Finally, an Explanation for the “String of Pearls” in Supernova 1987A

Not long after the explosion of Supernova 1987a, astronomers were abuzz with predictions about how it might look in a few years. They suggested a pulsar would show up soon and many said that the expanding gas cloud would encounter earlier material ejected from the star. The collision would light up the region around the event and sparkle like diamonds.

Today, astronomers look at the site of the stellar catastrophe and see an expanding, glowing ring of light. Over the years, its shape has changed to a clumpy-looking string of pearls. What’s happening to affect its appearance? The answer lies in something called the “Crow Instability.” We see this aerodynamical process when vortexes off the wingtips of airplanes interact with the contrails from their engines. The instability breaks up the contrail into a set of vortex “rings”.

University of Michigan graduate student Michael Wadas says this type of instability could explain why Supernova 1987a formed a string of pearls. “The fascinating part about this is that the same mechanism that breaks up airplane wakes could be in play here,” said Wadas, who is now doing post-graduate work at CalTech. If that’s true, it will go a long way toward explaining why those ghostly pearls exist.

About 1978a and its String of Pearls

Light and neutrinos from Supernova 1987a reached Earth on February 23, 1987. The original star, Sanduleak -69 202, lay about 168,000 light-years away in the Large Magellanic Cloud. It exploded as Type II, the first one in modern times to show astronomers the details of a core-collapse supernova. Since then, astronomers watched as a ring of ejected material and a shockwave from the explosion itself spread to space. It slammed into the material shed earlier in the star’s life. It does have a neutron star in the center. Astronomers detected it in 2019 and observed it using X-ray and gamma-ray observatories.

Several months after the explosion, astronomers used the Hubble Space Telescope to image bright rings surrounding the explosion site. That material came from the stellar wind of the progenitor star. Ultraviolet light from the explosion ionized the gases in the cloud. The inner ring lay about 2/3 of a light-year from the original star. The expanding ejecta from the supernova eventually collided with it in 2001. That heated it further. The shockwave has now expanded beyond the rings, leaving behind pockets of warm dust and glowing clouds of gas. The turbulence of that shockwave and the damage it did to regions of the inner ring is created the “pearls”.

Competing Theories for the String

So, what physics underlies the appearance of the pearls? Astronomers have tried to explain the string using something called a Rayleigh-Taylor instability. That occurs when two fluids (or plasmas) of different densities interact with each other. Think of oil and water trying to mix, or a heavy pyroclastic flow streaming out of a volcano. The interaction forms interesting and predictable shapes in the fluids. For 1978a, the denser “fluid” is the material ejected during the supernova explosion. It is colliding with a less dense cloud of material ejected earlier that has spread out to space. However, there are issues with using the Rayleigh-Taylor instability to explain what we see at the supernova site.

“The Rayleigh-Taylor instability could tell you that there might be clumps, but it would be very difficult to pull a number out of it,” said Wadas, who suggested the Crow Instability in a paper just published in Physical Review Letters. Jet contrails are a better comparison because the wingtip vortices break up the long smooth line of a jet contrail. The vortices flow into each other, leaving gaps that can be predicted.

To explore that idea, Wadas and his colleagues simulated the way winds push a model cloud outward while also dragging on its surface. The top and bottom of the cloud got pushed out faster than the middle. That caused it to curl in on itself, triggering a Crow Instability that broke the cloud apart into 32 even clumps similar to the string of pearls at 1987a (which has 30-40 clumps). That predictable number of clumps is why the team suggested the Crow Instability as a formation agent for the string. They also think it could help predict the formation of more beaded rings around the explosion site or when dust around a star coalesces to form planets. Recent JWST infrared images seem to show even more clumps that have appeared in the ring, and it will be interesting to see if more of them appear in the future.

For More Information

Explaining a Supernova’s “String of Pearls”
Hydrodynamic Mechanism for Clumping along the Equatorial Rings of SN1987A and Other Stars