The James Webb Space Telescope has used its infrared vision to dissect the dusty remains of a storm that erupted in Apep, an extreme Wolf-Rayet binary system.
The newly discovered companion is a hulking blue supergiant, and its existence shifts the way astronomers think about how massive stars mature, cast off material, and prepare the stage for cataclysmic explosions.
Apep reveals a secret third star bound to its core
Scientists sifting through Webb data announce that a third star, gravitationally bound, orbits the duo of Wolf-Rayet stars at Apep’s core. This upstart is probably about 40 to 50 times the Sun’s mass. On the other hand, the central binary — once an even more colossal pair — is now about 10 to 20 solar masses after having lost enormous amounts of material.
Wolf-Rayet stars are vanishingly rare. According to tallies collected by professional catalogs and NASA, the Milky Way probably holds only about 1,000 of them, a mere fraction of its hundreds of billions of stars. Still more unusual are binaries in which both stars are Wolf-Rayets; Apep is the most remarkable such system. The Webb observations confirm that this already odd system has been hiding a third player all along.
Dust shells chart a tightly knit 193-year orbit precisely
Webb’s mid-infrared imaging unveiled a stack of four nested shells around the pair, created from fierce stellar winds colliding and squeezing carbon dust. Instead of one fog, the dust materializes in consecutive waves — like a pond frequently disturbed. Each shell retains its complex structure as it expands outward to nearly 2 light-years from the central star.
By recording this separation and how fast the shells were expanding — a pell-mell 1,200 to 2,000 miles per second — they inferred that the central stars must complete an orbit around each other about every 193 years.
Mild asymmetries in the shells suggest some kind of oblong orbit, and winds that are stronger in one direction than another. These discrepancies create a natural laboratory in which to test models of colliding-wind physics.
The third star carves a recurring wedge through the dust
The outer companion doesn’t only trail along — it leaves a distinctive mark. As the dust waves spread, the supergiant runs into them and clears a neat, V-shaped void that returns in each layer at the same location. That repeating notch was the “smoking gun” that the researchers needed to prove that this star is really part of the system, rather than just a background object by chance.
How Webb observed Apep and why these findings matter
Described in two studies in The Astrophysical Journal, the effort takes advantage of Webb’s Mid-Infrared Instrument and Near-Infrared Camera, which were built with our international partners NASA–ESA–CSA.
The telescope’s exceptional sensitivity to warm dust and to fine structure allowed Apep’s architecture to come into sharp view, after years of alluring but incomplete glimpses via ground-based observatories.
Triple systems of massive stars are relatively common, but mapping their dynamics directly is challenging. “With Apep, we now have a very short registry of benchmark systems where the astronomers can actually see — and follow over time — dust production get turned on and off with orbital motion, use that to calibrate mass-loss rates and hone models for what happens in these stars’ lives,” Crowther said. For context, Webb has made analogous “dust clock” measurements for the Wolf-Rayet system WR 140 in which a chain of concentric arcs that wax and wane with time exposed discrete episodes of dust-making. Apep takes that idea to an even more extreme regime.
Distance estimates for Apep and the next steps ahead
Brightness measurements, dust temperatures, and the separation of shells indicate that Apep lies farther away than previously thought, most likely some 15,000 light-years from Earth. The final big uncertainty is nailing down the distance; revised astrometry and re-imaged Webb data tracking shell motion will constrain the estimate. High-resolution spectroscopy will additionally nail down the stars’ masses and shapes of their orbits.
However, the final numbers shake out, Apep is already rewriting the rulebook for massive-star systems. Its triple-star minuet, dust-making shocks, and century-long strut provide an unparalleled glimpse into the short, violent prologue to supernova — and a reminder that some of the brightest objects in the night sky can still contain mysteries hiding in plain sight.
