APOD: The Horsehead Nebula (2024 Nov 25)

[APOD Robot]

The Horsehead Nebula

Explanation: One of the most identifiable nebulas in the sky, the Horsehead Nebula in Orion, is part of a large, dark, molecular cloud. Also known as Barnard 33, the unusual shape was first discovered on a photographic plate in the late 1800s. The red glow originates from hydrogen gas predominantly behind the nebula, ionized by the nearby bright star Sigma Orionis. The darkness of the Horsehead is caused mostly by thick dust, although the lower part of the Horsehead's neck casts a shadow to the left. Streams of gas leaving the nebula are funneled by a strong magnetic field. Bright spots in the Horsehead Nebula's base are young stars just in the process of forming. Light takes about 1,500 years to reach us from the Horsehead Nebula. The featured image was taken from the Chilescope Observatory in the mountains of Chile.

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[Ann]

Have you seen the "ghost girl" superimposed on the Horsehead? Maybe she fell off the horse?

APOD 25 November 2024 annotated.png

The Horsehead Nebula. Image Credit & Copyright: Alex Lin (Chilescope)

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On a slightly more serious note: The Horsehead Nebula is a pillar, one of those ubiquitous structures that we see where there is nebulosity near hot stars. The hot star that is sculpting the Horsehead Nebula is Sigma Orionis:

Sigma Orionis Chris Kjeldsen.png

Sigma Orionis with surrounding nebulosity and pillars.
Note the Horsehead Nebula and other protrusions as well,
all pointing at Sigma Orionis. Credit: Chris Kjeldsen.

Pillars are formed when the harsh ultraviolet light and strong winds from a hot star blows away the nebular cloud that created it, but denser clumps of gas and dust resist destruction for a longer time. The clumps protect the area directly below them, creating the pillar-like shape.

We may note that Sigma Orionis may have had a nasty outburst in the direction of the Horsehead so that it smashed the original pillar's top down and made it concave, making one end of it hang down, creating the Horsehead effect.

APOD 25 November 2024 annotated 2.png

Ann

[AVAO]

Have you seen the "ghost girl" superimposed on the Horsehead? Maybe she fell off the horse?

APOD 25 November 2024 annotated.png

The Horsehead Nebula. Image Credit & Copyright: Alex Lin (Chilescope)

---

On a slightly more serious note: The Horsehead Nebula is a pillar, one of those ubiquitous structures that we see where there is nebulosity near hot stars. The hot star that is sculpting the Horsehead Nebula is Sigma Orionis:

Sigma Orionis Chris Kjeldsen.png

Sigma Orionis with surrounding nebulosity and pillars.
Note the Horsehead Nebula and other protrusions as well,
all pointing at Sigma Orionis. Credit: Chris Kjeldsen.

Pillars are formed when the harsh ultraviolet light and strong winds from a hot star blows away the nebular cloud that created it, but denser clumps of gas and dust resist destruction for a longer time. The clumps protect the area directly below them, creating the pillar-like shape.

We may note that Sigma Orionis may have had a nasty outburst in the direction of the Horsehead so that it smashed the original pillar's top down and made it concave, making one end of it hang down, creating the Horsehead effect.


APOD 25 November 2024 annotated 2.png


Ann

Cool discovery, the thing with the "ghost girl". In IR I would call her a mermaid and snake tamer.

Click to view full size image

artistical comosit jac berne (flickr)

[Christian G.]

Nice APOD, this Horsehead is darker than dark! I also like how the streams of gas leaving the nebula come out.

Horsehead_Lin_2596.jpg

And - wow! - we can really see Sigma Orionis "at work" in your image, Ann!

Sigma Orionis Chris Kjeldsen.png

[Dellok]

I cannot help but reflect on the heroic work of Horace Hedd whenever this silhouette is shown!

[johnnydeep]

Ok, where's this shadow that the horse's neck is casting? And which stars are the ones "in the process of forming" at the base of the horse's neck?

[Ann]

Ok, where's this shadow that the horse's neck is casting? And which stars are the ones "in the process of forming" at the base of the horse's neck?

Don't know about the shadow. And the only newborn star, or protostar, that I'm aware of in the Horsehead Nebula is this one:

APOD 25 November 2024 annotated 3.png

Oh, and - fun fact. In every reasonably "true-color" image, we see the Horsehead against a red, hydrogen alpha background. Well, years ago I read that the only chance that humans have to observe the Horsehead visually is to observe it with a hydrogen beta filter, which is this color, ███.

That's because the red hydrogen alpha emission is always mixed with a bit of cyan-colored hydrogen beta, too. The thing about us humans is that our eyes are quite insensitive to faint red light, but we are good at picking up green or blue-green light. That is why it is possible - very hard, but possible - to spot the dark shape of the Horsehead against a cyan-colored hydrogen beta background!

Ann