JWST combs through the Horsehead’s mane

Infrared light reveals how fiercely luminous stars are destroying the iconic nebula

April 30, 2024 Issue #715

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Where I’ll be doing things you can watch and listen to or read about

Pic o’ the Letter

A cool or lovely or mind-bending astronomical image/video with a description so you can grok it

Of all the gas and dust clouds in the sky, there is one that absolutely looks like what it’s named for: The Horsehead Nebula. If anything it resembles a gigantic celestial chess piece even more closely, but Horsehead it is.

In visible light (the kind our eyes see) it’s black, because it’s made of dust, tiny silicaceous (rock) and carbonaceous (sooty) grains that are excellent at absorbing light. Behind it from our viewpoint is a glowing cloud of hydrogen gas, pinkish-red, enhancing its equine silhouette.

But that’s in visible light. Warm dust glows in the infrared, making the nebula a prime target for telescopes that see in those wavelengths, like, say JWST.

Scientists just released a new image from that observatory, and while it’s not exactly horse-like, it’s still extremely cool and very interesting.

What you’re seeing here is the top of the horse’s head, the mane if you will. It really looks like the top of a cloud, doesn’t it? It also looks like it’s being illuminated by that bright star near the top, but that’s an illusion; that star is very bright in infrared, but the Horsehead is warmed by the light of two other stars, both outside this field of view… which play a role in why this image was taken.

The two stars are HD 37699 and Sigma Orionis. The former is a bright B-type star, and the latter a multiple-star system that includes two incredibly luminous massive stars that orbit each other. They flood the space around them with decently high-energy ultraviolet light. Although Sigma Orionis is about 10 light-years away from the Horsehead, that light energizes the atoms and molecules in the cloud, making the gas I mentioned earlier behind the Horsehead glow.

But they also hit the dust in the Horsehead, zapping it. This heats the dust up, which then flows away from the nebula. That eats away at the nebula, causing it to evaporate; estimates are it will last just another few million years before boiling away completely (though to be honest I’d give even odds one or both of the massive stars in the Sigma Ori system will probably go supernova before then anyway; while that will remove the persistent UV light coming from the system, it does add a handful of other, far more catastrophic situations for the nearby Horsehead).

The JWST image was taken to study this evaporation process. It’s a composite of images taken by both the Near Infrared Camera and the Mid-Infrared Instrument, covering a wide range of infrared wavelengths. The filters used emphasize light emitted by dust, cold hydrogen, water ice, complex carbon molecules called polycyclic aromatic hydrocarbons (or PAHs, which is the soot I mentioned above), and carbon dioxide. By looking at all these components, the astronomers can try to figure out in detail what’s happening to the dust in the nebula [link to journal paper].

They found direct evidence of material flowing away from the Horsehead: long, fine, thin lines of material extending perpendicular to the top of the cloud, similar to wisps of fog over a warm lake on a cool day. This is material getting hit by the light of Sigma Ori and streaming away, what’s called a photoevaporative flow, seen most strongly in the filters looking at PAHs. Using some complicated physics involving how bright the flow appears, how much light is hitting it, and how big the teeny grains of dust are, they find that the entire flow contains about five one-millionths (0.000005) of the Sun’s mass. That doesn’t sound like much, does it? Well think of it this way: that’s more than the mass of the entire Earth! So imagine taking our whole planet, grinding it into microscopic dust, and then scattering it over a few trillion kilometers of space.

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