A Stellar Explosion Mimicked the End of the Death Star

In 1997, George Lucas released the special edition remaster of his Star Wars trilogy, making changes to the special effects that both delighted and divided fans worldwide (did Han shoot first?). Among the myriad additions was a visually spectacular ring-shaped shockwave emanating from the exploding Death Star, and Alderaan too. Star Trek fans will delightedly point out that this particular special effect had previously been used in the 1991 film Star Trek VI: The Undiscovered Country, in which the Klingon moon Praxis explodes, giving the effect its name: the Praxis ring. Hollywood lore suggests Lucas was so impressed with the effect in that film that he added it to Star Wars too.

Praxis rings look cool on the silver screen, but we all know explosions in space are supposed to be spherical. With no preference for up, down, left, or right, the material blown off an exploding star should push outward in all directions.

Except when it doesn’t. A paper published at the end of March described the flattest explosion ever observed, with a disc of material ejecting out from the core along one plane, just like the Praxis effect.

The explosion is AT2018cow, nicknamed ‘The Cow’ because of the letters in its procedurally generated designation. The Cow is a Fast Blue Optical Transient (FBOT), a rare class of explosion of which only five have ever been seen. FBOTs tend to be in the blue end of the spectra range, and they happen quickly on astronomical time scales: The Cow exhausted most of its energy in 16 days. But FBOTs are also incredibly bright – brighter than most supernovas.

The Location of AT2018cow within its galaxy in the constellation Hercules, as seen by the Sloan Digital Sky Survey. Credit: SDSS

Justyn Maund, the lead author of the paper, said in a press release that “very little is known about FBOT explosions – they just don’t behave like exploding stars should, they are too bright and they evolve too quickly. Put simply, they are weird, and this new observation makes them even weirder.”

At 180 million light years away, The Cow, which was observed in 2018, is the nearest FBOT ever seen. Maund’s team has been pouring over the observation data, and by studying the polarization of the light received from the explosion, they were able to discern details about its shape.

Polarization is the property of light waves that describes the orientation of the waves. Sunglasses use this principle to filter out sunlight by blocking waves in one orientation while allowing waves in other orientations to pass through.

In a stellar explosion, a uniform, spherical debris cloud will produce light that is not polarized in any particular direction, effectively being canceled out across the sphere. But if there are irregularities in the shape of the debris cloud, the light will be partially polarized.

Observations of The Cow show a bright peak in polarization 5.7 days after the start of the explosion, which declined rapidly within a day, followed by a second peak on day 12. “Such a high polarization requires an extremely aspherical geometry,” the paper suggests, “such as shock breakout through an optically thick disk.”

Artist Rendering of AT2018cow, showing a flat disc-shaped explosion. Image credit: Philip Drury, University of Sheffield.

In other words, The Cow seems to have exploded like Praxis.

What causes FBOTs like The Cow? It’s going to take more work to find out for sure. New sky survey instruments like the Vera Rubin Observatory (slated for first light later this year) will be able to capture a bigger sample size of FBOTs and make the task of characterizing them easier.

For now, Maund and his team think that the most likely explanation is that there is an accreting object like a neutron star or black hole at the center of these objects driving the explosion, but the exact mechanism is unclear. It does not appear to be a typical supernova – the collapse of a star into a black hole or neutron star – because there is a lack of radioactive nickel, which supernovas usually produce in great quantities.

The Cow isn’t the only explosion to produce ring shapes. Supernova 1987a is seen here in x-ray and visible light. A similar mechanism may be at work. Image Credit: X-ray: NASA/CXC/U.Colorado/S.Zhekov et al.; Optical: NASA/STScI/CfA/P.Challis.

But there are clues that suggest that the explanation for the disc shape lies in what happened before the explosion. Astronomers have seen similar, though not identical, ring structures in some supernovae. During the last stages of a star’s life, it can grow into a blue supergiant, and the speed of its solar wind picks up dramatically. The fast wind collides with the slower wind, piling up material and grouping it around the poles and equator due to the star’s magnetic field. Later, when the star explodes, the explosion impacts the concentrated material, lighting it up in visible rings.

Whether the disc shape of The Cow is the result of a similar mechanism remains to be seen. Maund thinks it might also be a failed supernova, or the result of disruptions as a star passes a black hole.

Either way, its shape is highly unusual, but not impossible. So the next time you watch Star Wars, you’ll have to resist the urge to tell your friends that the Death Star should have a spherical explosion. If The Cow tells us anything, it’s that sometimes space explosions are just weird like that.

Learn More:

Scientists observe flattest explosion ever seen in space.”The University of Sheffield.

J. Maund et al. “A flash of polarized optical light points to an aspherical ‘cow’Monthly Notices of the Royal Astronomical Society.