Stars determine their own masses

Last year, a team of astrophysicists including key members from Northwestern University launched STARFORGE, a project that produces the most realistic, highest-resolution 3D simulations of star formation to date.

Now, the scientists have used the highly detailed simulations to uncover what determines the masses of stars, a mystery that has captivated astrophysicists for decades.

In a new study, the team discovered that star formation is a self-regulatory process. In other words, stars themselves set their own masses. This helps explain why stars formed in disparate environments still have similar masses. The new finding may enable researchers to better understand star formation within our own Milky Way and other galaxies.

The study was published last week in the Monthly Notices of the Royal Astronomical Society. The collaborative team included experts from Northwestern, University of Texas at Austin (UT Austin), Carnegie Observatories, Harvard University and the California Institute of Technology. The lead author of the new study is Dávid Guszejnov, a postdoctoral fellow at UT Austin.

“Understanding the stellar initial mass function is such an important problem because it impacts astrophysics across the board — from nearby planets to distant galaxies,” said Northwestern’s Claude-André Faucher-Giguère, a study co-author. “This is because stars have relatively simple DNA. If you know the mass of a star, then you know most things about the star: how much light it emits, how long it will live and what will happen to it when it dies. The distribution of stellar masses is thus critical for whether planets that orbit stars can potentially sustain life, as well as what distant galaxies look like.”

Faucher-Giguère is an associate professor of physics and astronomy in Northwestern’s Weinberg College of Arts and Sciences and a member of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA).

Outer space is filled with giant clouds, consisting of cold gas and dust. Slowly, gravity pulls far-flung specks of this gas and dust toward each other to form dense clumps. Materials in these clumps fall inward, crashing and sparking heat to create a newborn star.

Surrounding each of these “protostars” is a rotating disk of gas and dust. Every planet in our solar system was once specks in such a disk around our newborn sun. Whether or not planets orbiting a star could host life is dependent on the mass of the star and how it formed. Therefore, understanding star formation is crucial to determining where life can form in the universe.

“Stars are the atoms of the galaxy,” said Stella Offner, associate professor of astronomy at UT Austin. “Their mass distribution dictates whether planets will be born and if life might develop.”

Every subfield in astronomy depends on the mass distribution of stars — or initial mass function (IMF) — which has proved challenging for scientists to model correctly. Stars much bigger than our sun are rare, making up only 1% of newborn stars. And, for every one of these stars there are up to 10 sun-like stars and 30 dwarf stars. Observations found that no matter where we look in the Milky Way these ratios (i.e., the IMF) are the same, for both newly formed star clusters and for those that are billions of years old.

This is the mystery of the IMF. Every population of stars in our galaxy, and in all the dwarf galaxies that surround us, has this same balance — even though their stars were born under wildly different conditions over billions of years. In theory, the IMF should vary dramatically, but it is virtually universal, which has puzzled astronomers for decades.

“For a long time, we have been asking why,” Guszejnov said. “Our simulations followed stars from birth to the natural endpoint of their formation to solve this mystery.”

The new simulations, however, showed that stellar feedback, in an effort to oppose gravity, pushes stellar masses toward the same mass distribution. These simulations are the first to follow the formation of individual stars in a collapsing giant cloud, while also capturing how these newly formed stars interact with their surroundings by giving off light and shedding mass via jets and winds — a phenomenon referred to as “stellar feedback.”

The STARFORGE project is a multi-institutional initiative, co-led by Guszejnov and Michael Grudić of Carnegie Observatories. Grudić was a CIERA postdoctoral fellow at Northwestern when the project was initiated. STARFORGE simulations are the first to simultaneously model star formation, evolution and dynamics while accounting for stellar feedback, including jets, radiation, wind and nearby supernovae activity. While other simulations have incorporated individual types of stellar feedback, STARFORGE puts them all together to simulate how these various processes interact to affect star formation.

The collaboration is funded by the National Science Foundation, NASA, the Research Corporation for Science Advancement, the Extreme Science and Engineering Discovery Environment, CIERA and Harvard’s Institute for Theory and Computation. Research was completed on two supercomputers at UT Austin’s Texas Advanced Computing Center.

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