Order Amongst the Chaos of Apep

According to ancient Egyptian mythology, Ra - the Sun God - was the bringer of light and order, while Apep, his eternal nemesis, embodied chaos and darkness. What the ancients could never have imagined is that all the stars themselves, including the WR 70-16 system - nicknamed ‘Apep’ - are the harbingers of light across the Universe. Nor could they have foreseen that, several thousand years later, humans on their small, insignificant, blue planet would build orbiting telescopes capable of seeing light in invisible wavelengths, like the infrared.

Now, a new study using the JWST and data from the European Southern Observatory’s Very Large Telescope (VLT) in Chile has revealed fresh details about the chaos of light and darkness around the Apep system, offering the clearest view yet of the extraordinary spirals of dust it produces. JWST’s mid-infrared capabilities have uncovered not one, but four nested, corkscrewing dust shells emitted by this rare triple-star system. Before this, the outer shell was only hypothesised.

The new research, co-led by Ryan White, a PhD student at Macquarie University in Sydney and Yinuo Han (an undergraduate from University of Sydney, and currently at Caltech), shows that the two Wolf-Rayet stars in the system orbit one another roughly every 190 years, creating dense spirals of carbon-rich dust during close approaches that last 25 years. A third massive supergiant star, orbiting farther out, is carving cavities into each shell, slicing through the expanding material and leaving a visible signature in each shell.

“I was in disbelief when I first saw the JWST images of Apep,” said White. “I had been looking at the original VLT image for months and months and had gotten so used to seeing the structure of a single shell that now seeing four shells staring back at me was throwing me off - like seeing an old friend with a new and dramatic haircut.”

“The structure of the concentric shells were exactly what we were expecting from our geometric models, just further apart! The spacing between shells was essential in refining our estimate of the orbital period of the stars at the nebula's centre.” 

The new images and refined orbital models confirm that all three stars are gravitationally bound, revealing Apep to be a highly dynamic system that lives up to its mythological name.

“Much of this work would not have been possible without continued monitoring of Apep with VLT / VISIR. Our team had access to 8 years of VISIR data (covering 2016 to 2024), which showed the expansion of the Apep nebula clearly across the four imaging epochs.”

Located several thousand light-years away in the constellation Norma, Apep is a rare kind of celestial object for astronomers to study. In its centre lies a binary pair of Wolf-Rayet stars - massive, evolved stars that have shed their outer hydrogen layers and now burn helium in their cores, producing fierce stellar winds and temperatures exceeding 30,000 K, and in some cases reaching as high as 210,000 K. These stars are among the most luminous and hottest in the Universe, shining with tens of thousands of times the Sun’s brightness.

Only around 1,000 Wolf-Rayet stars are known in our entire Milky Way Galaxy, which hosts hundreds of billions of stars. Rarer still are binary Wolf-Rayet systems - and Apep is currently the only known example in the Milky Way where two Wolf-Rayet stars of these specific types are bound together in a binary. A third massive star, likely a 40 - 50 solar mass supergiant, orbits at a much greater distance, completing the triple system.

“Apep is the only confirmed Wolf-Rayet binary that hosts a WC (carbon-type Wolf-Rayet) star, probably within the whole galaxy since these binary systems are so stonkingly bright that we can see them from many kiloparsecs away,” said White.

“The Wolf-Rayet phase is, at least on astrophysical timescales, an incredibly short phase of stellar evolution of only the most massive stars - those born with masses of about 30 solar masses or more.”

“These stars live for only a few million years, where the Wolf-Rayet phase is roughly the final one hundred thousand years or so of their lives, and to catch two stars in this same, short-lived phase at the same time is incredibly lucky,” he said.

“Massive stellar binaries are quite common - and their binary evolution reasonably well understood - but how to make two Wolf-Rayet stars in the same system at the same time is an unsolved puzzle for astronomers.”

“For these stars to be coincidentally at this stage at the same time is unlikely at best, and so more modelling is needed to backtrack their evolutionary histories to see if they may have interacted in the past.”

The discovery that this third star is gravitationally tied to the central binary engine of this system - and that it disturbs the dust shells being emitted - adds another layer of complexity to Apep’s uniqueness. The combination of the Wolf-Rayet winds, their long orbital period, and the interaction with the third companion makes Apep a one-of-a-kind laboratory for studying stellar death and the shaping of the interstellar medium by these types of stars. 

“Since the discovery paper on Apep, our team knew that there was a third, hot supergiant star that showed up near the Apep Wolf-Rayet binary in the imagery,” said White.

“Confirming that this star was a part of the Apep system and not just a chance alignment is difficult, and previously, we only had hints that it was truly associated with the Apep system.”

“Our identifying the cavity it carved in the innermost shell at least suggested it was close to the Apep binary - 1700 astronomical units at present - but it did not confirm that it was gravitationally bound.”

“The JWST imagery, showing several concentric shells of dust going back roughly 600 years, was our smoking gun. By looking at how the position of the cavity ridges changed over time, we could confirm that this tertiary star is not whizzing by the system, but is moving quite slowly, as you might expect from a star in a stable orbit.”

As the two central Wolf-Rayet stars orbit each other, their stellar winds - travelling at speeds between 2,000 and 3,000 km per second - collide and compress. This turbulent region becomes a hot zone for the formation of dust. Unlike most other Wolf-Rayet binaries, which produce dust for only a few months during their short orbits, Apep forms dust in sustained bursts that last 25 years during each close approach, a process that repeats roughly every 190 years.

These dust-making events produce spirals of carbon-rich material, which cool as they expand and emit faint infrared light. Thanks to the sensitivity of JWST’s Mid-Infrared Instrument, astronomers could trace these shells much farther out than ever before, revealing four concentric spirals that span over 700 years of history of the emissions forming.

“Since both of the stars producing Apep's spiral nebula are Wolf-Rayet stars, the winds that feed the dust are unusually chemically enriched with elements like carbon and nitrogen,” said White.

“When these winds smash together at the wind-collision shock between the Wolf-Rayet stars, you get pockets of higher density mixed material that are shielded from the ionising radiation of the stars and efficiently form fragile carbon dust.”

“Most other Wolf-Rayet colliding wind binaries have a WC star together with a main-sequence O-class star whose wind is not so chemically enriched nor dense; Wolf-Rayet winds can be hundreds or thousands of times denser than that of O stars, and so Apep having two Wolf-Rayet stars means that its dust is the product of two uniquely powerful winds.”

As this dust spreads outward, it’s intercepted by the third star’s powerful stellar wind, which carves a V-shaped cavity into each shell. These "slashes" align in each spiral, confirming the star’s consistent orbital influence over centuries. This interaction paints a picture of a stellar skirmish where winds collide, materials merge and disperse, and chaos reigns in a surprisingly ordered fashion.

“What we think is happening here is that there is a dust-forming shock from Wolf-Rayet binary in Apep, making the dust shells which then go on to expand into the wind of the O supergiant to yield another shock,” said White.

“That is, there isn't just one shock in this contact wind binary, but two together with the O star. The geometry of the cavity is consistent with what we'd expect from the winds of each star.”

“In the study, we make some suggestions as to what physics could be taking place to destroy the dust grains in this cavity region, but more modelling is needed to be sure.”

The dramatic nature of this system in the presence will continue into the future of Apep. Wolf-Rayet stars are the final evolutionary stage of the most massive stars before they explode as core-collapse supernovae. Both stars at the centre of Apep are expected to end their lives in such spectacular detonations, potentially forming black holes.

If the binary remains gravitationally bound after these supernovae, it could become a rare binary black hole system - one that might eventually spiral inward and merge, releasing a burst of gravitational waves detectable by the observatories of the future, much like the LIGO-VIRGO-Kagara observatories observe these events today.

One of the two Wolf-Rayet stars is also rotating rapidly, a key trait of stars thought to be progenitors of long-duration gamma-ray bursts, some of the most energetic explosions in the Universe. If such a burst were directed toward Earth, even from a distance of thousands of light-years, it could have profound implications for our planet’s atmosphere and life. 

Although Apep poses no known threat to Earth, it serves as an invaluable example of the extreme environments and cataclysmic endings faced by massive stars. It also provides a natural laboratory for testing models of dust formation, stellar wind interactions, and the final fates of massive stellar systems.

The battle of chaos and light continues in the darkness of deep space. These days, we don’t rely on ancient Egyptian mythology to tell its story - instead, the signature of two battling Wolf-Rayet stars, revealed through the primary eye of the JWST, shows us the light.

Title video credit: NASA, ESA, CSA, STScI; Simulation: Yinuo Han (Caltech), Ryan White (Macquarie University); Visualization: Christian Nieves (STScI); Image Processing: Alyssa Pagan (STScI)

Sub-section video credit: The University of Sydney