Spinning Stars and Stellar Secrets: A Look at Fast Rotators in Magellanic Cloud Clusters

In recent years, astronomers using the Hubble Space Telescope have discovered surprising patterns in certain star clusters located in our neighboring galaxies, the Magellanic Clouds. These clusters show odd features in their color-magnitude diagrams (CMDs), which are like star charts plotting how bright and hot stars are. The weirdness includes extended turn-off points (where stars begin to age), split main sequences (indicating stars of the same age looking very different), and doubled red clumps (regions where stars burn helium in their cores). At first, scientists thought these patterns might mean that stars in these clusters were born at different times. But another theory, called the “stellar rotation scenario,” suggests something more exciting: stars are spinning at wildly different speeds.

Stellar Rotation: A Game Changer for Star Evolution

Stars that spin quickly behave very differently from those that don’t. Fast spinning causes stars to bulge at the equator and flatten at the poles—just like how pizza dough flattens when spun. This changes how gravity and temperature are spread across a star’s surface, leading to what’s called “gravity darkening.” A spinning star looks brighter and hotter if viewed from the top (pole-on) and dimmer if seen from the side (equator-on). The researchers behind this study, led by Greta Ettorre, used computer models to see how this rotation affects the appearance of stars in four clusters. They found that most stars in these clusters are spinning close to their “break-up speed,” the fastest they can go before gravity can no longer hold them together.

The Four Clusters in Focus

The clusters studied are NGC 419 (in the Small Magellanic Cloud) and NGC 2203, NGC 1831, and NGC 1866 (all in the Large Magellanic Cloud). Each cluster has unique properties, but all show signs that many of their stars are rotating quickly. For example, NGC 419 and NGC 1831 have clear signs of extended main sequences and red clumps, while NGC 1866 shows a "split main sequence"—likely due to a mix of slow and fast rotators. These features make these clusters perfect laboratories for testing theories about stellar rotation.

How the Models Work

To understand the rotation rates in these clusters, the team used isochrones—models that track how stars evolve over time—for stars with different initial spin rates. These were based on the parsec v2.0 code, which includes detailed physics about how rotation changes a star’s brightness and temperature. They created “partial models,” each representing a group of stars with a specific range of rotation speeds, and combined them using a fitting algorithm. By comparing these models with actual Hubble observations, they figured out what mix of spin speeds best explained each cluster’s CMD.

Key Findings: Fast Rotators Everywhere

Across all four clusters, a large portion of the stars were found to be spinning fast. In NGC 419, for instance, over 95% of stars had rotation rates above 70% of the break-up speed. NGC 1831 and NGC 1866 showed similarly high numbers, while NGC 2203 had a lower, but still significant, fraction (above 50%). These results strongly support the rotation-based explanation for the strange CMD features. They also suggest that fast rotation is a normal part of life for stars in these types of clusters.

Challenges and Caveats

Even with these strong results, the models aren’t perfect. For example, in some clusters, the red clump region in the CMD wasn’t matched as well as expected. This might mean the models underestimate the number of stars in that phase of life, or that rotation alone doesn’t explain everything. Also, the assumption that star spins are randomly oriented may not be accurate—some stars might align their axes during formation. These are open questions that future studies could explore.

Conclusion: Spinning Towards New Insights

This study adds to growing evidence that rotation plays a major role in how stars evolve and what they look like in star clusters. By modeling four clusters with strong rotation signatures, Ettorre and her team show that fast rotators are more common than previously thought. Their work helps refine our understanding of stellar physics and paves the way for even more detailed models in the future. For young astronomers, this research is a reminder that stars are not just twinkling lights—they’re dynamic, spinning, and full of surprises.

Source: Ettorre