled as a rotating fluid under its own gravity. The speed at which stars and gas rotate depends on the gravitational force pulling them toward the center. This circular velocity \(v_c(R)\) is related to the slope of the gravitational potential. According to the most precise measurements to date, based on maser parallaxes, the Sun is located about 8.15 kpc (≈ 26,600 light-years) from the galactic center and orbits at a circular speed of about 236 km/s, or nearly 247 km/s when accounting for its proper motion relative to the local reference frame.

The Toomre stability criterion

To determine whether the disk is stable or will locally collapse (forming arms or clusters), a criterion called the Toomre parameter \(Q\) is used. It depends on the matter density, the speed of sound in the gas (which reflects pressure or turbulence), and orbital dynamics. When \(Q\) is less than 1, the disk becomes unstable and can form structures such as arms.

Theories on the Formation and Dynamics of Galactic Arms

The existence of spiral arms has long puzzled astronomers. Two main theories attempt to explain their formation and persistence.

The density wave theory

According to this theory, galactic arms are not physical objects rotating together, but rather gravitational shock waves propagating through the galactic disk. These waves compress gas and dust, triggering the formation of new stars. These young massive stars are very luminous, which makes the arms more visible.

Self-propagation of star formation

In this model, the formation of new stars in one region of the galactic disk causes shock waves that in turn trigger the formation of new stars in neighboring regions. This process then propagates step by step, creating a spiral structure.

These two theories are not mutually exclusive, and it is possible that both contribute to the formation and dynamics of galactic arms.

Two Arms or Four? A Still-Vivid Scientific Debate

The 2008 Spitzer study: two major arms

The exact structure of the Milky Way remains debated, precisely because we observe it from the inside. In 2008, an analysis of infrared images from the Spitzer Space Telescope, counting stars in each direction, concluded that the Galaxy had only two major arms, Scutum-Centaurus and Perseus, extending directly from the ends of the central bar; Sagittarius and Norma, previously considered major, were demoted to secondary arms, rich in gas but poor in old stars.

The BeSSeL and VERA surveys: four spiral arms

Since then, the large international BeSSeL radio survey, combined with the Japanese VERA project, has measured by interferometry the trigonometric parallaxes of about 200 masers associated with young massive stars in star-forming regions. These measurements, significantly more direct than star counting, show that the Milky Way is in fact a four-arm spiral — Norma-Outer, Scutum-Centaurus, Sagittarius-Carina, and Perseus — the difference with the Spitzer study coming from the fact that young, massive stars do trace all four arms, while older stars are concentrated only in the two most massive. The two results are therefore not contradictory: they describe different stellar populations.

The Orion Arm, an Arm in Its Own Right

A local arm long considered a spur

The Sun does not reside in any of the four great arms: it is located in the Local Arm, also called the Orion Arm, a structure long considered a simple secondary "spur" wedged between the Sagittarius and Perseus arms. This modest classification was thoroughly revised starting in 2013, when a team led by astronomer Ye Xu measured by VLBI the parallax of masers located in this arm and discovered a number of star-forming regions much higher than expected.

The 2016 study: an arm in its own right

A follow-up study, published in 2016 in the journal Science Advances, identified eight new masers in the Orion Arm and raised its estimated length to about 25,000 light-years, compared with a little over 16,000 light-years estimated in 2013 — nearly double. Its pitch angle and star formation rate proved comparable to those of the great Sagittarius and Perseus arms. As astronomer Mark Reid (Harvard-Smithsonian), co-author of the study, summarized: the Milky Way does have four major arms, but also significant asymmetric structures like this local arm, neither quite a spur nor quite an arm in its own right.

Recent Observations

Precision measurements

Thanks to very precise techniques, such as position and motion measurements of stars with the Gaia satellite (whose observation mission ended in January 2025 after more than three trillion measurements) or very fine radio observations of masers from the BeSSeL survey, astronomers can now build a 3D map of the Milky Way and precisely measure the velocities of objects within it.

A complex structure

These new data show that our galaxy possesses both stable spiral arms, which resemble global waves, and more temporary structures that appear locally, such as the spurs connecting the Local Arm to neighboring arms. This gives a complex but coherent picture, where several phenomena combine to shape the spiral arms we observe.

Key Takeaways

A compromise between two mechanisms

From a mechanical point of view, spiral arms result from a compromise between collective modes (density waves) and transient local phenomena amplified by shear and dissipation in the gas. The Milky Way appears to combine these two mechanisms, explaining the complexity of its spiral structure. The multi-tracer and multi-scale approach is essential for deciphering galactic dynamics.

The Main Spiral Arms of Our Galaxy

Four major arms, plus a local arm

According to the best-constrained model to date, based on maser parallaxes from the BeSSeL survey, the Milky Way has four major spiral arms — Norma-Outer, Scutum-Centaurus, Sagittarius-Carina, and Perseus — to which is added the Local Arm (or Orion Arm), where our Solar System is located, now recognized as a structure in its own right rather than a simple spur.

Stars in motion

Contrary to a common misconception, stars do not remain fixed in these arms but pass through them during their galactic orbit, at a speed of around 230 km/s. This perpetual motion explains why the arms are not frozen structures but density waves in constant evolution.

N.B.:
The distances indicated correspond to the galactocentric radius of the reference segment used to fit each arm to a logarithmic spiral, and not to a total "length" of the arm, a poorly defined notion for a structure that winds over several tens of thousands of light-years. They are calculated for a Sun-galactic center distance of 8.15 kpc.

The Future of Our Galaxy

The collision with Andromeda

In about 4 billion years, the Milky Way will enter into a collision with the Andromeda galaxy. This major gravitational interaction will radically alter the structure of our spiral arms, probably transforming them into an elliptical galaxy. Numerical simulations suggest that the probability of direct stellar collision remains tiny, on the order of \(10^{-12}\) per star.