A pair of stars weighing 93 and 70 solar masses orbit each other every just 3.8 days – providing a rare glimpse into the processes that lead to the formation of binary black holes and gravitational waves
NGC 3603-A1 is a pair of giant stars locked in a rapid 3.8-day orbit. Their dynamical relationships hint at how massive stars evolve and form black holes.
A team of astronomers combined archival data from the Hubble Space Telescope with new observations to obtain precise measurements of the binary star system NGC 3603-A1. One star has a mass of about 93 solar masses, and the other about 70 solar masses. Together, they form one of the most massive binary systems identified in the Milky Way.
This system is unusual because of its orbital speed. The two large stars orbit each other every 3.8 days. For comparison, in the time it takes Earth to complete a full year around the Sun, these stars have completed nearly a hundred orbits. Their close proximity and enormous size create strong interactions that actively shape both stars.
New discovery from old data
Discovering this system required years of investigation and relied on insight from an unexpected source. In the summer of 2020, while working remotely at the Lowell Observatory during the pandemic, student Sarah Bodensky noticed details in old Hubble data that had previously eluded analysis.
"Sarah's work made it possible to advance this project. She noticed something that everyone else missed: 'Some of the spectral features were doubled when the stars were at their largest motions toward us and away from us,'" said Dr. Phil Massey of the Lowell Observatory.
This distinction was important because it revealed the binary nature of what appeared to be a single, faint star. The stars of the system, located in the densely populated star cluster NGC 3603, one of the most active star-forming regions in our galaxy, could only be resolved using Hubble's exceptional brightness.
Young stars impersonating giants
Both stars are so massive and energetic that they mimic Wolf-Eye stars, which are typically older dying giants that have shed their outer layers by powerful stellar winds. But the stars in NGC 3603-A1 are actually still young, exhibiting the extreme conditions that can make massive stars appear much more developed than they actually are.
The interaction between the two stars tells a fascinating story of stellar evolution. The smaller of the two appears to have stolen mass from its larger companion, causing it to spin faster as a result. This type of mass transfer is critical to understanding how massive stars change over time and provides insights into their ultimate fate.
Massive binary systems like NGC 3603-A1 are the source of binary black holes, which can eventually merge and create gravitational waves that scientists have been detecting since 2015. Understanding these connections between stars helps astronomers predict where and when such collisions will occur.
More of the topic in Hayadan:
