Supernova Echoes Probe Clouds in the Milky Way

The Cassiopeia A supernova remnant is one of the best-studied objects in the heavens, but that doesn’t mean it’s given up all its secrets. At last week’s meeting of the American Astronomical Society (AAS) in National Harbor, Maryland, astronomers once again showed how turning new telescopes toward old targets can advance our understanding in unanticipated ways.

New images from the James Webb Space Telescope not only offer fresh insights into the Cas A supernova itself, which occurred in the late 17th century, but also reveal previously unknown structure in the interstellar medium — the rarefied clouds of gas and dust that fill the space between stars in the Milky Way and serve as the birthplaces of new stars.

When the progenitor star of Cas A exploded some 350 years ago, it sent a pulse of X-rays and ultraviolet light into its surroundings. As that energetic light expands through space, it encounters dusty interstellar clouds, warming them and causing them to glow at infrared wavelengths. The Spitzer Space Telescope, an infrared observatory active from 2003 to 2020, first spotted these “light echoes” as a series of brightenings, one after the other, from a succession of clouds at increasing distances from the supernova remnant.

But Spitzer’s telescope was relatively small, 85 centimeters in diameter, so it couldn’t resolve any detail in the clouds. The NEOWISE infrared observatory monitored the echoes until its mission ended late last year, but its telescope was barely half as wide as Spitzer’s.

In an AAS press conference, Jacob Jencson (Caltech/IPAC) and Joshua Peek (Space Telescope Science Institute) showed what a difference aperture makes. Zooming in to the infrared echoes from two small clouds southwest of Cas A, Webb’s 6.5-meter telescope resolves myriad coiled knots and bundles of filaments that trace the magnetic fields threading through them.

“Structure like this has never been seen before,” says Jencson. “We were pretty shocked to see this level of detail.”

An animation created from three sets of Near Infrared Camera images separated by weeks shows different features within the clouds lighting up as the radiation from Cas A sweeps through them. Combining these images with spectra from Webb’s Mid-Infrared Instrument, Jencson, Peek, and their colleagues traced the 3D structure of the clouds, which they liken to crumpled ribbons or tightly packed sheets.

Even at the clouds’ distance of approximately 230 light-years, the researchers detect features as small as a few light-days across. Using that information, they can pin down the duration of the Cas A supernova’s ultraviolet pulse to mere days.

By studying the brightness of the infrared echoes, the researchers hope to constrain the intensity of the supernova blast. “This will give us important information about the star that exploded,” explains Jencson. Astronomers know it was a massive star, but they haven’t yet reached consensus on whether it was a red, yellow, or blue supergiant, so the evolutionary path that led to the supernova remains uncertain.

Peek points out that most observations of the interstellar medium focus on nebulosity illuminated, heated, and ionized by stars. But the lion’s share of the material between stars is cold and neutral, and that’s true of the clouds seen echoing the light from Cas A. “We are seeing the beautiful interstellar medium as it usually is,” says Peek, “not as it interacts with stars.”

Computer simulations of the competition between gravity and magnetic fields in cold, neutral clouds sometimes produce features similar to the whorls and ribbons seen in the clouds near Cas A. But pressure from magnetic fields resists the gravitational collapse of gas and dust to form new stars, so astronomers know they’re missing something about how the transition from gas and dust to stars works in the real universe.

Peek expects that further study of the fine-scale structures revealed by Webb — which are comparable in size to typical star-forming clouds — could answer the question of how clumps of interstellar material overcome magnetic resistance to transform themselves into active stellar nurseries.