The Orion Nebula may be a familiar and well-studied celestial object, but new images from the James Webb Space Telescope (JWST) show this star-forming cloud of gas and dust in an incredibly new and vibrant light.
The Orion Nebula, also known as "Messier 42" (M42), is located around 1,500 light years from Earth toward the constellation of Orion. This makes it the closest large star-forming and stellar nursery to our solar system.
Visible to the naked eye under dark skies, the Orion Nebula has been studied throughout human history, but the JWST images show it in unprecedented detail. In particular, the powerful space telescope zoomed in on the diagonal, ridge-like feature of gas and dust at the lower left quadrant of M42 called "the Orion Bar."
The images collected as part of the JWST's PDRs4All program are valuable for more than their stunning beauty. This treasure trove of data will allow scientists to delve into the often messy and chaotic conditions that accompany star formation.
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"These images have such incredible detail that we will be scrutinizing them for many years to come. The data are incredible and will serve as benchmarks for astrophysics research for decades to come," Western University astrophysicist and PDRs4All principle investigator Els Peeters said in a statement. "So far, we have explored only a tiny fraction of the data, and this has already resulted in several surprising and major discoveries."
Star birth is messy in the Orion Nebula
Star formation occurs when overdense patches in gigantic clouds of gas and dust collapse under their own gravity. This forms a "protostar" wrapped in a natal cocoon of gas and dust left over from its formation.
Protostars continue to gather material from their natal envelopes until they have gathered enough mass to trigger the nuclear fusion of hydrogen to helium in their cores. This process defines a main-sequence star like our sun, which will have gone through this process around 4.6 billion years ago.
The situation is more complicated than it may initially sound, though, because these overdense patches aren't all the same size or mass, and they don't all collapse at the same time.
"The process of star formation is messy because star-forming regions contain stars of varying masses at different stages of their development while still embedded in their natal cloud and because many different physical and chemical processes are at play that influence one another," Peeters said.
One of the most important aspects of understanding the gas and dust between stars or "interstellar medium" from which other stars are created is the physics of photo-dissociation regions or "PDRs" (the PDR in PDRs4All). The chemistry and physics of PDRs are determined by how ultraviolet radiation from hot young stars interacts with gas and dust.
In the Orion Nebula, this bombardment of radiation is creating structures like the Orion Bar, which is essentially the edge of a large bubble carved out by some of the massive stars that power the nebula.
"The same structural details that give these images their aesthetic appeal reveals a more complicated structure than we originally thought – with foreground and background gas and dust making the analysis a bit harder," PDRs4All team member Emile Habart from the University of Paris-Saclay said. "But these images are of such quality that we can separate these regions well and reveal that the edge of the Orion Bar is very steep, like a huge wall, as predicted by theories."
The JWST allowed the researchers to not only see the structure of the Orion Bar like never before, but the spectrum of light from the Orion Bar also let them determine how its chemical composition varies throughout it. This is possible because chemical elements absorb and emit light at characteristic wavelengths, leaving their fingerprints on the spectrum of light passing through gas and dust.
This helped to reveal the widescale chemical makeup of M42, allowing the PDRs4All team to see how temperature, density, and radiation field strength change through the Orion Nebula.
The detection of over 600 chemical fingerprints in the spectra of the Orion Nebula over the course of this investigation could vastly improve models of PDRs.
"The spectroscopic dataset covers a much smaller area of the sky compared to the images, but it contains a ton more information," Peeters said. "A picture is worth a thousand words, but we astronomers only half-jokingly say that a spectrum is worth a thousand images."
James Webb Space Telescope leaves other telescopes in the dust
The PDRs4All team also tackled a longstanding problem with previous observations of the Orion Nebula, namely a steep variation in emissions from dust in the Orion Bar, the origin of which couldn't be explained. This investigation revealed that this variation in emission was the result of a destructive process in the Orion Bar spark by radiation from massive young stars.
"The sharp hyperspectral JWST data contains so much more information than previous observations that it clearly pointed to the attenuation of radiation by dust and the efficient destruction of the smallest dust particles as the underlying cause for these variations," team member and Institut d’Astrophysique Spatiale postdoctoral researcher Meriem El Yajouri said.
The PDRs4All team was also able to tease out details about emissions from the Orion Nebula that come from large carbon-bearing molecules known as polycyclic aromatic hydrocarbons (PAHs). These happen to be among the largest reservoirs of carbon-based materials in the cosmos, thought to account for as much as 20% of the carbon in the universe.
Because the only life in the cosmos we are aware of is carbon-based, the study of PAHs is hugely relevant to our understanding of the existence of life on planets that form around young stars.
"We are studying what happens to carbonaceous molecules long before the carbon makes its way into our bodies," Cami added.
PAH molecules are long-lasting due to their sturdiness and resilience. Their emissions are bright, and the JWST is able to use these to determine that even with the toughness of PAHs, ultraviolet light from young stars can alter these emissions.
"It really is an embarrassment of riches," said Peeters. "Even though these large molecules are thought to be very sturdy, we found that UV radiation changes the overall properties of the molecules that cause the emission."
This revealed that ultraviolet radiation breaks apart smaller carbon molecules while larger molecules have their emissions changed. These effects are seen in different extremes across the Orion Nebulas, moving from shielded environments to more exposed regions.
"What makes the Orion Bar truly unique is its edge-on geometry, giving us a ring-side seat to study in exquisite detail the different physical and chemical processes that happen as we move from the very exposed, harsh ionized region into the much more shielded regions where molecular gas can form," Jan Cami, PDRs4All team member and Western University researcher said.
Using machine learning to assess PAHs revealed that even when ultraviolet light doesn't break these molecules down, it can cause their structure to be changed.
"These papers reveal some sort of survival of the fittest at the molecular level in the harshest environments in space," Cami concluded.
The team's research is published across a series of six papers in the journal Astronomy & Astrophysics
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