The secret to a galaxy's shape lies in its dense core

Space · Published 25 October 2026 Swipe more like this →

In short: Why do some spiral galaxies boast dozens of arms while others, like our own Milky Way, only have two? Astronomers are finding that the answer depends on how compact a galaxy's center is. This structural blueprint reveals deep secrets about how matter organizes itself across the cosmos.

For generations, the variation in spiral arm counts has remained a mystery to astronomers. New research from East Tennessee State University and Iowa State University suggests that the primary driver is the 'central bulge'—the dense collection of stars at a galaxy's core. By analyzing thousands of images from the European Space Agency’s Euclid space telescope, researchers identified a consistent pattern: galaxies with larger, more compact central bulges almost always feature two dominant spiral arms.

Conversely, galaxies with smaller, flatter centers tend to develop numerous spiral arms. This correlation suggests that the concentration of mass at the center dictates the geometry of the surrounding structure. The study confirms that these observations align closely with computer simulations of galaxy formation. Additionally, a galaxy's total mass influences the specific length and prominence of its arms, while the overall distribution of matter shapes the final appearance.

This discovery has significant implications for our understanding of dark matter, which NASA describes as the 'invisible glue' of the universe. Since spiral shapes are formed by both visible and dark matter, researchers suggest that tallying a galaxy's arms might offer an alternative technique for gauging central dark matter density. By looking back billions of years—to when the universe was only half its current age—astronomers can now map how these systems evolved over eons.

Furthermore, simulations show that specific masses, such as black holes, can create ripples in stellar orbits. These gravitational disturbances contribute to the formation of classic spiral structures, highlighting a complex interplay between central mass concentrations and the orbital paths of stars.

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