The James-Webb telescope reveals spectacular images of the remains of the supernova Cassiopeia A

2023-12-12 14:20:13

Cassiopeia A (Case A) is the remnant of a supernova in the Milky Way in the form of a kind of very hot plasma bubble about 13 light years in diameter and which telescopes in space can observe in the constellation Cassiopeia. The James Webb’s piercing infrared gaze reveals new details about Cas A’s spectacular remains.

Cassiopeia A is a powerful astronomical radio source, second in intensity in a wavelength band after the Sun, which was discovered in 1947 during the rise of radio astronomy. A counterpart in the visible was only discovered in 1950, a supernova remnant forming a sort of bubble of hot plasma at around 30 million degrees and expanding in the Milky Way around 11,000 light years from the Solar System .

Upon studying it more closely, astronomers were amazed because the speed of expansion of the bubble caused by the explosion of a massive star was of the order of 4000−6000 km/s. Knowing its current size and somehow rewinding the film of the explosion, we could deduce that it probably occurred around 1667. However, there is no proof that it was reported by observers on Earth like a new star on the celestial vault. It is assumed that the emitted light must have been strongly absorbed by interstellar dust, producing a light source of magnitude too weak to note despite its proximity to the Sun.

This video presents the NIRCam image (Near-Infrared Camera) of Webb of the remnant of the supernova Cassiopeia A (Case A). NIRCam’s high resolution detects tiny knots of gas resulting from the star’s explosion, as well as light echoes scattered across the field of view. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Automatically translate”. Choose “French”. © Danielle Kirshenblat (STScI),NASA, ESA, CSA, STScI, Danny Milisavljevic (Purdue University), Ilse De Looze (UGent), Tea Temim (Princeton University)

Cassiopeia A, a window on the explosion of stars

The supernova at the origin of Cassiopeia A is called SN 1667, sometimes followed by a question mark (SN 1667?) to mean that the date is not known with certainty. The study of its remains can give us information on the massive stars which are responsible for the nucleosynthesis of many of the elements which make us stardust, as the late Hubert Reeves liked to say.

This remnant has therefore long been one of the objects of choice for astronomers who have already studied it with telescopes in space at various wavelengths, X-rays with Chandra and IXPE, infrared with Spitzer or visible with Hubble. This is the second time today that the gaze of the James Webb Space Telescope (JWST) has been turned towards it by NASA and the ESA, which reveal new images taken on this occasion.

« Thanks to the resolution of NIRCam, we can now see how the dying star completely shattered when it exploded, leaving behind filaments like tiny shards of glass. It’s truly incredible, after all these years studying Case A, to now be able to resolve these details, which provide us with groundbreaking insight into how this star exploded. », explains on this occasion Danny Milisavljevic from Purdue University who leads the research team behind the new false-color images, indicating wavelength bands that allow JWST’s instruments to make observations in infrared invisible to the eye.

Cassiopeia A under the piercing gaze of the JWST

In the image above in four close-ups in the boxes, we can see several interesting features of the remains of the supernova Cassiopeia A, seen with NIRCam (Near-Infrared Camera) by Webb. A NASA press release gives the following descriptions:

In this box, the high resolution power of NIRCam can detect tiny concentrations of gas containing ionized atoms of sulfur, oxygen, argon and neon, originating from the star itself when it was performing nucleosynthesis of elements. Debris filaments too small to be resolved have a diameter comparable to or less than 10 billion kilometers (about 100 astronomical units). Researchers say this represents how the star shattered like glass when it exploded.

The circular holes visible in the boxed image are this time from Miri (Mid-Infrared Instrument) inside what astronomers have called the Green Monster, a loop of green light in the internal cavity of Case A, are slightly outlined by white and purple emission in the NIRCam image — this represents ionized gas. Researchers believe this is due to supernova debris passing through and sculpting gas ejected by flares from the star before it explodes.

This is one of the few light echoes visible in the NIRCam image of Cas A. A light echo occurs when light from the star’s explosion emitted long ago reaches and warms distant dust, which glows as it cools.

NIRCam captured a particularly complex and large light echo, dubbed Baby Cas A by researchers. Measurements indicate that it is located about 170 light years behind the supernova remnant.

The regions scattering with white color betray the light of synchrotron radiation, which is emitted over a large part of the electromagnetic spectrum, including in the near infrared accessible to the JWST. As Nobel Prize winner in Physics Richard Feynman explains in his famous Physics courses, this radiation is generated by charged particles moving at extremely high speeds spiraling around magnetic field lines. Synchrotron radiation is also visible in the bubble-shaped shells located in the lower half of the internal cavity.

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