Before a massive star explodes as a supernova, it convulses and sends its outer layers into space, signalling the explosive energy about to follow. When the star does explode, it sends a shockwave out into its own ejected outer layer, lighting it up as different chemical elements shine with different energies and colours. Intermingled with this is any pre-existing matter near the supernova. The result is a massive expanding shell with filaments and knots of ionized gas, populated by even smaller bubbles.
“With NIRCam’s resolution, we can now see how the dying star absolutely shattered when it exploded, leaving filaments akin to tiny shards of glass behind.”
Danny Milisavljevic, Purdue University
Cassiopeia A exploded about 10,000 years ago, and the light may have reached Earth around 1667. But there’s much uncertainty, and it’s possible that English astronomer John Flamsteed observed it in 1680. It’s also a possibility that it was first observed in 1630. That’s for historians to determine.
But whenever the exact date is, the light has reached us and continues to reach us, making Cassiopeia A an object of astronomical fascination. It’s one of the most-studied SNRs, and astronomers have observed it in multiple wavelengths with different telescopes.
The SNR is about 10 light-years across and is expanding between 4,000 and 6,000 km/second. Some outlying knots are moving much more quickly, with velocities from 5,500?14,500 km/s. The expanding shell is also extremely hot, at about 30 million degrees Kelvin (30 million C/54 million F.)
The JWST’s NIRCam high-resolution image of Cass. A reveals intricate detail that remains hidden from other telescopes. Image Credit: NASA, ESA, CSA, STScI, Danny Milisavljevic (Purdue University), Ilse De Looze (UGent), Tea Temim (Princeton University)
But none of our prior images are nearly as breathtaking as these JWST images. These images are far more than just pretty pictures. The cursive swirls and knotted clumps of gas reveal some of nature’s detailed interactions between light and matter.
The JWST sees in infrared, so its images need to be translated for our eyes. The wavelengths the telescope can see are translated into different visible colours. Clumps of bright orange and light pink are most noticeable in these images, and they signify the presence of sulphur, oxygen, argon, and neon. These elements came from the star itself, and gas and dust from the region around the star are intermingled with it.
The image below highlights some parts of the Cassiopeia A SNR.
1 shows tiny knots of gas comprised of sulphur, oxygen, argon, and neon from the star itself. 2 shows what’s known as the Green Monster, a loop of green light in Cas A’s inner cavity, which is visible in the MIRI image of the SNR. Circular holes are outlined in white and purple and represent ionized gas. This is likely where debris from the explosions punched holes in the surrounding gas and ionized it. 3 shows a light echo, where light from the ancient explosion has warmed up dust which shines as it cools down. 4 shows an especially large and intricate light echo known as Baby Cas A. Baby Cas A is actually about 170 light-years beyond Cas A. Image Credit: NASA, ESA, CSA, STScI, Danny Milisavljevic (Purdue University), Ilse De Looze (UGent), Tea Temim (Princeton University)
The JWST’s MIRI image shows different details. The outskirts of the main shell aren’t orange and pink. Instead, it looks more like smoke lit up by campfire flames.
Seeing the NIRCam image (L) and the MIRI image (R) tells us about the SNR and the JWST. First of all, the NIRCam image is sharper because of its higher resolution. The NIRCam image also appears less colourful, but that’s because of the wavelengths of light being emitted that are more visible in Mid-Infrared. In the MIRI image, the outer ring is lit up more brightly than in the NIRCam image, while the MIRI image also shows the ‘Green Monster,’ the green inner ring that is invisible in the NIRCam image. Image Credit: NASA, ESA, CSA, STScI, Danny Milisavljevic (Purdue University), Ilse De Looze (UGent), Tea Temim (Princeton University)
The Hubble Space Telescope, the Spitzer Space Telescope, and the Chandra X-Ray Observatory have all studied Cas A. In fact, Spitzer’s first light image back in 1999 was of Cas A.
This X-ray image of the Cassiopeia A (Cas A) supernova remnant is the official first light image of the Chandra X-ray Observatory. The bright object near the center may be the long-sought neutron star or black hole that remained after the explosion that produced Cas A. Image Credit: By NASA/CXC/SAO – http://chandra.harvard.edu/photo/1999/0237/, Public Domain, https://commons.wikimedia.org/w/index.php?curid=33394808
The Hubble has imaged Cas A too. This image is from 2006 and is a composite of 18 separate images. While interesting and stunning at the time, the JWST’s image surpasses it in both visual and scientific detail.
This NASA/ESA Hubble Space Telescope image provides a detailed look at the tattered remains of Cassiopeia A (Cas A). It is the youngest known remnant from a supernova explosion in the Milky Way. Image Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration. Acknowledgement: Robert A. Fesen (Dartmouth College, USA) and James Long (ESA/Hubble)
The JWST’s incredible images are giving us a more detailed look at Cas A than ever. Danny Milisavljevic leads the Time Domain Astronomy research team at Purdue University and has studied SNRs extensively, including Cas A. He emphasizes how important the JWST is in his work.
“With NIRCam’s resolution, we can now see how the dying star absolutely shattered when it exploded, leaving filaments akin to tiny shards of glass behind,” said Milisavljevic. “It’s really unbelievable after all these years studying Cas A to now resolve those details, which are providing us with transformational insight into how this star exploded.”
