Galaxies may look serene from afar, but their lives are shaped by powerful internal structures and by the neighbors they keep.
A new study led by Professor Woong-bae Zee of Sejong University shows that a galaxy does not follow just one evolutionary path.
Instead, the same galaxy can grow brighter with new stars or slowly quiet down—depending on the nature of the nearby galaxy it is interacting with.
Across the universe, galaxies often come in pairs. These “galaxy pairs” are bound by gravity and strongly influence one another as they slowly move toward an eventual merger.
Even our own Milky Way is destined to collide with the Andromeda galaxy billions of years from now.
Astronomers have long known that galaxies in pairs tend to form stars more actively than isolated galaxies, because gravitational forces stir up gas and funnel it into star-forming regions.
But interactions with neighbors are only part of the story. Many galaxies also contain a prominent bar-shaped structure across their centers.
The Milky Way itself has a bar stretching tens of thousands of light-years.
These bars can act like cosmic conveyor belts, driving gas from the outer regions of a galaxy toward its center, where it can ignite bursts of star formation. At the same time, some observations have suggested the opposite effect: bars may quickly use up or expel gas, leaving a galaxy unable to form many new stars.
For years, astronomers have debated whether bars help galaxies grow or push them toward decline.
Professor Zee’s new research offers a clear explanation for why the answer has been so confusing. The effect of a bar, it turns out, depends strongly on the properties of the neighboring galaxy.
Using detailed observations from the Dark Energy Spectroscopic Instrument, the research team examined about 4,000 galaxy pairs in which the central galaxy contains a bar.
They carefully categorized each pair based on how actively the neighboring galaxy was forming stars, which also reflects how much gas it still holds.
The researchers then studied how the bar’s size and strength were linked to the central galaxy’s rate of star formation.
The results revealed two very different outcomes. When the neighboring galaxy is rich in gas and actively forming stars, longer and stronger bars in the central galaxy are linked to higher star-formation rates. In these cases, the bar appears to efficiently pull in fresh gas—possibly even gas influenced by the neighbor—fueling the birth of new stars.
However, when the neighboring galaxy is gas-poor and forms few new stars, the pattern flips. As the bar grows longer, the central galaxy’s star formation drops. Without an external supply of gas, the bar seems to accelerate the use of the remaining fuel, effectively shutting down future star formation.
This discovery helps explain why earlier studies reached conflicting conclusions. Bars are not inherently “good” or “bad” for star formation. They can do both, depending on the cosmic environment. A galaxy’s future is shaped not only by its own structure, but also by the condition of the neighbor tugging at it through gravity.
Professor Zee likens the result to a cosmic version of Schrödinger’s cat. The bar can either promote growth or cause decline, and both possibilities exist.
Which one becomes reality is decided by the neighboring galaxy. The study highlights how deeply interconnected galaxies are, and how their destinies are written not in isolation, but through their relationships across the universe.
