Cassiopeia A (Cas A) is a supernova remnant located about 11,000 light-years from Earth in the … [+]
Exquisite new mid-infrared Webb Space Telescope images of Cassiopeia A (Cas A) —- our Milky Way Galaxy’s youngest known supernova remnant —- may finally enable astronomers to understand this colorful agglomeration of dust and debris. Such remnants are thought to be crucial in 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. Best estimates are that light from the supernova that generated Cas A reached Earth in August of 1680, some 340 years ago.
But there were no eyewitnesses at the time. And astronomers still debate the details of what generated the explosion. Was it a single super red giant star that underwent a core collapse explosion? Or was the supernova caused by the merger of some sort of binary companion?
These images hold an overwhelming amount of information about the poorly understood evolutionary phases the star went through prior to explosion, Purdue University astronomer Danny Milisavljevic, principal investigator of the Webb program that captured the observations, told me via email.
It’s hard to underestimate the scale of such stellar remnants. Located some 11,000 light-years from Earth in the constellation of Cassiopeia, Cas A stretches over 10 light years. That’s more than twice the distance between here and Alpha Centauri, Earth’s next star system over.
Spectroscopic measurements of the “light echoes” from the original supernova made it possible for previous astronomers to classify it as a “Type IIb,” says Milisavljevic. That means that the progenitor star was stripped of its hydrogen envelope before the explosion, he says. It can be hard for a single star to do this independently, so it’s likely that two stars were involved, says Milisavljevic.
What’s most striking about this early release NASA image?
The remnant’s central region remains the most intriguing/puzzling, says Milisavljevic. In many color composites made from multiple filters, it appears as green, and it quickly gained the moniker “Green Monster,” he says. It has a long wisp of filaments running North/South (up/down) and East/West (left/right) that are pockmarked with bubbles of beautiful, confounding symmetry, says Milisavljevic.
These formations were completely unexpected, and we have a “Dream Team” of experts coming up with theories to explain their origin, says Milisavljevic. I’m also taken aback by the red/orange glow the surrounds the bright ring of stellar debris, he says. It’s detailing mass loss from the star prior to its explosion, he notes.
It’s thought that such mass loss data likely holds key information about the supernova’s progenitor star or stars.
Milisavljevic leads a team of some 50 international scientists awarded approximately 45 hours of Webb Telescope time.
Our project seeks to understand how the original explosion occurred and determine what type of star was there, says Milisavljevic. These questions hinge on knowing precisely how much gas and dust is present today, especially near the center of the explosion. We also want to understand how much stellar debris is transformed into dust.
Because our data set is made up of hundreds of individual images that must be carefully combined and stitched together, data processing takes days, says Milisavljevic.
Even so, years of analysis is planned to constrain how dust and molecules were formed and destroyed by the supernova.
As for when Cas A’s progenitor star or stars actually exploded?
Using images of Cas A obtained over the decades since it was originally discovered, astronomers have been able to follow the remnant’s gas as it moves radially outwards, says Milisavljevic. Precise measurements of this motion can be used to trace the ballistic path 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 likely 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 both ground- and space-based telescopes, I’ve 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 Cas A in their mind when thinking about galaxies evolving through cosmic time,” said Milisavljevic.
