Free Astronomy Magazine March-April 2025

MARCH-APRIL 2025 into the surrounding space. About 350 years later, that pulse of light has reached interstellar material, il- luminating it, warming it, and caus- ing it to glow in infrared light. NASA’s James Webb Space Telescope has observed that infrared glow, re- vealing fine details resembling the knots and whorls of wood grain. These observations are allowing as- tronomers to map the true 3D struc- ture of this interstellar dust and gas (known as the interstellar medium) for the first time. “We were pretty shocked to see this level of detail,” said Jacob Jencson of Caltech/IPAC in Pasadena, principal investigator of the science program. “We see lay- ers like an onion,” added Josh Peek of the Space Telescope Science Insti- tute in Baltimore, a member of the science team. “We think every dense, dusty region that we see, and most of the ones we don’t see, look like this on the inside. We just have never been able to look inside them before.” The team presented their findings in a press conference at the by NASA/ESA/CSA Christine Pulliam O nce upon a time, the core of a massive star collapsed, cre- ating a shockwave that blasted outward, ripping the star apart as it went. When the shock- wave reached the star’s surface, it punched through, generating a brief, intense pulse of X-rays and ul- traviolet light that traveled outward Webb reveals intricate Cassiopeia A light echoes I mage of light echoes near Cassiopeia A captured by Webb’s NIRCam instrument, with compass arrows, scale bar, and color key for refer- ence. The north and east compass arrows show the orientation of the image on the sky. Note that the rela- tionship between north and east on the sky (as seen from below) is flipped relative to direction arrows on a map of the ground (as seen from above). The scale bar is la- beled in light-years, which is the distance that light travels in one Earth-year. (It takes one year for light to travel a distance equal to the length of the bar.) One light-year is equal to about 5.88 trillion miles or 9.46 trillion kilome- ters. This image shows invisi- ble near-infrared wave- lengths of light that have been translated into visible- light colors. The color key shows which NIRCam filters were used when collecting the light. The color of each filter name is the visible light color used to represent the infrared light that passes through that filter. [NASA, ESA, CSA, STScI, Jacob Jenc- son (Caltech/IPAC)]