(CN) — Scientists at NASA announced Monday that they believe they've clinched the origin of a strange green shape lurking within a famous nebula.
The nebula, dubbed Cassiopeia A and located some 11,000 light-years away in the Cassiopeia constellation, is the remnant of a supernova with light that first reached Earth in the late 1600s. Scientists have studied the nebula for decades, but in April 2023 new observations using the James Webb Space Telescope's infrared cameras revealed a feature never before seen — a rough "C" or "G" shaped patch of filaments, colored green and obscured within the rest of the gas cloud.
"We were amazed," said Princeton University research astronomer Tea Temim, one of the researchers studying Cassiopeia A who discovered the strange green structure in 2023.
Temim and other "Cas A" scientists dubbed the structure the "green monster" in honor of the left field wall at Boston's Fenway Park. But they weren't sure exactly what it was or how it came to be. It had "nearly perfectly" circular holes running through its structure, Themim said, and its green color — an artifact of the filters used for the collated infrared images of the nebula — indicated that its constituent gas and dust was relatively lukewarm.
Given these features and the fact that from Earth's perspective the green monster sits near the center of the nebula, Themim said her team Initially thought it was material left over from the supernova that created Cas A.
"We thought we might be looking at material from the explosion that hadn't yet been reheated by collisions with the surrounding material," Themim said.
But this hypothesis fell apart when the researchers noticed that the green monster was only expanding outward at the rate of a few kilometers per second. Practically standing still on a cosmic scale, and not at all what one expects of material rocketed out by an exploding star.
"The velocity was very low, practically zero," Themim said. "If it was material from the explosion, we would expect much higher velocities."
A new theory emerged after comparing Webb's infrared data to observations taken by the Chandra X-ray Observatory. As its name suggests, the Chandra X-ray Observatory is an orbital NASA telescope that captures images primarily in the much more energetic X-ray spectrum of light. Using Chandra, the researchers found that the "holes" present in the green monster in infrared were also present in structures corresponding to the green monster in X-ray.
It suggested that, rather than being a remnant of Cas A's original supernova, the green monster is actually a denser region of space sitting between Earth and Cas A that was caught up in the blast.
“We concluded that the Green Monster is also part of the blast wave and is photobombing the central part of Cas A rather than being part of it," Ilse De Looze, another Cas A researcher from from Ghent University in Belgium said in a prepared statement.
Themim clarified that the green monster's "holes" in both the Webb and Chandra images may indicate where supernova ejecta punched through the surrounding gas and dust of the region, possibly gas and dust the original star shed in the years prior to its explosive death.
"Those ejecta shrapnel... they went out and punched through the surrounding material," Themim said. "It's still probably material from the star, just material that was shed prior to the explosion."
NASA released a composite image of Cas A, including the "photobombing" green monster, Monday. The new multicolored photo combines X-ray and infrared data captured by Chandra and Webb respectively, as well as by another of NASA's infrared orbital telescopes known as Spitzer, and the Hubble Space Telescope which photographs primarily in the visible spectrum.
Bluer hues in the image represent warmer, more energetic material, Themim said, while red and orange colors represent cooler areas of the nebula.
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