Young Stars' Feedback Emerges as Major Force Shaping Galaxy Evolution

Across billions of years, galaxies have been shaped by two great forces: the violent collisions between them, and the quiet work of stars being born inside them. A team of researchers at Ohio State University, led by graduate student Debosmita Pathak, set out to understand just how much weight that second force carries. They examined 18,000 star-forming regions scattered across nearby spiral galaxies, using data from the Hubble Space Telescope, the James Webb Space Telescope, and the Atacama Large Millimeter/Submillimeter Array to see what happens when stars ignite.

The story begins in clouds of hydrogen gas called HII regions, where gravity pulls matter together until protostars form and eventually burst into light. But a newborn star is not a quiet thing. It floods its surroundings with radiation and heat. It sends out jets of material that carve caverns into the clouds that birthed it. This cascade of energy—what astronomers call stellar feedback—reshapes the neighborhood around every young star. Pathak's team found that the pressure from ionized gas driven by these hot young stars plays a decisive role in how star-forming regions expand or contract. Whether a region continues to grow depends heavily on what lies around it. "When young massive stars are born," Pathak explained, "they're very energetic and pump out a ton of photons into their surroundings. In that process, they disrupt their local environments and start to drive interstellar material out of the area."

This feedback can cut two ways. The shockwaves and radiation from newborn stars can trigger star formation in nearby clouds, compressing gas and igniting new generations of stellar birth. But the same process can also destroy the raw material needed for planets to form, or scatter gas so thoroughly that star formation simply stops. A galaxy's chemical makeup gets rewritten by these events. The Milky Way, for instance, births roughly one star per year on average, a steady hum of creation spread across its spiral arms. But other galaxies produce stars at vastly higher rates, often because they have undergone violent mergers with other galaxies. When two spiral galaxies collide, gravitational forces tear them apart and send shock waves racing through their gas clouds. Those shock waves compress the gas, and the result is often a frenzy of star formation that can last for millions of years.

The starburst galaxy NGC 3256 offers a vivid example. This system consists of two spiral galaxies in the process of merging, their collision igniting regions of intense star formation. The survey revealed something striking: stellar feedback pressures in NGC 3256 are roughly 100 times stronger than in normal spiral galaxies like the Milky Way. The young, massive star clusters in NGC 3256's densest regions are constrained by this immense pressure, though most are powerful enough to keep expanding anyway. The galaxy also churns with high turbulence—the gas being compressed has not settled into the flat disk where star formation typically occurs. This is an extreme environment, a laboratory for understanding how stars shape galaxies when conditions are pushed to their limits.

Pathak's work matters because it bridges two scales of cosmic time. In the early universe, when galaxies were young and collisions were common, stellar feedback would have been a dominant force in determining which galaxies grew and which remained small. Understanding how stars regulate their own birth and death—how they pump energy into their surroundings and reshape the material around them—is essential to building models that actually work in those distant, violent epochs. "It's important to study environments in normal parts of the universe, but also how things deviate in the extremes," Pathak said. "Without this type of research, we wouldn't know if the physics that we're working with and the models that we're building actually hold true in such extreme places."

In a few billion years, the Milky Way itself will collide with the Andromeda Galaxy. When that happens, both will deform under each other's gravity. Shock waves will compress gas clouds. A new generation of stars will ignite, and those young stars will begin their own work of reshaping the galaxy that contains them. The collision will be slow—a cosmic dance played out over millions of years—but the feedback from all those newborn stars will help determine what kind of galaxy emerges from the merger. Understanding that process, in all its complexity, is what Pathak and her team are working toward.