Once upon a time, the core of a massive star collapsed, creating a shockwave that blasted outward, tearing the star apart as it passed. When the shockwave reached the star’s surface, it erupted from it, creating a brief, intense pulse of X-ray and ultraviolet light that traveled outward into surrounding space. About 350 years later, that pulse of light has reached interstellar matter, illuminating it, heating it, and making it glow in infrared light.

NASA’s James Webb Space Telescope has seen that infrared light, revealing fine details like knots and swirls in the grain of wood. The observations are allowing astronomers to map the true 3D composition of this interstellar dust and gas (known as the interstellar medium) for the first time.

‘We were really surprised to see this level of detail,’ said Jacob Jensen of Caltech/IPAC in Pasadena, principal investigator of the science program.

‘We see layers like an onion,’ said science team member Josh Peake of the Space Telescope Science Institute in Baltimore. ‘We think every dense, dusty region we see, and most of the ones we don’t, looks like this from the inside. We’ve never been able to see inside them before.’

The team is presenting their findings at a press conference at the 245th meeting of the American Astronomical Society in Washington.

‘Even when a star dies, its light lives on – echoing throughout the universe. It has been an extraordinary three years since we launched NASA’s James Webb Space Telescope. Every image, every discovery, paints a picture not only of the splendor of the universe but also of the power of NASA’s team and the promise of international partnership. NASA’s largest international space science collaboration, this unprecedented mission is a true testament to NASA’s ingenuity, teamwork, and pursuit of excellence,’ said NASA Administrator Bill Nelson. ‘What a privilege it is to observe this magnificent endeavor, shaped by the tireless dedication of thousands of scientists and engineers around the world. This latest image beautifully encapsulates Webb’s enduring legacy – a fortress of the past and a mission that will inspire generations to come.’

Images from Webb’s NIRCam (Near-Infrared Camera) produce light echoes in a phenomenon known as light echoes. When a star explodes or collapses, a light echo is created, which bounces light off surrounding dust clouds and causes them to glow in an ever-expanding pattern. Light echoes at visible wavelengths (such as those seen around the star V838 Monocerotis) are caused by light reflecting off interstellar matter. In contrast, light echoes at infrared wavelengths occur when dust is heated by energetic radiation and then glows.

The researchers targeted a light echo that was previously observed by NASA’s retired Spitzer Space Telescope. It is one of dozens of light echoes seen near the Cassiopeia A supernova remnant — the remains of an exploded star. The light echo is coming from unrelated matter behind Cassiopeia A, not from material ejected by the star’s explosion.

The most obvious features in the Web images are the tightly packed sheets. These filaments show structure on a remarkably small scale, less than about 400 astronomical units, or one-hundredth of a light-year. (An astronomical unit, or AU, is the average Earth-Sun distance. Neptune’s orbit is 60 AU in diameter.)

‘We didn’t know that the interstellar medium had structure on such a small scale, let alone that it was sheet-like,’ Peake said.

These sheet-like structures could be affected by interstellar magnetic fields. The images also show dense, tightly wound regions that look like knots in the grain of wood. These could indicate magnetic ‘islands’ filling the interstellar medium in more orderly magnetic fields.

‘This is the astronomical equivalent of a medical CT scan,’ explained science team member Armin Rest of the Space Telescope Science Institute. ‘We have three slices taken at three different times, which allows us to study the true 3D structure. It will completely change the way we study the interstellar medium.’

Future Work
The team’s science program also includes spectroscopic observations using Webb’s MIRI (Mid-Infrared Instrument). They plan to target the light echo multiple times, weeks or months apart, to see how the light echo evolves as it travels.

‘We can observe the same dust particle before, during and after it is illuminated by the echo and try to detect any changes in the structure or state of the molecules, including whether some molecules or even tiny dust particles are destroyed,’ Jensen said.

Infrared light echoes are also extremely rare, because they require a specific type of supernova explosion with a short pulse of energetic radiation. NASA’s upcoming Nancy Grace Roman Space Telescope will conduct a survey of the galactic plane that will allow Webb to find evidence of additional infrared light echoes for detailed study.

The James Webb Space Telescope is the world’s premier space science observatory. Webb is unraveling the mysteries of our solar system, looking at distant worlds around other stars, and exploring the mysterious structure and origin of our universe and our place in it. Webb is an international program led by NASA that includes its partners, ESA (the European Space Agency) and CSA. Yes.