Stunning Pinwheel Nebula Is a Cosmic Cataclysm in the Making

By George Dvorsky on at

This image of a dusty, gas-rich nebula looks pretty, but appearances can be deceiving. Known as a Wolf-Rayet star system, it’s poised to unleash a catastrophic gamma-ray burst when it finally goes supernova. What’s remarkable about this particular Wolf-Rayet system, however, is that it’s the first to be discovered in our own galaxy. Cue the ominous music...

This Wolf-Rayet star system is formally known as 2XMM J160050.7-514245, but to the researchers who recently investigated this enigmatic object, it’s simply “Apep” – an exotic object named for the serpentine ancient Egyptian god of chaos. In a press release, Joseph Callingham, the lead author of the new study and an astronomer at the Netherlands Institute for Radio Astronomy (ASTRON), said “it’s the first such system to be discovered in our own galaxy” – a system he never expected to find “in our own backyard.” The details of this research were published today in Nature Astronomy.

Indeed, astronomers have observed Wolf-Rayet stars before, but only in other galaxies. These massive star systems are on the verge of entering into their death throes, at which time they’ll generate a type of supernova that emits an extremely powerful and narrow jet of plasma – the dreaded gamma-ray burst.

Apep is one such gamma-ray progenitor system, featuring a massive triple star system at its core – a binary pair and a lone star – and vast spiral arms composed of gas and dust. The system is located around 8,000 light-years from Earth, which is uncomfortably close given its explosive potential.

“This was a very fun project to do in some ways, in the sense that Joe found this object and first showed it to me in 2012 when we were officemates as undergrads in Sydney – and it took us six years to gather all the data to reveal this surprising story,” Benjamin Pope, a NASA Sagan fellow at New York University’s Center for Cosmology and Particle Physics and a co-author of the new study, told Gizmodo. “Sometimes science is slow! But I remember when last year, the day before my PhD defence in Oxford, he was visiting and showed me the picture of the Apep spiral – I literally gasped, it was so shocking. There’s really nothing quite like this.”

Using the VISIR mid-infrared camera on the European Southern Observatory’s Very Large Telescope, Pope, Callingham, and their colleagues measured the velocity of the dust within the spiral arms. At this end-stage of their brief life cycle (these systems only last a few hundred thousand years – a blink of the eye in cosmological terms), stars within Wolf-Rayet systems spin rapidly, producing stellar winds that move at horrendous speeds. These winds carry significant portions of stellar material into space, and they’re responsible for forming the majestic plumes of dust particles. In the case of Apep, its spiral arms measure several light-years across.

By measuring the rotational speeds within this nebula, the researchers concluded that at least one of the three stars within the system is spinning fast enough such that it’ll trigger a long-duration gamma-ray burst when it finally explodes as supernova (the exact timing is still impossible to predict). The speed of gas within the nebula was clocked at 12 million kilometres per hour, but the dust is moving at “just” 570 million kilometres per hour. The researchers say this discrepancy is indicative of a star approaching near-critical rotation.

“Apep’s dust pinwheel moves much slower than the wind in the system,” said Callingham. “One way this can occur is if one of the massive stars is rotating so quickly that it is nearly tearing itself apart. Such a rotation means that when it runs out of fuel and begins to explode as a supernova, it will collapse at the poles before the equator, producing a gamma-ray burst.”

The significance of this finding, said Pope, is that nobody had observed rapidly rotating Wolf-Rayet systems in our galaxy before. Moreover, many astronomers assumed these objects couldn’t even exist in a galaxy like ours; the Milky Way is old and metal-rich, containing an abundance of heavy stars that should spin down quickly. The new result suggests our understanding of how massive stars die is still incomplete.

“Wolf-Rayet star systems are thought to be the progenitors of long gamma-ray bursts, so if there’s one in our galaxy that’s an exciting find,” Pope told Gizmodo. “Even if not – something deeply weird is happening to this star system and this is the best explanation we have.”

Artist’s impression of a gamma-ray burst. (Image: NASA/Dana Berry/Skyworks Digital)

As noted, gamma-ray bursts are among the most powerful explosions known to astronomers. Lasting from between two seconds and a few hours, long-duration gamma-ray bursts release as much energy as the Sun does over the course of its entire lifetime. Disturbingly, some scientists theorise that the Ordovician-Silurian extinction – a mass extinction event that happened on Earth some 440 million years ago – was caused by a gamma-ray burst within our own galaxy. Physicist Adrian Melott from the University of Kansas speculates that a “dangerously near GRB should occur on average two or more times per billion years.”

Pope said it would be “pretty bad” if one were to go off nearby, but he’s not particularly concerned.

“In terms of why we have nothing to worry about, the best I can offer is that it’s highly uncertain whether Apep will go off as a gamma-ray burst at all, and if it does, it is unlikely to be in the very near future.”

As another point of encouragement, gamma-ray bursters are highly directional, spewing their concentrated, high-energy rays in a specific direction. So for Apep to pose a threat, it would not only have to go supernova, it would also have to be pointed in our general direction.

Regardless, there’s nothing we can really do about it except to learn more about Wolf-Rayet systems. It might also be useful to speak the ancient Egyptian incantation to rid the world of Apep’s destructive powers:

Spitting upon Apep, Defiling Apep with the left foot, Taking a lance to smite Apep, Fettering Apep, Taking a knife to smite Apep, Putting fire upon Apep... [Nature Astronomy]