Milky Way’s hidden supernova revealed by JWST

In 1667, a core-collapse supernova happened right here in the Milky Way, invisible to all humans. ~350 years later, here's what JWST sees.

This NIRCam view of the supernova remnant in Cassiopeia was taken by JWST, and the data was publicly released on November 5, 2023. The structure of the supernova remnant appears impressive, but the science within is even more fascinating.

In 1604, a supernova appeared to skywatchers on Earth, between the constellations of Ophiuchus and Sagittarius. Known as Kepler’s supernova, on October 17, 1604, it made a brilliant “line” with Mars, Jupiter, and Saturn flanking it.

Not since 1604 have human eyes witnessed a supernova directly.

In 1604, the last naked-eye supernova to occur in the Milky Way galaxy happened, known today as Kepler’s supernova. Although the supernova faded from naked-eye view by 1605, its remnant remains visible today, as shown here in an X-ray/optical/infrared composite. The bright yellow “streaks” are the only component still visible in the optical, more than 400 years later.

However, two Milky Way supernovae occurred subsequently.

This image from NASA’s Chandra X-ray Observatory shows the location of different elements in the Cassiopeia A supernova remnant including silicon (red), sulfur (yellow), calcium (green) and iron (purple), as well as the overlay of all such elements (top). A supernova remnant expels heavy elements created in the explosion back into the Universe. Although it isn’t shown here, the ratio of U-235 to U-238 in supernovae is approximately 1.6:1, indicating that Earth was born from largely ancient, not recently, created raw uranium.

Cassiopeia A was a core-collapse supernova in the galactic plane, occurring between 1667-1680.


This X-ray image, taken by NASA’s Chandra X-ray observatory, showcases the remnant of supernova G1.9+0.3, spotted near the galactic center in our own Milky Way. Estimates of its age place its origin at about 1868, making it the youngest known supernova remnant within the Milky Way.

G1.9+0.3, near the galactic center, occurred around 1868.

Back sometime between 1667 and 1680, a core-collapse supernova occurred in the constellation of Cassiopeia, between 9000 and 11000 light-years away. First detected in 1947 in radio light, this VLA image maps out the radio emission coming from this supernova remnant.

Discovered only in 1947, Cassiopeia A is the brightest radio source beyond our Solar System.

Shown here in optical light as revealed by the Hubble Space Telescope, the supernova remnant from the Cassiopeia A event looks like there are only a few sparse filaments of light coming from this portion of the sky. In reality, there’s a wide variety of light being emitted from this region of space, but only a tiny fraction of it is still around in the visible light portion of the spectrum. Infrared, radio, and X-ray views are far more revealing.

In visible light, there’s very little to see: like a fast-fading firework.

This image of the Cassiopeia A supernova remnant shows the aftermath of a type II, core-collapse supernova that occurred more than 350 years ago. The supernova remnant glows in a variety of electromagnetic wavelengths, including in various X-ray bands, as shown here. The color-coding reveals the diversity of elemental signatures found within.

In X-ray light, its heated gases shine brilliantly.

This 2008 image, released by NASA’s Spitzer showcases not only the supernova remnant of Cassiopeia A in infrared light, but also highlights three regions, in color, where the light from the supernova is reflecting off of the ejecta and only now arriving in our Solar System. These light-echoes illuminate different areas of gas and dust over time.

Infrared views, however, are most revealing, showcasing various elements and turbulent knots.

The data from JWST’s NIRCam observations of the Cassiopeia A supernova remnant were first released on November 5, 2023, and have since been stitched together using multiple photometric filters, creating this wonderful view of the nebula in near-infrared light.

At long last, JWST’s views, with both NIRCam and MIRI, are now public.

This image was taken with JWST’s MIRI instrument, and reveals details never before seen within the supernova remnant of Cassiopeia A. From end to end, this image corresponds to a size of about 10 light-years at the distance of Cassiopeia A, indicating how fast its material is expanding.

At just ~350 years old, the remnant is already 10light-years across.

This detailed section of JWST’s NIRCam view of the supernova remnant in Cassiopeia A shows a dense gaseous filament, likely driven by ejecta from the supernova, with sparse, heated material that showcases bubbles and cavities left behind after the blast wave passed over them.

The “shell” of ejecta expands at 1.5% the speed of light, with jets reaching nearly ~5% of lightspeed.

At the top-right of the image, wisps of material seem to point in different directions, highlighting various features of low-temperature material ejected from the supernova and also from the circumstellar material that predated the supernova. Bright, knotted filaments and bubbles blown within the remnant gas are all highlighted in this MIRI view of the detail of this supernova remnant.

Internally, “bubbles” appear, providing evidence for theunderlying shape of the gas distribution.

As seen with both NIRCam (monochrome) and MIRI (color), a smattering of what look like small “bubbles” can be found throughout the interior of the supernova remnant of Cassiopeia A. These bubbles likely represent the shape of the gas, with the interior either being empty of matter or simply too dense for the heat to have penetrated the bubbles’ interior.

Incredibly bright, near-infrared regions highlight dense filaments of heated material.

This NIRCam detail of the supernova remnant Cassiopeia A represents the highest-resolution view of these gaseous features in the near-infrared ever taken. There’s a temperature gradient throughout these filaments, as highlighted by the various colors, with dense knots separated by sparsely populated cavities.

Layers of the exploded star currently collide with the surrounding circumstellar matter.

The tenuous gas seen at the edge of the supernova remnant also shows what looks like “streaks” of material, which is evidence for the supernova ejecta plowing into circumstellar material that was ejected long before the supernova event occurred. These features help astronomers measure the speed of the ejecta and determine how long this material has been expanding for.

The explosion’s ejecta reveals numerous elements: oxygen, neon, argon, calcium, and phosphorus among them.

By looking for specific spectral signatures, various elements can be revealed in a wide variety of wavelengths of light. Here, Chandra X-ray data is used to identify different elements, where color-coded iron (orange), oxygen (violet), titanium (light blue, as seen by NuSTAR), and silicon-vs-magnesium (green) all indicate.

The mid-infrared “wisps” provide clues for how collapsing stars expel material.

Near the center of the JWST MIRI view of the supernova remnant Cassiopeia A, a variety of features all abound. In the upper left, bright, knotted gaseous filaments appear. To the right, diffuse streams of gas tangle up owing to the pressure from the supernova’s ejecta. To the top-right and lower portions (especially the lower-left) of the image, dark, gas-rich regions persist that seem to be opaque to even MIRI’s views. A lot of science remains to be extracted from these images.

With so little hydrogen present, it shows how varied supernovae can be.

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