Dust emanates from the cosmic plumes near the Cassiopeia A supernova remnant in this spectacular image from the James Webb Space Telescope. (Image credit: NASA, ESA, CSA, STScI, J. Jencson (Caltech/IPAC))
What is it: Interstellar medium near the supernova remnant Cassiopeia A.
Location: About 11,000 light years away, in the constellation Cassiopeia.
When published: January 14, 2025.
Why it's remarkable: This stunning set of images from the James Webb Space Telescope (JWST) illustrates the glowing interstellar medium — the gas and dust that fills the space between stars — near the Cassiopeia A supernova remnant. Light from this supernova is thought to have reached Earth in the 1660s. More than 350 years later, its bright light has exposed complex layers of glowing material around the long-dead star.
These images show a fascinating phenomenon called a light echo — faint reflections of the light emitted during a supernova explosion. When a star reaches the end of its life and explodes, it emits powerful radiation that illuminates the surrounding gas and dust, creating an “echo” visible in visible wavelengths. Sometimes, the energetic radiation from a supernova also heats nearby gas and dust in the interstellar medium, causing them to emit their own glow, resulting in a rare type of light echo seen in infrared wavelengths — the type of light that JWST excels at detecting. According to NASA, the infrared light echo in these images actually comes from material behind Cas A, not material ejected during the explosion.
JWST captured three separate images of the Cassiopeia A supernova remnant, showing how the gas and dust components changed over time. Note that the bottom two images are slightly tilted compared to the others due to the tilt of the JWST telescope at the time.
The images reveal a densely packed, sheet-like structure in the interstellar medium that looks a bit like the layers of an onion. These filamentary structures have been captured in unprecedented detail, with measurements taken on scales of about 400 astronomical units (AU), or 400 times the average distance between the Earth and the Sun. Astronomers have previously detected structures in the interstellar medium at distances of parsecs (1 parsec is about 206,000 AU, or 3.2 light years). The discovery that these structures can exist in significantly
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