Astronomers Reveal One of the Milky Way’s Most Massive Binary Star Systems

FLAGSTAFF, Ariz. — Astronomers at Lowell Observatory have unveiled one of the most massive binary star systems ever discovered in the Milky Way, providing unprecedented insight into the lives and deaths of giant stars.

The system, known as NGC 3603-A1, lies within a spectacular star cluster 25,000 light-years from Earth. Using previously unpublished Hubble Space Telescope (HST) data alongside new high-precision observations, researchers measured the system’s properties with unmatched accuracy. Their results will be published in The Astrophysical Journal.

A Pair of Stellar Heavyweights

NGC 3603-A1 consists of two colossal stars orbiting each other once every 3.8 days. The larger star weighs about 93 times the mass of the Sun, while its companion measures around 70 solar masses. Both stars burn with blistering intensity, releasing powerful winds and radiation that mimic the appearance of Wolf-Rayet stars, a rare and short-lived stellar phase. However, unlike true Wolf-Rayet stars, these giants are still relatively young.

“This is a rare and exciting find,” said Dr. Phil Massey, lead researcher from Lowell Observatory. “These stars are among the heaviest we’ve ever directly measured in our galaxy. Understanding them helps us unlock the story of how massive stars evolve and die.”

Breakthrough Through Archival Data

While astronomers had long suspected the binary nature of NGC 3603-A1, it took a fresh analysis of archival Hubble data to confirm it. Sarah Bodansky, then an undergraduate at Carleton College working remotely with Lowell Observatory in 2020, identified spectral features that revealed the stars’ orbital dance.

Her observation of “doubled” spectral signatures — showing the stars’ greatest movements toward and away from Earth — was key to determining their masses. “Without Sarah’s discovery, this project might never have advanced,” Massey noted.

Stellar Interactions and Cosmic Implications

The research also revealed that the smaller star in the pair appears to have siphoned material from its larger companion, spinning up in the process. Such mass transfer events are crucial for understanding how stars evolve, collapse into black holes, or explode as supernovae.

Massive binary systems like NGC 3603-A1 are also central to the study of gravitational waves — ripples in space-time created when binary black holes merge. This system offers astronomers a valuable look at the early stages of that cosmic journey.

Unlocking the Universe’s Secrets

Because NGC 3603 is densely packed with stars, only Hubble’s sharp vision could separate A1 from its neighbors. The findings not only advance stellar astrophysics but also reinforce the critical role of space telescopes and long-term data archives.

Other contributors to the project include Laura Penny (College of Charleston), Nidia Morrell (Las Campanas Observatory), Kathryn Neugent (Lowell Observatory), and Bodansky, now a Ph.D. student at the University of Massachusetts, Amherst.

About Lowell Observatory

Founded in 1894, Lowell Observatory is one of America’s oldest astronomical institutions and home to landmark discoveries, including the identification of Pluto. Today, its researchers study stars, galaxies, and planets using a mix of ground-based observatories and space telescopes, while welcoming more than 100,000 visitors annually for tours and telescope viewing.