If the universe had an instruction manual, University of Missouri Assistant Professor Charles Steinhardt believes some sections might be absent. It appears either the cosmos has followed distinct laws throughout or humans have misunderstood the guidelines.
Historically, astronomers have classified galaxies into two main types: blue ones, which are youthful and currently generating new stars, and red ones, which are more mature and have stopped producing stars. However, Steinhardt is now disrupting this conventional view by suggesting a third classification: red but still forming stars. These galaxies do not clearly fall under either typical category; rather, they occupy an intermediate position.
He explained that “red, star-forming galaxies mainly generate low-mass stars, which gives them their reddish hue even as new stars continue to form.” This concept emerged to tackle discrepancies related to the conventional observed proportions between black hole mass and stellar mass, along with variations in the initial mass function among blue and red galaxies—issues that couldn’t be resolved solely through processes like aging or mergers. Yet, our findings suggest that many of the stars visible now may have originated under circumstances quite distinct from those initially presumed.
Steinhardt's research, published in The Astrophysical Journal , indicates that red, star-forming galaxies may have had a significantly larger impact on the universe’s evolution than previously thought. This revelation has the potential to alter our present comprehension of galaxy development, the mechanisms influencing their structure, and the methods used for measuring star formation across cosmic time.
These galaxies' presence suggests that the universe may have produced considerably more stars than we initially thought," he stated. "This finding bolsters the notion that the evolution of galaxies is far more intricate than just transitioning from blue to red and ceasing star formation.
A fresh perspective on post-starburst galaxies
Historically, galaxies are recognized for their evolution via slow maturation processes or by merging, with these mergers often sparking waves of newborn stars. Consequently, scientists have struggled to understand post-starburst galaxies—these systems abruptly cease forming new stars following a brief phase of vigorous star creation. It’s generally thought that when two galaxies crash into each other, this interaction leads to an immediate surge in star birth but eventually exhausts available resources, resulting in silence.
However, Steinhardt proposes an alternative explanation. It could be possible that certain galaxies were gradually producing tiny, reddish stars rather than undergoing a rapid increase in star formation. He noted that should this be accurate, we might have to reconsider our classification of post-starburst galaxies since some might actually fall into a separate group of galaxies characterized by their ongoing creation of red stars.
In the future, Steinhardt along with his students from Mizzou's Department of Physics intend to carry out more sophisticated experiments to delve deeper into the study of star-forming galaxies. Junior Mathieux Harper and a group of undergraduates will seek additional proof to back the notion that certain post-starburst galaxies might belong to this new classification.
In the meantime, sophomore students Carter Meyerhoff and Zach Borowiak will head up a research initiative utilizing information gathered by the European Space Agency’s Gaia satellite to examine more than two billion stars within our Milky Way galaxy.
More information: Charles L. Steinhardt, Do Red Galaxies Produce More Stars Than Blue Galaxies? The Astrophysical Journal (2025). DOI: 10.3847/1538-4357/adb95b
Furnished by the University of Missouri
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