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 vibrant light.
The Orion Nebula, also known as “Messier 42” (M42), is located about 1,500 light-years from Earth toward the constellation Orion. This makes it the closest major star-forming and stellar nursery to our solar system.
The Orion Nebula, visible to the naked eye under dark skies, 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 in the lower left quadrant of M42, called “the Orion Bar.”
The images collected as part of JWST’s PDRs4All program are valuable not only for their stunning beauty. This wealth of data will allow scientists to delve into the often messy and chaotic conditions associated with star formation.
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“These images have such incredible detail that we will be examining them for many years to come. The data is incredible and will serve as benchmarks for astrophysical research for decades to come,” Western University astrophysicist and PDRs4All principal investigator Els Peeters said in a statement. . “So far we have examined only a small portion of the data, and this has already resulted in several surprising and important discoveries.”
Star birth is messy in the Orion Nebula
Star formation occurs when overdense patches in giant clouds of gas and dust collapse under their own gravity. This forms a ‘protostar’, wrapped in a birthing cocoon of gas and dust left over from its formation.
Protostars continue to accumulate material from their natal envelopes until they accumulate enough mass to initiate nuclear fusion of hydrogen to helium in their cores. This process defines a main sequence star like our Sun, which would have gone through this process about 4.6 billion years ago.
However, the situation is more complicated than it initially seems, because these overcrowded areas are not all the same size or weight, and they do not all collapse at the same time.
“The process of star formation is messy because star-forming regions contain stars of different masses that are at different stages of their development while still embedded in their natal cloud, and because there are many different physical and chemical processes at play that influence each other,” he said. Peeters. .
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 way ultraviolet radiation from hot young stars interacts with gas and dust.
In the Orion Nebula, this radiation bombardment creates 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 reveal a more complicated structure than we initially thought – with gas and dust in the foreground and background making the analysis a little more difficult,” said PDRs4All team member Emile Habart from the University of Paris -Saclay . “But these images are of such quality that we can separate these regions well and show that the edge of Orion Bar is very steep, like a huge wall, as predicted by theories.”
The JWST not only allowed the researchers to see the structure of the Orion Bar like never before, but also use the light spectrum of the Orion Bar to determine how its chemical composition varies. 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 reveal the large-scale chemical composition of M42, allowing the PDRs4All team to see how temperature, density and radiation field strength change through the Orion Nebula.
The detection of more than 600 chemical fingerprints in the Orion Nebula spectra over the course of this study could greatly improve models of PDRs.
“The spectroscopic dataset covers a much smaller part of the sky compared to the images, but contains much more information,” said Peeters. “A picture is worth a thousand words, but we astronomers only half-jokingly say that a spectrum is worth a thousand pictures.”
James Webb Space Telescope leaves other telescopes in the dust
The PDRs4All team also addressed a long-standing problem with previous observations of the Orion Nebula, namely a steep variation in dust emissions in the Orion Bar, the origin of which could not be explained. This study showed that this variation in emission was the result of a destructive process in the Orion Bar spark, caused by radiation from massive young stars.
“The sharp hyperspectral JWST data contain so much more information than previous observations that they clearly pointed to the attenuation of radiation by dust and the efficient destruction of the smallest dust particles as the underlying cause of these variations,” says team member and Institut d’Astrophysique. Spatial postdoctoral researcher Meriem El Yajouri said.
The PDRs4All team was also able to reveal details about the Orion Nebula’s emissions that come from large carbon-containing molecules known as polycyclic aromatic hydrocarbons (PAHs). These are among the largest reservoirs of carbon-based materials in the cosmos, believed to represent as much as 20% of the universe’s carbon.
Because the only life we are aware of in the cosmos 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 study what happens to carbon-containing molecules long before the carbon enters our bodies,” Cami added.
PAH molecules are long lasting due to their strength and resilience. Their emissions are bright, and the JWST can use them to determine that even with the toughness of PAH’s ultraviolet light from young stars can change these emissions.
“It really is an embarrassment of riches,” said Peeters. “Although 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 showed that ultraviolet radiation breaks down smaller carbon molecules, while larger molecules change their emissions. These effects are visible in varying extremes in the Orion Nebulae, moving from shielded environments to more exposed areas.
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“What really makes the Orion Bar unique is its ‘edge-on’ geometry, which gives us a ringside seat where we can study in great detail the various physical and chemical processes that take place as we move from the highly exposed , hard ionized area to the much more shielded areas where molecular gas can form,” said Jan Cami, PDRs4All team member and researcher at Western University.
Using machine learning to assess PAHs has shown that even if ultraviolet light does not break down these molecules, it can cause their structure to change.
“These papers reveal a kind of survival of the fittest at the molecular level in the harshest environments in space,” Cami concluded.
The team’s research has been published in a series of six articles in the journal Astronomy & Astrophysics