A strange lack of stellar orbits around the core of the Small Magellanic Cloud (SMC) puzzled astronomers for decades. In addition, the SMC has a strange, irregular shape and exhibits a tidal current. Now a team of observers led by graduate student Himansch Rathore from the University of Arizona has discovered the reason why the stars don’t orbit. This is because the SMC crashed in the distant past directly through its neighbor, the Large Magellanic Cloud (LMC). This massive collision disrupted the star’s movements and sent them on completely different trajectories. It also disrupted the gas clouds within the SMC, creating a gas tail that stretched across space.
The team’s work provides unique insights into the way galaxies change over time. “We see a galaxy changing in live action,” Rathore said. “The SMC gives us a unique front-row view of a very transformative process that is critical to the evolution of galaxies.”
A closer look at the SMC
The Small Magellanic Cloud is a member of a trio of interacting galaxies that includes the Large Magellanic Cloud and our own Milky Way. The SMC is about 200,000 light-years away, while the LMC is about 158,000 light-years away. Both have sites of active star formation. The SMC is categorized as a dwarf irregular galaxy and has a mass equivalent to about 7 billion solar masses. However, not all of the mass is contained in stars. Most of the SMC’s mass is contained in huge clouds of gas that eventually become star formation sites. This happens when the clouds cool and gather. When conditions are right, this process creates hot, young stars that astronomers can study to understand the star formation process.
This visible light mosaic shows the LMC and SMC in relation to the plane of our own galaxy, the Milky Way. Dusty filaments create dark trails on the Milky Way’s bright midplane, visible at the top of the image. Below, about 21 degrees apart, lie the LMC and SMC, the closest large galaxies to our galaxy. The LMC and SMC orbit each other as well as our own Milky Way Galaxy. Photo credit: Axel Mellinger, Central Michigan University (via NASA Goddard Scientific Visualization Studio).
Astronomers have measured the movement of stars existing in the SMC using data from the Hubble Space Telescope and the Gaia mission. They discovered that the stars of the SMC do not orbit around the center of this galaxy like the stars in most other galaxies. This lack of orbital activity was puzzling until Rathore’s team thought about the impact of a collision on the SMC and LMC. A few hundred million years ago, the SMC crashed directly through the disk of the LMC. The LMC’s gravity destroyed the SMC’s internal structure and sent its stars into random, disordered motion. In addition, the gas in the LMC exerted enormous pressure on the gas of the SMC, destroying its gas rotation.
Because the LMC, SMC, and the Milky Way interact with each other, astronomers want to understand how this interaction affects all three galaxies. Astronomers found a gas bridge between the LMC and SMC, which was likely pulled from one of the galaxies during tidal interactions between the two galaxies. This bridge is busy forming stars in the shocked gas.
Large and small Magellanic clouds from GAIA data. Image credit: ESA/Gaia/DPAC – CC BY-SA 3.0 IGO.
Solve the mystery of disturbed stellar orbits
According to Gurtina Besla, a senior author of an article on the finding, the crash between LMC and SMC caused great damage to both. “The SMC experienced a catastrophic crash that injected a lot of energy into the system. It is by no means a ‘normal’ galaxy,” Besla said. To understand this crash, the team turned to computer simulations. First, they compared the known properties of the SMC and the LMC – their gas content, the star’s total mass and their positions relative to the Milky Way. They combined the simulations with theoretical calculations of how the collision affected the SMC’s gas as it plowed through the LMC’s dense gas environment. They also developed new methods for reading the scrambled stellar motions in a post-collision galaxy, tools that can now be used to properly interpret what telescopes are actually measuring in the SMC.
This is important because the SMC is small, gas-rich and poor in heavy elements – properties that made it a standard benchmark for the types of galaxies that existed early in the universe. A galaxy still affected by a collision may not be a clean reference point, Besla said. However, it can shed light on what impact collisions and interactions have had on galaxies over time.
This diagram shows the simulated gas distribution of the Magellanic System resulting from the tidal encounter between the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) as they orbit our home galaxy, the Milky Way. The solid line shows the calculated path of the LMC and the dotted line is the path of the SMC. Plot by G. Besla, background image of the Milky Way by Axel Mellinger (used with permission)
Other effects of a collision
Another study published by the team in 2025 showed that the collision also left a physical trace on the LMC that could help scientists study dark matter. At the center of the LMC is a rod-shaped structure that has tilted out of the plane of the galaxy due to the collision. Rathore, the lead author of the 2025 study, said the degree of tilt depends on how much dark matter the SMC contains, giving researchers a new way to measure a substance that has never been directly detected but could only be inferred from its gravitational effects.
“We are used to thinking of astronomy as a snapshot of time,” Rathore said. “But these two galaxies came very close together, passed right through each other and turned into something else.”
In addition, the interaction between the LMC, SMC and the Milky Way influences the shape of our galaxy. It appears that the LMC causes a deformation in the shape of the Milky Way’s star disk. It also pulls on our galaxy’s core, disrupting the halo and accelerating its speed through space. The SMC also contributes to this warping and pulling, contributing to the formation of the Magellanic Current. This is a trail of gas and stars that contributes to the colonization of the Milky Way.
More information
A Galactic Transformation – Understanding the Structural and Kinematic Imbalance of the SMC
A galaxy next door is changing, and astronomers can see it happening
