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News — For nearly six months, during the year 1181, people looked up to the skies to find a new star glittering in the constellation Cassiopeia. Chinese and Japanese astronomers recorded the rare event, an explosion of a star, or supernova. 

But it was not until 2013 that the remains of the explosion were finally found. As part of a citizen scientist project, amateur astronomer Dana Patchick—who had sifted through images taken by the now-retired Wide-field Infrared Survey Explorer, or WISE—found a nebula at the site where the supernova had occurred.

Further observations convinced astronomers that this nebula, called Pa 30, was in fact the leftover ejected material from the 1181 supernova. Later, in 2023, strange filaments emanating from the supernova remnant were discovered, which resemble the wispy petals of a dandelion flower. 

Now, with the help of the Caltech-built Keck Cosmic Web Imager at the W.M. Keck Observatory in Hawai‘i, astronomers have mapped the location and speed of those unusual filaments in three dimensions for the first time. 

The results are described in a new paper in The Astrophysical Journal Letters. The co-lead authors are Tim Cunningham, a NASA Hubble Fellow at the Center for Astrophysics |Harvard & Smithsonian (CfA) and Ilaria Caiazzo, a former Caltech postdoctoral scholar who recently became an assistant professor at the Institute of Science and Technology Austria.

"A standard image of the supernova remnant would be like a static photo of a fireworks display," says co-author Caltech Professor of Physics Christopher Martin, who led the team that built KCWI. "KCWI gives us something like a 'movie' since we can measure the motion of the explosion's embers as they streak outward from the central explosion." 

The 1181 supernova is thought to have occurred when a thermonuclear explosion was triggered on a dense, dead star called a white dwarf. Typically, the white dwarf would be completely destroyed in this type of explosion, but in this case some of the star survived, leaving behind a sort of "zombie star." This type of partial explosion is called a Type Iax supernova. "Because this was a failed explosion, it was fainter than normal supernovae, which has been shown to be consistent with the historical records," Caiazzo says. 

Astronomers know that the material in the nebula surrounding the remaining star was ejected in the explosion, but how the peculiar filaments formed is unclear.

To probe the three-dimensional structure of the supernova remnant, the astronomers turned to KCWI, an instrument that can capture spectral information for every pixel in an image. This enabled the team to measure the motions of the filaments poking out from the center of the explosion and ultimately create a 3-D map of the structure. Light from material that is flying toward us will be shifted toward the blue end of the spectrum (blue-shifted), while light from material moving away from us will be shifted towards the red end of the spectrum (red-shifted). 

This is analogous to the doppler shift one can hear as a blaring firetruck races by. As the vehicle moves toward us, the sound waves from its horn become squeezed into higher frequencies; as the truck moves away from us, the sound waves become elongated to lower frequencies.

Specifically, the "red arm" of the KCWI instrument, which was installed at Keck last summer was used for this study. KCWI consists of two halves: one captures light wavelengths at the blue end of the visible spectrum, and the other half covers the red end of the spectrum, in addition to infrared light. 

"The addition of the red arm more than doubled the spectral coverage of KCWI and made these observations possible," says Caltech graduate student and co-author Nikolaus Prusinski. "This 3-D map comprises the most sensitive spatial and spectral measurements of Pa 30 to date and holds the current record for the largest contiguous region surveyed with the red channel."

The results showed that the material is speeding along at approximately 1,000 kilometers per second. 

"We find the material in the filaments is expanding ballistically," says Cunningham. "This means that the material has not been slowed down, or sped up since the explosion. From the measured velocities, looking back in time you can pinpoint the explosion to almost exactly the year 1181." 

The 3-D information also revealed a large cavity inside the spindly, spherical structure, in addition to some evidence that the supernova explosion of 1181 occurred asymmetrically. 

As to how the filaments formed after the blast, the scientists are still puzzled. "A reverse shock wave may be condensing surrounding dust into filaments, but we don't know yet," says Cunningham. "The morphology of this object is very strange and fascinating."

The paper titled "Expansion properties of the young supernova type Iax remnant Pa 30 revealed," was funded by NASA, the National Science Foundation, and the National Fund for Scientific and Technological Development in Chile (FONDECYT). 

The CfA’s David Charbonneau, John Raymond, and Scott Kenyon are also co-authors on this paper. Other authors include graduate student Nikolaus Prusinski, Professor of Theoretical Astrophysics James Fuller, George Ellery Hale Professor of Astronomy and Planetary Science Shri Kulkarni, Software Group Lead James (Don) Neill, graduate student Zeren Lin, Senior Instrument Scientist Mateusz Matuszewski of Caltech; Paul Duffell of Purdue University; Odette Toloza of the Federico Santa Maria Technical University; Rosalie McGurk of the Keck Observatory; Abigail Polin of the Observatories of Carnegie Institution for Science; and Phillippe Yao of Princeton University.