For more than half a century, astronomers have relied on a basic assumption when estimating the number of stars inside distant galaxies: stars of different sizes form in roughly the same proportions everywhere in the universe.
New research suggests that assumption may not always be correct.
Scientists at the University of Missouri have found evidence that the proportion of large and small stars can vary depending on the environment where the stars formed.
The discovery could change how astronomers calculate the mass, age and evolution of galaxies and may even help explain some puzzling observations from the James Webb Space Telescope.
The research focuses on a mathematical tool called the initial mass function, or IMF. Astronomers use it because directly counting every star in a distant galaxy is impossible.
Small, low-mass stars are especially difficult to see because they are much fainter than larger stars.
Instead, scientists count the brighter stars they can observe and use the IMF to estimate how many smaller stars should also be present. For decades, astronomers have generally assumed that the same relationship can be applied across different regions of space.
The new study suggests the universe may be more complicated.
To investigate, the researchers used observations from the European Space Agency’s Gaia mission, which has mapped the positions and other properties of nearly 2 billion stars in our Milky Way galaxy.
The team focused on star clusters, which are groups of stars that formed together under similar conditions. These clusters provide a useful natural laboratory because researchers can compare populations of stars that developed in different environments.
If the traditional assumption were correct, different clusters should contain roughly the same proportions of low-mass and high-mass stars.
But that is not what the researchers found.
Instead, the mix of stars varied significantly among different clusters. This provides direct evidence that local conditions can influence what kinds of stars are produced.
The researchers are not suggesting that astronomers abandon the initial mass function. Instead, they argue that scientists may need different versions of it for different environments.
This could have important consequences for understanding distant galaxies. If astronomers underestimate or overestimate the number of faint stars in a galaxy, their calculations of its total mass could also be wrong.
The finding may be particularly relevant to observations from NASA’s James Webb Space Telescope. Webb has discovered surprisingly bright and apparently massive galaxies that existed relatively early in cosmic history. Some have seemed difficult to explain using existing models of how quickly galaxies could grow.
If stars formed differently in those early environments, however, some galaxies might only appear more massive when calculated using the traditional assumptions.
The research therefore offers a possible way to improve one of astronomy’s most important measuring tools.
Rather than discovering that distant galaxies somehow violate known physics, astronomers may simply need a better cosmic yardstick—one that recognizes that the universe does not make stars exactly the same way everywhere.
