James Webb Telescope: Double Perspective of an Exploding Star

James Webb, the largest telescope ever launched into space, has captured a new ultra-high resolution infrared image of the remnant of the supernova Cassiopeia A. This close-up view reveals details of the expanding shell of matter crashing into the surrounding gas released by the star just before its explosion.

NASA James Webb Space Telescope’s new view of Cassiopeia A (Cas A) captured by **NIRCam**
*Credits: NASA, ESA, CSA, STScI, D. Milisavljevic (University of Purdue), T. Temim (University of Princeton), I. De Looze (University of Gent)*

This is an image captured in infrared wave frequencies, so outside the frequencies visible to the human eye. Since infrared light is imperceptible to the human eye, scientists use color representation to communicate information. NIRCam assigns colors to different filters, each suggesting distinct activities inside Cas A.

Cassiopeia A: a dead star

A supernova remnant, such as Cassiopeia A (Cas A), is what remains after the explosion of a massive star in a supernova. During the explosion, the energy released is so intense that the star can temporarily become brighter than an entire galaxy and the outer layers of the star are ejected into space, forming what we call a Supernova remnant.

Cas A is one such remnant, located in the constellation Cassiopeia. Studying such remnants provides important information on the dynamics of stellar explosions and the formation of elements in the universe.

Cas A is particularly interesting because it is one of the youngest supernova remnants that we can observe from Earth, with the explosion occurring around 340 years ago. It is also one of the best-studied supernova remnants, and over the years several space and ground-based observatories, including NASA/ESA’s Hubble Space Telescope, have contributed to the study of this cosmic object at different wavelengths.

A New Chapter with the James Webb

This is not the first time the James Webb has observed Cas A: in April 2023, Webb’s mid-infrared instrument (MIRI) opened a new chapter in the exploration of Cas A, revealing unexpected features. However, the recent NIRCam image raises questions, as some features visible in MIRI’s infrared spectrum are completely absent. Astronomers are now investigating this discrepancy.

View of Cassiopeia A (Cas A) captured by **MIRI** . *Credits: NASA, ESA, CSA, STScI, D. Milisavljevic (University of Purdue), T. Temim (University of Princeton), I. De Looze (University of Gent)*

When observing the images of Cassiopeia A (Cas A) captured by the James Webb Space Telescope, we notice that although the NIRCam image appears less colorful than the MIRI image, it contains a lot of precious information. This is due to the specific wavelengths emitted by the material that makes up the supernova remnant.

Analyzing the colors in the near-infrared view, we see bright areas of orange and pink representing the inner shell of Cas A. These colors indicate the presence of gas, containing elements such as sulphur, oxygen, argon and neon, from the exploded star. This gas also contains dust and molecules, which will play a role in the formation of new stars and planetary systems.

Comparison of near- and mid-infrared views. Credits: NASA, ESA, CSA, STScI, D. Milisavljevic (University of Purdue), T. Temim (University of Princeton), I. De Looze (University of Gent)

Comparing the NIRCam and MIRI images, we can obtain a deeper understanding of the dynamics of Cas A. We note that in the new image, the inner cavity and outermost shell are curiously colorless.

Colors in Cas A Supernova Remnant

The absence of color in some parts of the NIRCam image highlights the impact zones, where the shock wave of the supernova that started propagating 340 years ago meets the surrounding material ejected by the dying star before the explosion. The dust in the surrounding material is too cold to be detected directly at near-infrared wavelengths, but it glows in the mid-infrared.

In addition, the NIRCam image also reveals areas of white light that the researchers have associated with synchrotron radiation. Synchrotron radiation is also visible in the bubble-like shells in the lower half of the inner cavity. This phenomenon is caused by charged particles moving at high speed along magnetic field lines.

Another difference that immediately catches the eye when looking at the two images is that the beams of green light, that illuminate the central cavity of Cas A in mid-infrared light, are not visible in near-infrared vision. For now, the researchers have found this feature, called the “Green Monster” difficult to understand.

Finally, a point of great interest is the discovery of “Baby Cas A” in the bottom right corner of the NIRCam image. This is a light echo, the distant dust is illuminated and heated by the light of the explosion that occurred centuries ago, the dust is in fact about 170 light years away from the main supernova remnant. In this way, the image gives us a fascinating window into the history and complexity of this celestial event.