Keck and JWST team up to watch as massive stars blast away gas in the Orion Nebula

One of the most truly spectacular objects in the entire sky is the celebrated Orion Nebula, a star-birth factory 1,300 light-years from Earth. It's so bright that it can be seen with the naked eye; it looks like a star, the middle of three that form Orion's "dagger" that hangs down from his Belt. With a telescope it reveals itself to be a brilliant and colorful display of hot gas a couple of dozen light-years across.

But our view of it is changing, because it’s eating itself up from the inside out. The question is, will this process destroy the gas and starve stars of building material, or will it help trigger the formation of new stars?

To find out more, a team of astronomers used both the ground-based 10-meter Keck telescope with a near-infrared camera and the space observatory JWST to observe the Orion Nebula, the closest site of massive star formation to the Sun. It’s actually a small part of the much larger Orion Molecular Cloud, a cold, dense, and dark cloud of gas. 

Hubble (left) versus JWST (right) image of the Orion Bar. The bar (bright diagonal line) is the border between a cavity in the Orion Nebula and the material in the cavity wall. Hubble sees warm atomic gas which is closer to the cavity center, versus JWST which sees cooler molecular gas that is farther out. Photo: (JWST): NASA, ESA, CSA, PDRs4All ERS Team; image processing Olivier Berné; (HST): NASA/STScI/Rice Univ./C.O’Dell et al.

They want to examine the process of stellar feedback. In the center of the nebula are several recently created big stars, including the beast Theta1 Ori C, a binary system where one star has 11 times the mass of the Sun and the other 33. That bigger one blasts out 200,000 times as much energy as the Sun! That radiation is intense, and is heating and eroding the molecular gas around it. The cooler gas in the molecular cloud is made mostly of hydrogen molecules, two attached hydrogen atoms. But the ultraviolet light from Theta1 Ori C blasts them apart, creating neutral hydrogen atoms. It also ionizes those atoms, profoundly changing how they behave. This is carving a growing cavity around the stars with Theta1 Ori C at the center, like a worm eating its way out of an apple from the center outwards. 

This makes it much harder for stars to form in the nebula. But, this process also drives intense waves of pressure into the gas, which can compress it and cause more stars to be born. 

So stars born in the nebula affect that nebular gas — that’s the stellar feedback bit — but which wins? The forces of creation or destruction? 

Details of the JWST image of the Orion Bar. A young star in a dense knot of gas has just switched on (upper left), while the bright star Theta2 Orionis A (lower left) shines just outside the bar. A still-forming star is surrounded by a protoplanetary disk (upper right), and filaments of gas are created as shock waves move through the material. Photo: NASA, ESA, CSA, Data reduction and analysis : PDRs4All ERS Team; graphical processing S. Fuenmayor & O. Berné

As Theta1 Ori C eats away at the material around it, it creates a roughly spherical wall around it. We see one part of that wall edge-on so it looks more like a line, called the Orion Bar. This is what the astronomers targeted and obtained the highest-resolution near-infrared images of it ever taken.

What they saw is chaos. As light from the star slams into the denser material the gas is heated and flows away toward the star — this is like mist rising from a warm puddle of water as it’s heated by the Sun. Over time the gas becomes layered: On the outside of the bar — farther from Theta1 Ori C and the cavity center — is the cold, dense, molecular gas; closer to the center the gas is blown apart into atoms by the light — called photodissociation — and closer to the center yet those atoms are ionized. The borders between these layers are called fronts, so there’s a photodissociation front and an ionization front.

Left: A Hubble image of the Orion Bar shows the direction to young massive stars (upper right) that are flooding the nebula with ultraviolet light. The light ionizes the gas (HII region), creating a boundary with denser gas (ionization front), and breaks up molecular gas (dissociation front) as it penetrates deeper. The yellow box inset shows the area imaged using the Keck telescope. Photo: Habart et al. (2022) / W. M. Keck Observatory

They found that the two fronts are very close together, separated by only 750 billion kilometers or so. That’s less than a tenth of a light-year, which is a short distance compared to the 25 light-year-wide cavity making up the Orion Nebula.

The densest part of the region is along the photodissociation front, where the gas is being heated and compressed. That creates a lot of pressure inside the gas, causing it to expand. It can’t expand well into the dense gas behind it, so it moves toward the star, flowing toward it along a vast plane. This creates ridges and filaments of gas, long thin structures. These tend to lie parallel to the fronts, and are very narrow indeed, just 50 – 100 billion kilometers or so thick. 

A near-infrared image of the Orion Bar, a wall of material in the Orion Nebula being eroded away by the powerful light of nearby massive stars. Ridges, filaments, dense knots of gas, and forming stars are seen in this image. Photo: Habart et al. (2022) / W. M. Keck Observatory

They also found knots, also called globules, of very dense gas embedded in the less dense material. That’s expected, since lower density gas will erode away more rapidly than denser material. Some of those globules are where stars are being born, the gas collapsing down and feeding nascent stars. In fact they may have found protostars, stars in the earliest stages of formation, in some of these globules.

Just above the ionization front, those globules get blasted by the starlight. Eroding away, they form long, comet-like tails as the gas flows around them, and we call these protoplanetary disks, or proplyds. Two of these were seen in their observations, so the astronomers will be able to study just how the light affects these forming stars in detail.

Much of these observations confirm and extend previous observations, showing us how stars affect their environment once they form. The details of star formation and stellar feedback are still being learned, but these observations and ones coming up soon will give us a huge boost in understanding this fiercely complex — and quite beautiful — process.