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SYFY WIRE Bad Astronomy

ORION. Holy Wow, Orion.

By Phil Plait

At a distance of 1,300 light-years—just 13,000 trillion kilometers, which is close on a galactic scale—the Orion Nebula is one of the most magnificent objects in the sky. It’s so luminous that you can see it by eye even in mildly light-polluted areas, and when you use binoculars you can tell it’s not a star, but something fuzzy and big, hinting at its true nature.

That nature comes into clarity when the nebula is photographed using a telescope. That reveals it to be an immense cloud of gas and dust, light-years across, a factory for creating stars, colorful and spectacular.

But why describe it when I can show you? Behold!

The magnificent Orion Nebula, in infrared light taken using the HAWK-1 camera on the Very Large Telescope. Credit: ESO/H. Drass et al., Additional Processing Robert Gendler

This image of the Orion Nebula is one of the largest and deepest ever taken. It was done using the HAWK-1 infrared camera attached to the Very Large Telescope in Chile, an 8.2 meter telescope that can see celestial objects in extraordinary detail. This image is not exactly what was released by the European Southern Observatory originally; the observations were remastered by astrophotographer Robert Gendler to bring out more detail and to really shine a light (so to speak) on the phenomenal beauty of this immense stellar nursery.

There’s some important science lurking in this image, but there’s something I want to point out first. The glowing part of the nebula is actually just a small part of a much larger complex called the Orion Molecular Cloud. It’s a dense, cold cloud of gas and dust, invisible to the eye, and stars are forming in it. A clutch of stars happened to form near the edge of the cloud, and once they switched on after birth their intense radiation began carving enormous cavities in the gas, chewing away at the material in the cloud.

Because they’re near the edge, they eventually ate a hole on the side of the cloud. In a sense they popped the bubble, blowing out the cloud at their location, which happened to be on the side of the cloud facing us. When we look at the Orion Nebula, we’re actually seeing a dimple or divot scooped out of the denser material. The lower density and much hotter gas filling that dimple glows brilliantly, creating the nebula we see.

This image actually shows that extremely well. Redder material is denser, and the blue glow suffusing the nebula is lower density, hotter gas, tracing the shape of the cavity. It’s an extraordinary glimpse literally inside the nebula.

You can easily see hundreds of stars clustered in the center of the nebula. In fact, that’s why these observations were made: To literally count the newborn stars there, determine their mass and luminosity (the amount of energy they emit), and use that information to determine just how stars are formed.

This can be hard to do, because at the faint end stars can be hard to see. Worse, objects with less mass than stars—brown dwarfs and planetary-size objects—are so faint they fade to invisibility with distance.

But not to the HAWK-1 coupled with the VLT. This duo is so sensitive that all these objects can be found, providing a complete census of the stellar (and substellar) population.

The results are actually pretty intriguing. Before I reveal that, let me explain what the astronomers involved with this research were looking for.

The most obvious, brightest stars in the Orion Nebula are monsters, a hundred times the mass of the Sun and hundreds of thousands of times more luminous—put one of them in the center of our solar system and the Earth would be fried to a crisp.

Only a handful of these behemoths can be found in the nebula. In general, that’s always true: Massive, luminous stars are rare, and lower mass, cooler stars more common. Think of it this way: Take a rock, and hit it with a hammer. You get one or two big pieces, more medium-size pieces, and zillions of tiny shards. But the number of stars versus their mass tells you a lot about how they’re born, and so astronomers study this in as much detail as they can, and they call it the Initial Mass Function, or IMF.

Understanding it means understanding under what conditions stars are born, and that’s a key goal. That’s why the Orion Nebula was observed: It’s close enough that the faintest objects born there can be seen. Because of this, astronomers can be reasonably sure they’re seeing everything, and not missing the faintest objects, throwing off their results.

In general, out in the galaxy at large, the IMF peaks around stars with 0.25 times the Sun’s mass then drops off with lower mass. The big questions is: Does this trend continue, such that even lower mass objects like brown dwarfs and exoplanets* are rare?

For the Orion Nebula the answer is “no.” The astronomers saw two peaks in the IMF, one around 0.25 times the Sun’s mass, and another at 0.025. In other words, Orion really likes making objects around those two masses, with fewer object at more or less mass than that.

That second peak is pretty surprising. It shows that Orion like to make substellar objects at ten times the rate previously seen in other surveys of the sky. Fully half the objects seen are substellar, objects up to 70 or so times the mass of Jupiter; too low mass to ignite fusion in their cores to become true stars. They found ~160 planetary mass objects and ~760 brown dwarf objects (I’ll note these are candidates; they need to be confirmed).

The dip between the two peaks occurs right around the mass of objects that can start fusion. Why? There are two ways to form objects from a gas cloud: Direct collapse from the gas itself, or clumping of material around the disk of a forming star.

Stars are thought to form from direct collapse; a knot of material in the gas can be big enough that gravity causes it to collapse, forming a star. Around that star you get a flattened disk of material, and planets form from that. They build from the bottom up, as grains clump together to form rocks, and these aggregate to form larger and larger objects.

If a star forms from the gas, it tends to blow away the gas around it. That’s why you expect a drop in the IMF at that mass; the birth of the stars themselves cuts it off. But that second peak means something else is going on. It’s not clear what, but one explanation is that brown dwarfs form along with stars, gathering mass from the disk around the young star. Then, after some time, they get ejected, perhaps due to the gravity of other nearby stars (especially if they form in multiple star systems, where such encounters would be more common).

This sort of thing is pretty new; it’s only been recently that we’ve even had the capability to make these observations down to such low masses. Astronomers have been looking into the IMF for decades, but with powerful tools like VLT and HAWK-1 they can now peer more deeply than ever before, and uncover these faint, low-mass objects (I’ll note the first brown dwarf wasn’t even found until 1995).

In astronomy, our growth in knowledge is directly tied to our prowess in creating more powerful technology to observe the heavens. And when we do the Universe rewards us with more understanding, and a deep appreciation for the laws of physics and Nature.

That’s a pretty good deal.

* Usually, exoplanets form with stars, but under some conditions they can also form on their own. These objects are what we call rogue exoplanets, wandering space with no host star to warm them. Most rogue exoplanets may have formed around stars and gotten ejected from their parent systems, but some formed that way in the first place. Understanding how many of each exist in the galaxy would help us understand the dynamics of how they’re formed in the first place.

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