Most Close Pairs of Stars Are Born as Cosmic Twins

New research from ALMA suggests disk fragmentation may form close-companion protostellar systems

A new study of infant stars in the Perseus and Orion star-forming regions suggests that most close pairs of stars are born as twins in the same disk, rather than drifting together later from larger distances. By watching powerful streams of gas blasting away from baby stars, a team of researchers has shown that most close pairs of stars likely form side-by-side in the same spinning disk of gas and dust.

Many stars in our galaxy don't live alone like the Sun. Roughly half of Sun-like stars are part of a pair or even a small family of stars that orbit each other. Young stars are even more likely to have companions, which tells astronomers that forming in multiples is a normal part of how stars are born.

What hasn't been clear is how close pairs of stars-separated by only a few times the width of our solar system-actually come together. Do they form together in the same disk of gas and dust, or do they start far apart and slowly move closer over time?

This new research, led by undergraduate student Ryan Sponzilli of the University of Illinois Urbana-Champaign, tests two leading ideas for how close-companion protostars form:

1. A single, massive disk of gas and dust around a newborn star becomes unstable and breaks into two or more clumps, each collapsing to form a star. This disk fragmentation tends to produce close pairs in an organized, aligned configuration.
2. Turbulence in a larger cloud core causes it to break into widely separated clumps that form stars far apart, which are later pulled inward through complex gravitational interactions. This process of turbulent fragmentation and migration should leave stellar spins and orbits in more random orientations.

"Figuring out which process is more common in the formation of these 'twins' will help us understand more about not only stars, but also what kinds of planetary systems might form around them," shares Sponzilli.

To test these ideas, the research team studied 51 very young protostellar star systems that host close companion stars in the Perseus and Orion molecular clouds, some of the nearest stellar nurseries to Earth. ALMA observations mapped both the dust surrounding the stars and jets of molecular gas blasting away from them.

In 38 of the systems, fast, narrow streams of outflowing gas were clearly observed. These outflows show which way the system is spinning. The outflows usually shoot out at right angles to the disk of material around each star, so their direction is a good guide to how the system is oriented in space.

The researchers compared the direction of each outflow to the line connecting the two stars in a pair. This let them work out whether the system looked organized, as expected if the stars formed together in a disk, or more random, as expected if they formed separately and later moved closer.

The team also built simple computer models of what they should see in the sky for each of the two formation scenarios. When they compared these models to their 42 outflow measurements, the real data matched best with a picture where the outflows tend to line up at right angles to the line between the stars, which is expected if the stars formed together in a single disk.

"The results point to disk fragmentation as the main way that close pairs of baby stars form, at least in the young regions studied here," adds co-author Leslie Looney, Sponzilli's professor at the University of Illinois Urbana-Champaign.

By showing that many close stellar twins are likely born together in a single spinning disk, this study strengthens the link between the earliest stages of star formation and the later evolution of planetary systems around multiple stars. Understanding these early alignments will help astronomers predict how common aligned planetary orbits might be in binary systems and how stable those planetary systems can become over time.

About NRAO

The National Radio Astronomy Observatory (NRAO) is a facility of the U.S. National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

About ALMA

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Southern Observatory (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science and Technology Council (NSTC) in Taiwan and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

This news article was originally published on the NRAO website on April 13, 2026.