Free Astronomy Magazine March-April 2025

MARCH-APRIL 2025 his background image of the region around super- nova remnant Cassiopeia A was released by NASA’s Spitzer Space Telescope in 2008. By taking multiple im- ages of this region over three years with Spitzer, re- searchers were able to exam- ine a number of light echoes. Now, NASA’s James Webb Space Telescope has imaged some of these light echoes in much greater detail. Insets at lower right show one epoch of Webb observations, while the inset at left shows a Webb image of the central supernova remnant released in 2023. [Spitzer Image: NASA/JPL-Caltech/Y. Kim (Univ. of Arizona/Univ. of Chicago). Cassiopeia A Inset: NASA, ESA, CSA, STScI, Danny Milisavljevic (Purdue University), Ilse De Looze (Ghent University), Tea Temim (Princeton Univer- sity). Light Echoes Inset: NASA, ESA, CSA, STScI, J. Jencson (Caltech/IPAC)] T hese shimmering cosmic curtains show interstellar gas and dust that has been heated by the flashbulb explosion of a long-ago supernova. The gas then glows infrared light in what is known as a thermal light echo. As the supernova illumina- tion travels through space at the speed of light, the echo appears to expand. NASA’s James Webb Space Tele- scope observed this light echo in the vicinity of the supernova rem- nant Cassiopeia A three separate times, in essence creating a 3D scan of the interstellar material. Note that the field of view in the top row is rotated slightly clockwise relative to the middle and bottom rows, due to the roll angle of the Webb telescope when the observations were taken. [NASA, ESA, CSA, STScI, Jacob Jencson (Caltech/IPAC)] stellar medium had structures on that small of a scale, let alone that it was sheet-like,” said Peek. These sheet-like structures may be influenced by interstellar magnetic fields. The images also show dense, “This is the astronomical equivalent of a medical CT scan,” explained Armin Rest of the Space Telescope Science Institute, a member of the science team. “We have three slices taken at three different times, which will allow us to study the true 3D structure. It will completely change the way we study the interstellar medium.” The team’s science program also in- cludes spectroscopic observations using Webb’s MIRI (Mid-Infrared In- strument). They plan to target the light echo multiple times, weeks or months apart, to observe how it evolves as the light echo passes by. “We can observe the same patch of dust before, during, and after it’s il- luminated by the echo and try to look for any changes in the compo- sitions or states of the molecules, in- cluding whether some molecules or even the smallest dust grains are de- stroyed,” said Jencson. Infrared light echoes are also ex- tremely rare, since they require a specific type of supernova explosion with a short pulse of energetic ra- diation. NASA’s upcoming Nancy Grace Roman Space Telescope will conduct a survey of the galactic plane that may find evidence of ad- ditional infrared light echoes for Webb to study in detail. tightly wound regions that resemble knots in wood grain. These may rep- resent magnetic “islands” embed- ded within the more streamlined magnetic fields that suffuse the in- terstellar medium. !