Researchers Use Simulations to Reveal What Determines the Masses of Stars

STARFORGE, a project creating the most accurate, highest-resolution 3D simulations of star formation to date, was launched last year by a team of astrophysicists that include key participants from Northwestern University. Currently, the researchers have employed extremely thorough simulations to reveal what determines the masses of stars, which is a mystery that has fascinated astrophysicists for several years.

The team found that star formation is a self-regulatory process in a new study. Put differently, the stars set their own mass. This sheds more light on why stars that are formed in disparate environments still possess similar masses. The new result might allow scientists to better comprehend star formation inside the Milky Way and other galaxies.

The research was published in the Monthly Notices of the Royal Astronomical Society. The joint team comprised specialists from Northwestern, University of Texas at Austin (UT Austin), Carnegie Observatories, Harvard University, and the California Institute of Technology. The new study’s lead author is Dávid Guszejnov, who is 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. 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.

Claude-André Faucher-Giguère, Study Co-Author and Associate Professor, Physics and Astronomy, Weinberg College of Arts and Sciences, Northwestern University

Faucher-Giguère is also a member of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA).

Made of cold gas and dust, outer space is packed with massive clouds. To form dense clumps, gravity gradually pulls far-flung fragments of this dust and gas toward one another. Components in these clumps fall inward, crashing and sparking heat to form a newborn star.

A rotating disk of gas and dust surrounds every “protostar.” Each planet in the solar system was once fragments in such a disk surrounding the newborn sun. The star’s mass and the way it was formed determines if planets orbiting a star can host life or not. Hence, to find where life could be formed in the universe, knowledge of star formation is important.

Stars are the atoms of the galaxy. Their mass distribution dictates whether planets will be born and if life might develop.

Stella Offner, Associate Professor, Astronomy, UT Austin

In astronomy, every subfield relies on the stars’ mass distribution or initial mass function (IMF)—which has proved difficult for researchers to model accurately. Stars much larger than the sun are exceptional, which make up only 1% of newborn stars.

For each star, there are maximum of 10 sun-like stars and 30 dwarf stars. Observations discovered that regardless of where it is spotted in the Milky Way, these ratios (i.e., the IMF) are the same for both starts that are billions of years old and for newly formed star clusters.

Stars have relatively simple DNA. If you know the mass of a star, then you know most things about the star.

Claude-André Faucher-Giguère, Study Co-Author and Associate Professor, Physics and Astronomy, Weinberg College of Arts and Sciences, Northwestern University

This is the IMF’s mystery. Every population of stars in the Milky Way, and in every dwarf galaxy that surrounds us, has the same balance—although their stars were formed under wildly diverse circumstances over billions of years. Hypothetically, the IMF should differ dramatically, but it is virtually universal, puzzling astronomers for years.

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

However, the new simulations revealed that stellar feedback, in an attempt to oppose gravity, pushes stellar masses toward the similar mass distribution. In a collapsing giant cloud, these simulations are the earliest to follow the individual stars’ formation, meanwhile also capturing the way these newly formed stars communicate with their environments by supplying light and shedding mass through jets and winds. This phenomenon is known as “stellar feedback.”

The STARFORGE project—a multi-institutional initiative— is co-headed by Guszejnov and Michael Grudić of Carnegie Observatories. Grudić was a CIERA postdoctoral fellow at Northwestern when this project was started.

STARFORGE simulations are the first to model star evolution, formation, and dynamics simultaneously while keeping up with stellar feedback, along with jets, wind, radiation, and surrounding supernovae activity. When individual types of stellar feedback are incorporated by other simulations, STARFORGE puts all of them together to simulate how these different processes communicate to impact star formation.

The association is financially supported 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. The research was accomplished on two supercomputers at UT Austin’s Texas Advanced Computing Center.

Pillars of Creation in STARFORGE [Narrowband, VR, 8k]. Video Credit: Northwestern University

Journal Reference:

Guszejnov. D., et al. (2022) Effects of the environment and feedback physics on the initial mass function of stars in the STARFORGE simulations. Monthly Notices of the Royal Astronomical Society. doi.org/10.1093/mnras/stac2060.