Exquisite new Webb Space Telescope mid-infrared images of Cassiopeia A (Case A) – the youngest known supernova remnant from the collapse of our Milky Way galaxy’s core – may finally allow astronomers to understand this colorful clump of dust and debris.* These remnants are thought to be crucial for seeding new generations of stars and planets with biologically important elements like calcium and iron.
Yet the time and date of the stellar explosion that created this collection of gas, dust, and debris has never been fully confirmed. The best estimates are that light from the supernova that generated Cas A reached Earth in August 1680, about 340 years ago.
But there were no eyewitnesses at the time. And astronomers are still debating the details of what generated the explosion. Was it a single red giant superstar that suffered a core collapse explosion? Or was the supernova caused by the merger of some sort of binary companion?
These images contain an overwhelming amount of information about the poorly understood evolutionary phases the star went through before the explosion, Purdue University astronomer Danny Milisavljevic, principal investigator of the Webb program, told me via email. who captured the sightings.
It is difficult to underestimate the magnitude of these stellar remnants. Located some 11,000 light years from Earth in the constellation Cassiopeia, Cas A spans 10 light years. That’s more than twice the distance between here and Alpha Centauri, Earth’s next star system.
Spectroscopic measurements of the “bright echoes” of the original supernova have allowed previous astronomers to classify it as “Type IIb,” says Milisavljevic. This means the progenitor star was stripped of its hydrogen envelope before the explosion, he says. It can be difficult for a single star to do this independently, so it’s likely that two stars are involved, says Milisavljevic.
What’s most striking about this first NASA image?
The central region of the remnant remains the most intriguing/puzzling, says Milisavljevic. In many color composites made from multiple filters, it appears green, and it quickly earned the nickname “Green Monster,” he says. It has a long trail of north/south (up/down) and east/west (left/right) filaments that are pockmarked with bubbles of disconcertingly beautiful symmetry, says Milisavljevic.
These formations were completely unexpected, and we have a “Dream Team” of experts offering theories to explain their origin, says Milisavljevic. I’m also surprised by the red/orange glow that surrounds the bright ring of stellar debris, he says. It details the star’s loss of mass before it exploded, he notes.
It is believed that this mass loss data likely contains key information about the supernova’s progenitor star or stars.
Milisavljevic leads a team of some 50 international scientists who received around 45 hours of time from the Webb Telescope.
Our project seeks to understand how the original explosion occurred and determine what type of star was there, says Milisavljevic. These questions are based on the precise knowledge of the quantity of gas and dust present today, in particular near the center of the explosion. We also want to understand how much stellar debris is turned into dust.
Because our dataset is made up of hundreds of individual images that must be carefully combined and stitched together, processing the data takes days, says Milisavljevic.
Even so, years of analysis are planned to determine how the dust and molecules were formed and destroyed by the supernova.
As to when the Cas A progenitor star or stars actually exploded?
Using images of Cas A obtained over the decades since its initial discovery, astronomers have been able to track the residual gas as it moves radially outward, says Milisavljevic. Accurate measurements of this movement can be used to trace the ballistic trajectory back to where the explosion originally took place, he says. This analysis leads to a date around the year 1680, says Milisavljevic.
The supernova that created Cas A was probably not seen at the time, because as Milisavljevic explains, there is considerable obscuring dust between Earth and the exploding star. In fact, it wasn’t until the 1950s that Cas A was first observed in the optical spectrum by astronomers Walter Baade and Rudolph Minkowski.
But clearly, these latest optical images of Cas A are the best the world has ever seen.
Using dozens of ground and space telescopes, I spent 17 years studying stars and their titanic explosions, says Milisavljevic. Yet I was still unprepared for this data, he says.
“People should have this image of Case A in mind when they think of galaxies moving through cosmic time,” Milisavljevic said.
*Correction: This post has been updated to note that Case A is the youngest known “core-collapse” supernova remnant in our galaxy..
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