Cassiopeia A’s many faces

Image: NASA/ESA/CSA/STScI/D. Milisavljevic (Purdue University)/T. Temim (Princeton University)/I. De Loose (University of Gent).

This is the James Webb Space Telescope’s (JWST’s) second view of the Cassiopeia A (Cas A) supernova remnant – its first was captured by its Mid-Infrared Instrument (MIRI) in April 2023. This new image was taken by the JWST’s Near-Infrared Camera (NIRCam) and shows significant differences in the details compared to the MIRI image.

Cas A is the remains of a massive star that exploded in the late 17th century, although no one noticed it at the time. Located 11,000 light years away, the bright clumps of orange and pink (both false colours) near the centre are composed of argon, neon, oxygen and sulphur – all innards of the mighty star that turned itself inside-out. In the MIRI image, the outer edge of the main shell of debris is bright, but to NIRCam it appears faint and wispy. The outer edge of the shell is where the supernova blast wave is crashing into circumstellar material expelled by the star before it exploded. The dust in this material is still too cool to be detected at shorter infrared wavelengths.

Also missing in the NIRCam image compared to MIRI’s view is the ‘Green Monster’ – the nickname given to a ring of green light (again, false colour) in the remnant’s central cavity. NIRCam can see ionised gas (the purple and white coloured emission) believed to be caused by the supernova debris pushing past and shaping the circumstellar material to form the Green Monster.

Image: NASA/ESA/CSA/STScI/D. Milisavljevic (Purdue University)/T. Temim (Princeton University)/I. De Loose (University of Gent).