Astronomers Have Been Weighing Distant Galaxies With a Ruler That Changes Length Depending on Where You Are

Astronomers cannot see most of the stars in a distant galaxy. The small, dim ones — which make up the overwhelming majority — are invisible at those distances, so the mass of a galaxy is not measured so much as extrapolated. You count the bright stars you can see, and then you apply a rule that tells you how many faint ones must be there with them. That rule is the initial mass function, and it has been assumed to hold everywhere in the universe for more than 50 years.

University of Missouri researchers report that it does not. In a study titled "Direct evidence for stellar initial mass function variation in the Milky Way," published in The Astrophysical Journal Letters, they find that the ratio of large to small stars depends on the environment in which those stars formed.

"One of astronomy's basic assumptions may be oversimplified," said Charles Steinhardt, an astronomy professor at Mizzou and a co-author of the study. "Other galaxies weren't breaking the laws of physics — we were measuring them with the wrong yardstick."

The test was built on data from the European Space Agency's Gaia mission, which has mapped nearly two billion stars in the Milky Way with precise positions and distances. The team focused on star clusters — groups whose members formed together, at the same time, out of the same gas, under the same conditions. That makes clusters the cleanest available laboratory for the question. If the initial mass function were genuinely universal, every cluster should contain the same mix of stellar masses. The researchers compared clusters against one another and found significant, systematic differences.

The implications run outward from there. Galaxy masses, ages and star-formation histories are all computed through the IMF, which means a variable IMF propagates into every one of those numbers. It may also resolve one of the sharper embarrassments of the past few years: the galaxies the James Webb Space Telescope found in the early universe that appeared too massive to have assembled in the time available. If those galaxies formed stars in different proportions than the standard rule assumes, they were never too heavy — the conversion was wrong.

The team is not proposing to discard the IMF. They are proposing to condition it. "The pattern we found is surprisingly clean," said Carter Meyerhoff, an undergraduate researcher and co-author. "Instead of applying the same model to every galaxy, astronomers could account for the conditions under which stars formed and select the IMF that best matches that environment."

That is a considerably larger amount of work than using one function for everything. It is also the difference between a measurement and an assumption.