NASA’s Webb research how the radiation explosions of huge stars have an effect on their environment.


In a nearby star nursery called the Orion Nebula, young, massive stars shine distant ultraviolet light on the cloud of dust and gas from which they were born. This intense flood of radiation violently disrupts the cloud by breaking apart molecules, ionizing atoms and molecules by stripping off their electrons and heating gas and dust. An international team deploying NASA’s James Webb Space Telescope, due to launch in October, will study part of the emitted cloud called Orion Bar to learn more about the impact of massive stars on their surroundings and even on the formation of our own Solar system.

The Orion Bar is a diagonal, comb-like feature of gas and dust in the lower left quadrant of this image of the Orion Nebula. Shaped by the intense radiation from hot young stars nearby, the Orion Bar appears to be shaped like a bar at first glance. It’s likely a prototype of a photodissociation region, or PDR.

“The fact that massive stars form the structure of galaxies through their explosions as supernovae has long been known. More recently, however, people have found that massive stars affect their environment not only as supernovae but also by wind and radiation throughout their lives, ”said one of the team’s lead researchers, Olivier Berné, a scientist at the French National Center for Scientific Research in Toulouse.

Why the Orion Bar?

While it sounds like a Friday night watering hole, the Orion Bar is actually a comb-like feature of gas and dust in the spectacular Orion Nebula. Slightly more than 1,300 light years away, this nebula is the closest region to the Sun with massive star formation. The Orion Bar is shaped by the intense radiation of hot young stars nearby and, at first glance, appears to be shaped like a bar. It is a “photodissociation region” or PDR in which ultraviolet light from young, massive stars creates a mostly neutral but warm region of gas and dust between the fully ionized gas that surrounds the massive stars and the clouds in which they are born , generated. This ultraviolet radiation has a strong influence on the gas chemistry of these regions and acts as the main source of heat.

PDRs occur where interstellar gas is dense and cold enough to remain neutral, but not dense enough to prevent the ingress of distant ultraviolet light from massive stars. Emissions from these regions provide a unique tool for studying the physical and chemical processes that are important for most of the mass between and around stars. The processes of radiation and cloud disturbance drive the development of interstellar matter in our galaxy and in the entire universe from the early era of powerful star formation to the present day.

“The Orion Bar is likely a prototype PDR,” explained Els Peeters, another lead investigator on the team. Peeters is a professor at the University of Western Ontario and a member of the SETI Institute. “It has been studied extensively and is therefore well characterized. It’s very close and it’s really seen edge. That means you can examine the different transition areas. And since it is nearby, this transition from one region to another is spatially different when you have a telescope with high spatial resolution. ”

The Orion Bar is representative of the harsh scientific conditions of PDRs in the universe billions of years ago. “We believe that at that time you had ‘Orion Nebulae’ all over the universe, in many galaxies,” said Berné. “We believe it can be representative of the physical conditions related to the ultraviolet radiation field in so-called ‘starburst galaxies’ that dominated the era of star formation when the universe was about half its age.”

The formation of planetary systems in interstellar regions irradiated by massive young stars remains an open question. Detailed observations would enable astronomers to understand the impact of ultraviolet radiation on the mass and composition of newly formed stars and planets.

Studies of meteorites in particular suggest that the solar system formed in a region similar to the Orion Nebula. Observing the Orion Bar is one way of understanding our past. It serves as a model to get to know the very early stages of the formation of the solar system.

This graphic shows the layered nature of a photodissociation region (PDR) like the Orion Bar. Previously viewed as homogeneous areas of warm gas and dust, PDRs now contain complex structures and four distinct zones. The field on the left shows part of the Orion Bar in the Orion Nebula. The box at the top right shows a massive star-forming region whose ultraviolet radiation affects a PDR. The lower right box enlarges a PDR to show its four distinct zones: 1) the molecular zone, a cold and dense region where the gas is in the form of molecules and where stars could form; 2) the dissociation front, on which the molecules disintegrate into atoms as the temperature rises; 3) the ionization front, where the gas is stripped of electrons and becomes ionized when the temperature rises dramatically; and 4) the fully ionized gas flow into a region of atomic ionized hydrogen. For the first time, Webb can separate and examine the physical conditions of these different zones. Credits: NASA, ESA, CSA, Jason Champion (CNRS), Pam Jeffries (STScI), PDRs4ALL ERS team

Like a layer cake in space

PDRs have long been considered homogeneous regions of warm gas and dust. Now scientists know that they are heavily layered, like a cake. In reality, the Orion Bar is not a “bar” at all. Instead, it contains a lot of structure and four different zones. These are:

  • The molecular zone, a cold and dense region where the gas is in the form of molecules and where stars could form;
  • The dissociation front at which the molecules disintegrate into atoms when the temperature rises;
  • The ionization front, where electrons are removed from the gas, is ionized when the temperature rises dramatically;
  • The fully ionized gas flow into an area of ​​atomic, ionized hydrogen.

“With Webb, we can separate and examine the physical conditions of the different regions, which are completely different,” said Emilie Habart, another of the team’s lead investigators. Habart is a scientist at the French Institute for Space Astrophysics and a lecturer at the University of Paris-Saclay. “We will study the transition from very hot to very cold regions. This is the first time we’ve been able to do this. ”

The phenomenon of these zones is similar to what happens to the heat from a fireplace. As you move away from the fire, the temperature drops. Similarly, the radiation field changes with distance from a massive star. The composition of the material also changes at different distances from this star. With Webb, scientists will for the first time resolve and fully characterize each individual region within this layer structure in the infrared.

Trailblazer for future observations

These observations will be part of the Director’s Discretionary Early Release Science program, which will provide observation time to selected projects at the start of the telescope mission. This program enables the astronomical community to quickly learn how to best use Webb’s skills while producing a solid science.

One goal of the Orion Bar work is to identify the features that will serve as “templates” for future studies of more distant PDRs. At greater distances, the different zones can become blurred. Information from the Orion bar is useful in interpreting this data. The Orion Bar observations will be available to the wider scientific community very soon after they are collected.

“Most of the light we get from very distant galaxies comes from ‘Orion Nebulae’ that are in those galaxies,” explained Berné. “It therefore makes a lot of sense to observe the Orion Nebula very closely in our vicinity in order to then understand the emissions of these very distant galaxies, in which there are many Orion-like regions.”

Only possible with Webb

With its position in space, infrared capability, sensitivity and spatial resolution, Webb offers a unique opportunity to study the Orion Bar. The team will study this region using Webb’s cameras and spectrographs.

It is really the first time that we have such good wavelength coverage and angular resolution, ”said Berné. “We are very interested in spectroscopy because there you can see all the ‘fingerprints’ that give you detailed information about the physical conditions. But we also want the images to see the structure and organization of matter. When you combine spectroscopy and imaging in this unique infrared region, you get all the information you need for the science we are interested in. ”

The study includes a core team of 20 members, but also a large international, interdisciplinary team of more than 100 scientists from 18 countries. The group includes astronomers, physicists, chemists, theorists, and experimenters.

The James Webb Space Telescope will be the world’s premier space science observatory when it launches in 2021. Webb will solve puzzles in our solar system, look beyond distant worlds around other stars and investigate the mysterious structures and origins of our universe and our location in there. Webb is an international program led by NASA with its partners ESA (European Space Agency) and Canadian Space Agency.

More information about Webb can be found at

Ann Jenkins / Christine Pulliam

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