The Orion Nebula may be a well-known 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 vivid light.
The Orion Nebula, also known as Messier 42 (M42), is located about 1,500 light-years from Earth in the constellation Orion. This makes it the closest major star formation and star formation site to our solar system.
The Orion Nebula is visible to the naked eye under dark skies and has been studied throughout human history, but the JWST images show it in unprecedented detail. In particular, the powerful space telescope magnified the diagonal, comb-like structure of gas and dust in M42’s lower left quadrant, called the “Orion Bar.”
The images collected through JWST’s PDRs4All program are valuable for more than just their breathtaking beauty. This trove of data will allow scientists to delve into the often chaotic and chaotic conditions that accompany star formation.
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“These images are so incredibly detailed that we will be studying them for many years to come. The data is incredible and will serve as a benchmark for astrophysics research for decades to come,” Els Peeters, Western University astrophysicist and PDRs4All principal investigator, said in a statement. “So far we have examined only a tiny fraction of the data, and this has already led to several surprising and important discoveries.”
Star birth in the Orion Nebula is chaotic
Star formation occurs when superdense areas in huge clouds of gas and dust collapse under their own gravity. This creates a “protostar” wrapped in a cocoon of gas and dust left over from its formation.
Protostars continue to collect material from their natal envelope until they have accumulated enough mass to trigger nuclear fusion of hydrogen into helium in their cores. This process defines a main sequence star like our Sun, which underwent this process about 4.6 billion years ago.
However, the situation is more complicated than it might initially sound because these overdense patches are not all the same size or mass, and they do not all collapse at the same time.
“The process of star formation is chaotic because star-forming regions contain stars of different masses at different stages of their evolution while still embedded in their natal cloud, and because there are many different physical and chemical processes at play that influence each other,” Peeters said.
One of the most important aspects of understanding the gas and dust between stars, or the “interstellar medium” from which other stars form, is the physics of photodissociation regions, or “PDRs” (the PDR in PDRs4All). The chemistry and physics of PDRs are determined by the interaction of ultraviolet radiation from hot young stars with gas and dust.
In the Orion Nebula, this radiation bombardment creates features like the Orion Bar, which is essentially the edge of a large bubble formed by some of the massive stars that power the nebula.
“The same structural details that give these images their aesthetic appeal reveal a more complicated structure than we originally thought – with gas and dust in the foreground and background making analysis a little more difficult,” said PDRs4All team member Emile Habart from the university Paris Saclay. “But these images are of such quality that we can separate these regions well and show that the edge of Orion’s bar is very steep, like a giant wall, as the theories predicted.”
The JWST allowed researchers to not only see the structure of the Orion Rod like never before, but also to use the Orion Rod’s light spectrum to determine how its chemical composition varies throughout the Orion Rod. This is possible because chemical elements absorb and emit light at characteristic wavelengths, leaving their fingerprints in the spectrum of light that penetrates gas and dust.
This helped reveal the long-range chemical composition of M42 and allowed the PDRs4All team to see how the temperature, density and radiation field strength change in the Orion Nebula.
The discovery of over 600 chemical fingerprints in the Orion Nebula spectra during this study could significantly improve PDR models.
“The spectroscopic data set covers a much smaller area of the sky compared to the images, but contains a lot 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 pictures.”
The James Webb Space Telescope is letting other telescopes down
The PDRs4All team also addressed a long-standing problem in previous observations of the Orion Nebula, namely a strong variation in dust emissions in the Orion bar, the origin of which could not be explained. This investigation revealed that this variation in emission was the result of a destructive process in the spark of the Orion bar caused by radiation from massive young stars.
“The sharp hyperspectral JWST data contain so much more information than previous observations that they clearly point to the attenuation of radiation by dust and the efficient destruction of the smallest dust particles as the underlying cause of these variations,” said team member and Institut d’Astrophysique Spatiale postdoctoral researcher Meriem El Yajouri said.
The PDRs4All team was also able to find out details about the Orion Nebula’s emissions, which come from large carbon-containing molecules known as polycyclic aromatic hydrocarbons (PAHs). This is one of the largest reservoirs of carbon-based materials in the cosmos, thought to make up up to 20% of the carbon in the universe.
Because the only life we know of in the cosmos is carbon-based, the study of PAHs is of great importance to our understanding of the existence of life on planets forming around young stars.
“We study what happens to carbon-containing molecules long before the carbon enters our bodies,” Cami added.
PAH molecules are long-lasting due to their robustness and resilience. Their emissions are bright, and JWST can use this to determine that ultraviolet light from young stars can alter these emissions, despite the toughness of PAHs.
“It really is an embarrassment of wealth,” Peeters said. “Although these large molecules are considered very robust, we found that UV radiation changes the overall properties of the molecules that cause the emission.”
It was shown that ultraviolet radiation breaks down smaller carbon molecules while changing the emissions of larger molecules. These effects can be observed at varying extremes in the Orion nebulae, moving from shielded environments to more exposed regions.
“What makes the Orion Bar truly unique is its edge geometry, which provides us with a ring-side seat to monitor the various physical and chemical processes that occur as we move from the very exposed, highly ionized region to the much more distant region. “To study in exquisite detail “shielded regions where molecular gas can form,” said Jan Cami, PDRs4All team member and Western University researcher.
Using machine learning to evaluate PAHs revealed that even if ultraviolet light does not break down these molecules, it can cause a change in their structure.
“This work reveals a kind of survival of the fittest at the molecular level in the harshest environments in space,” concluded Cami.
The team’s research is published in a series of six papers in the journal Astronomy & Astrophysics