One of the most spectacular objects in the whole sky is the famous Orion Nebula, a star birth factory 1,300 light years from Earth. It is so bright that it can be seen with the naked eye; it looks like a star, in the middle of three that form Orion’s “dagger” that hangs from his belt. With a telescope, it turns out to be a bright, colorful display of hot gas a few dozen light-years across.
But our view of her changes, because she devours herself from the inside. The question is: will this process destroy the gas and deprive the stars of building materials, or will it help trigger the formation of new stars?
To find out more, a team of astronomers used the two the 10-meter Keck terrestrial telescope with a near infrared camera and the JWST space observatory to observe the Orion Nebula, the site of massive star formation closest to the Sun. It is actually a small part of Orion’s much larger molecular cloud, a cloud of cold, dense, dark gas.
They want to look at the stellar feedback process. In the center of the nebula are several recently created large stars, including the beast Theta1 Ori C, a binary system in which one star has 11 times the mass of the Sun and the other 33. This larger one emits 200,000 times more energy than the Sun! This radiation is intense and heats and erodes the molecular gas that surrounds it. The cooler gas in the molecular cloud is mostly hydrogen molecules, two hydrogen atoms attached. But Theta1 Ori C’s ultraviolet light separates them, creating neutral hydrogen atoms. It also ionizes these atoms, profoundly modifying their behavior. This carves out a growing cavity around the stars with Theta1 Ori C at the center, like a worm worming its way out of an apple from the center outward.
This makes it much more difficult for stars to form in the nebula. But this process also drives intense pressure waves through 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 who wins? The forces of creation or destruction?
When Theta1 Ori C eats away at the material around it, it creates a roughly spherical wall around itself. We see part of this wall on the edge so it looks more like a line, called Orion’s bar. That’s what astronomers targeted and got the highest resolution near-infrared images ever taken.
What they saw was chaos. When the light from the star hits the denser material, the gas is heated and flows towards the star – it’s like mist rising from a hot puddle as it is heated by the Sun. Over time, the gas becomes stratified: outside the bar – further away from Theta1 Ori C and the center of the cavity – is the cold, dense molecular gas; closer to the center the gas is blown into atoms by light – called photodissociation – and closer to the center these atoms are ionized. The boundaries between these layers are called fronts, so there is a photodissociation front and an ionization front.
They found that the two fronts are very close to each other, separated by only about 750 billion kilometers. It’s less than a tenth of a light-year, which is a short distance from the 25-light-year cavity that makes up the Orion Nebula.
The densest part of the region is along the photodissociation front, where the gas is heated and compressed. This creates a lot of pressure inside the gas, causing it to expand. It cannot expand well in the dense gas behind it, so it moves towards the star, flowing towards it along a wide plane. This creates gas ridges and filaments, long thin structures. These tend to run parallel to the fronts and are very narrow indeed, only 50 to 100 billion kilometers thick.
They also found nodes, also called globules, of very dense gas embedded in the less dense material. This is normal, as low density gas will erode faster than denser materials. Some of these globules are where stars are born, the gas collapsing and powering infant stars. In fact, they may have found protostars, stars in the early stages of formation, in some of these globules.
Just above the ionization front, these globules are projected by starlight. As they erode, they form long comet-like tails as gas flows around them, and we call these protoplanetary disks, or proplyds. Two of them were seen in their observations, so astronomers will be able to study in detail how light affects these forming stars.
Many of these observations confirm and extend previous observations, showing us how stars affect their surroundings once they form. The details of star formation and stellar feedback are still being learned, but these and future observations will give us a huge boost in understanding this hugely complex – and quite beautiful – process.
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