Free Astronomy Magazine March-April 2025

49 MARCH-APRIL 2025 inspire generations to come.” The images from Webb’s NIRCam (Near- Infrared Camera) highlight a phe- nomenon known as a light echo. A light echo is created when a star ex- plodes or erupts, flashing light into surrounding clumps of dust and causing them to shine in an ever-ex- panding pattern. Light echoes at vis- ible wavelengths (such as those seen around the star V838 Monocerotis) are due to light reflecting off of in- terstellar material. In contrast, light echoes at infrared wavelengths are caused when the dust is warmed by energetic radiation and then glows. The researchers targeted a light echo that had previously been ob- served by NASA’s retired Spitzer Space Telescope. It is one of dozens of light echoes seen near the Cas- siopeia A supernova remnant – the remains of the star that exploded. The light echo is coming from unre- lated material that is behind Cas- siopeia A, not material that was ejected when the star exploded. The most obvious features in the Webb images are tightly packed sheets. These filaments show struc- tures on remarkably small scales of about 400 astronomical units, or less than one-hundredth of a light-year. (An astronomical unit, or AU, is the average Earth-Sun distance. Nep- tune’s orbit is 60 AU in diameter.) “We did not know that the inter- T his time-lapse video using data from NASA’s James Webb Space Telescope highlights the evo- lution of one light echo in the vicinity of the super- nova remnant Cassiopeia A. A light echo is created when a star explodes or erupts, flashing light into surrounding clumps of interstellar dust and causing them to shine in an ever-expanding pattern. Webb’s exquisite resolution not only shows incredible detail within these light echoes, but also shows their ex- pansion over the course of just a few weeks – a re- markably short timescale considering that most cosmic targets remain unchanged over a human life- time. [NASA, ESA, CSA, STScI, Jacob Jencson (Cal- tech/IPAC), Joseph DePasquale (STScI)] 245 th meeting of the American As- tronomical Society in National Har- bor, Maryland. “Even as a star dies, its light en- dures—echoing across the cosmos. It’s been an extraordinary three years since we launched NASA’s James Webb Space Telescope. Every image, every discovery, shows a por- trait not only of the majesty of the universe but the power of the NASA team and the promise of interna- tional partnerships. This ground- breaking mission, NASA’s largest in- ternational space science collabora- tion, is a true testament to NASA’s ingenuity, teamwork, and pursuit of excellence,” said NASA Administra- tor Bill Nelson. “What a privilege it has been to oversee this monumen- tal effort, shaped by the tireless ded- ication of thousands of scientists and engineers around the globe. This latest image beautifully captures the lasting legacy of Webb—a keyhole into the past and a mission that will