Astronomers Discover How the Small Magellanic Cloud's Stars Lost Their Rotation in a Galactic Collision

Astronomers have solved a decades-old cosmic mystery: why the stars in the Small Magellanic Cloud (SMC)—one of the Milky Way’s closest galactic neighbors—fail to orbit its center in an orderly, spinning disk like most galaxies. New research published in *The Astrophysical Journal* reveals that the SMC’s erratic star motions and disrupted gas structure are the direct result of a catastrophic collision with its larger companion, the Large Magellanic Cloud (LMC), a few hundred million years ago. The findings not only rewrite the SMC’s history but also force scientists to reevaluate the galaxy’s long-standing role as a cosmic benchmark for understanding star formation and galactic evolution across cosmic time.

How a Galactic Collision Shattered the Small Magellanic Cloud’s Structure

The Small Magellanic Cloud: A Galaxy Out of Sync

Visible to the naked eye from the southern hemisphere as a hazy patch of light near the constellation Tucana, the SMC is a small, irregular galaxy gravitationally bound to the Milky Way. Unlike spiral galaxies such as our own, where stars orbit a central hub in a flat, rotating disk, the SMC’s stars move in chaotic, disordered paths. For over 50 years, astronomers have mapped its stars, gas, and motion using ground-based telescopes and space observatories like NASA’s Hubble Space Telescope and the European Space Agency’s Gaia satellite. Yet the question of why its stars lack coherent rotation remained unanswered—until now.

The Collision That Redefined a Galaxy

Led by University of Arizona astronomer Himansh Rathore, a team of researchers traced the SMC’s structural disarray to a violent head-on collision with the LMC, its more massive sibling galaxy. The cosmic smashup occurred roughly 100 to 300 million years ago—a blink of an eye in cosmic terms—and left an indelible mark on both galaxies. The LMC’s immense gravitational pull tore through the SMC, destabilizing its internal gas dynamics and scattering its stars into random orbits. ‘We are seeing a galaxy transforming in live action,’ Rathore said. ‘The SMC gives us a unique, front-row view of something very transformative—a process critical to how galaxies evolve.’

Gas Under Pressure: The Invisible Force Reshaping the SMC

The collision didn’t just alter the SMC’s stars—it also stripped away its rotational motion entirely. The SMC contains far more gas than stars; in most galaxies, that gas cools, condenses under gravity, and settles into a rotating disk, much like the spiral structure of the Milky Way. But when the SMC plunged through the LMC’s dense gas disk, the LMC’s gravitational forces and ram pressure—a kind of cosmic wind—applied immense pressure to the SMC’s gas. ‘Imagine sprinkling water droplets on your hand and moving it through the air,’ Rathore explained. ‘As the air rushes past, the droplets get blown off because of the pressure it exerts. Something similar happened to the SMC’s gas as it punched through the LMC.’ The result? The SMC’s gas lost its rotation, and its stars, which form from that gas, inherited the disorder.

The possible reason, Rathore said, is a collision. A few hundred million years ago, the SMC crashed directly through the LMC's disk. The LMC's gravity disrupted the SMC's internal structure and sent its stars into random, disordered motion.

Why the Small Magellanic Cloud’s Gas Rotation Was an Illusion

Decades of Confusion Over the SMC’s Spin

For decades, astronomers believed the SMC’s gas was rotating based on telescope observations. This was puzzling because stars form from gas, and their motion should reflect that rotation. Gurtina Besla, an astronomy professor at the University of Arizona and senior author of the study, explained that the apparent rotation was actually an optical illusion created by the collision. ‘The collision is stretching the SMC,’ she said. ‘Gas moving toward and away from Earth along that stretch looks like rotation from certain perspectives.’ Using advanced computer simulations tailored to the SMC and LMC’s known properties—such as their gas content, star masses, and positions relative to the Milky Way—the team demonstrated how the collision’s aftermath distorted the SMC’s structure, making it appear as though its gas was spinning when it wasn’t.

A Cosmic Yardstick That May No Longer Measure Up

The SMC has long been a cornerstone for studying galaxy formation and star evolution, particularly because it resembles the kinds of small, gas-rich galaxies that populated the early universe. Its low abundance of heavy elements—products of stellar explosions—mirrors conditions in the primordial cosmos. But the new findings call that benchmark into question. ‘The SMC went through a catastrophic crash that injected a lot of energy into the system,’ Besla said. ‘It is not a ‘normal’ galaxy by any means.’ The collision’s energetic aftermath means the SMC is no longer a pristine example of an early-type galaxy, complicating its use as a reference model for cosmic archaeology.

The Collision’s Ripple Effects: Dark Matter and the Large Magellanic Cloud

The SMC-LMC collision didn’t just reshape the Small Magellanic Cloud—it left a lasting imprint on the Large Magellanic Cloud as well. A companion study by the same team, published in 2025, revealed that the collision tilted the LMC’s central bar—a dense, elongated region of stars and gas—out of the galaxy’s main plane. This tilt is directly tied to the amount of dark matter in the SMC, offering astronomers a novel way to probe the elusive substance that makes up 85% of the universe’s matter but has never been directly detected.

Astronomy in Motion: Why Real-Time Galactic Change Matters

Traditionally, astronomy has been a science of snapshots—a single moment frozen in time. But the SMC and LMC’s collision forces scientists to adopt a dynamic view of the cosmos. Rathore emphasized this shift: ‘We are used to thinking of astronomy as a snapshot in time. But these two galaxies have come very close together, gone right through one another, and transformed into something different.’ This transformation provides a rare opportunity to observe a galactic merger in progress, offering insights into similar events that have shaped the universe’s largest structures.

Key Takeaways: What the Discovery Means for Astrophysics

• The Small Magellanic Cloud’s stars lost their rotation after colliding with the Large Magellanic Cloud hundreds of millions of years ago, disrupting its structure and gas dynamics.
• Astronomers previously believed the SMC’s gas was rotating, but the new study shows this was an optical illusion caused by the collision’s stretching effect.
• The SMC’s role as a cosmic benchmark for galaxy formation is now in question, as its post-collision state makes it atypical compared to early-universe galaxies.
• The collision also tilted the LMC’s central bar, providing a new method to measure dark matter in the SMC by observing its gravitational influence.
• This discovery underscores the importance of dynamic, real-time observations in astronomy, challenging the traditional ‘snapshot’ approach to studying galaxies.

How Scientists Unraveled the Mystery of the SMC’s Chaotic Stars

The breakthrough came from combining high-precision data from the Hubble Space Telescope and ESA’s Gaia satellite with sophisticated computer simulations. The team modeled the SMC and LMC’s interaction, accounting for their gas reservoirs, star populations, and positions within the Milky Way’s gravitational field. They also developed new analytical tools to interpret the scrambled motions of stars in a post-collision galaxy, allowing them to separate the effects of the crash from the SMC’s original structure. ‘We paired the simulations with theoretical calculations of how the SMC’s gas was affected as it plowed through the LMC’s dense gas environment,’ Rathore noted. ‘These methods can now be used to properly interpret what telescopes actually measure in the SMC.’

Frequently Asked Questions About the Small Magellanic Cloud’s Galactic Collision