JWST uncovers new secrets within the Horsehead Nebula

The Horsehead Nebula is an iconic astronomical sight.

A dark molecular cloud of neutral gas sits in front of an active star-forming region: IC 434, creating the iconic sight of the Horsehead Nebula. Located about 1300 light-years away in the constellation of Orion, the dark cloud is quite large in extent, explaining why more stars are visible “above” the nebula than below it.

Most nebulae are luminous: shining red and/or blue.

This ground-based, wide-field image of the Eagle Nebula shows the star-forming region in all its glory, with new stars, the blue glow of reflected starlight, and the red glow of ionized atoms all present. Dusty, light-blocking features are also prominent. The reddish glow at the gaseous outskirts is a result of hydrogen atoms recombining, and a photon of precisely 656.3 nanometers being emitted every time an electron transitions from the n=3 to the n=2 energy state.

Energetic light ionizes hydrogen atoms, leading to red “emission” nebulae as electrons de-excite after recombining.

This portion of the Cosmic Reef composition highlights the blue reflection nebula created by winds blown off of a hot, massive, giant blue star that are then illuminated in reflected light by the original star that created it. The Wolf-Rayet star that powers it may be destined, in short order, for a stellar cataclysm such as a core-collapse supernova, but we can only see the presence of the cold, expelled gas from its outer layers as they reflect the starlight from the luminous, blue star powering it.

Neutral atoms also reflect bright starlight, creating bluish “reflection” nebulae.

Dark, dusty molecular clouds, like Barnard 59, part of the Pipe Nebula, appear prominent as they block out the light from background objects: stars, heated gas, and light-reflecting material. In the young Universe, prior to an age of ~550 million years, a large fraction of atoms were not ionized, and so should be very efficient at blocking the light even from hot, newly-formed stars. Why a galaxy like JADES-GS-z13-1-LA has a bright hydrogen emission line is currently a puzzle for astronomy.

But the Horsehead is a different beast: a dark nebula.

Near Orion’s Belt, the reflection nebula known as the Flame Nebula (left) as well as the star-forming emission nebula known as IC 434 (in red) are joined by a series of dark molecular clouds in the foreground that create spectacular silhouettes known as dark nebulae. The Horsehead Nebula (at center) is arguably the most famous dark nebula of them all.

Cool gas clouds in the foreground of stars and star-forming regions block the background light.

This optical image of the Horsehead Nebula was taken by the Hubble Space Telescope back in 2001, and showcases not only the shape and density of the light-blocking nebula itself, but reveals many faint, reddened stars located behind the nebula where the light still penetrates and escapes the light-blocking neutral matter.

Their silhouettes create intricate “dark nebulae” against the illuminated backdrop.

This view of the Horsehead Nebula comes courtesy of the ESA’s Euclid mission, which is an optical-and-infrared space telescope that took this panoramic, detailed view of this region of the constellation Orion, also known as Barnard 33. The Horsehead Nebula sits just to the south of star Alnitak, the easternmost of Orion’s famous three-star belt, and is part of the vast Orion molecular cloud.

With internal and external heating, infrared eyes reveal intricate features within the Horsehead Nebula.

Unlike visible-light images of the Horsehead Nebula, this near-infrared view is sensitive to the relatively warm gas within the optically dark nebula itself, lighting it up for Hubble’s infrared instruments. Intricate, wispy features within the gas, as well as partially obscured background and internal stars and proto-stars, all appear illuminated in this view.

Euclid, Hubble, and JWST’s NIRCam views all expose this neutral gas.

The infrared views of the Horsehead Nebula from Euclid (left), Hubble (center), and JWST (right). The smaller field-of-views of Hubble and JWST are a feature, not a bug, as they image regions of this nebula at far higher resolution than a wide-field telescope like Euclid.

Different features appear at longer, mid-infrared wavelengths in JWST’s MIRI instrument.

This image of the Horsehead Nebula from NASA’s James Webb Space Telescope focuses on a portion of the horse’s “mane.” It was taken with Webb’s MIRI (Mid-Infrared Instrument). Mid-infrared light captures the glow of substances like dusty silicates and soot-like molecules called polycyclic aromatic hydrocarbons. This image was assembled using nine different mid-infrared filters, ranging from 5.6 to 25 microns in wavelength.

But inside, with JWST’s high-resolution instruments, many new secrets are exposed.

The “mane” of the Horsehead Nebula, as highlighted by JWST’s NIRCam views, shows off simultaneously the intricate features of the gas at the edge of the nebula along with background stars and galaxies, plus wispy features at the edge likely shaped by magnetic fields. Within the gaseous nebula itself, various “lights” are visible inside.

Background galaxies are just as abundant near the Horsehead’s boundary as they are farther away.

With JWST’s unprecedented sensitivity, we can now clearly see that even though substantial amounts of light-blocking material are present near and at the boundary of the Horsehead Nebula itself, the background density of galaxies remains unchanged near the nebula itself. Features and shapes are more difficult to detect, but there are literally galaxies everywhere.

Individual stars appear to be forming inside the nebula, poking through the neutral gas in infrared light.

Many lights can be seen in this JWST NIRCam image of the gas within the Horsehead Nebula. Some are due to background galaxies while others are due to partially obscured stars that are either inside or behind the “horse’s head” itself, but unless the diffraction spikes (of a star) or the extended morphological features (of a galaxy) are clearly visible, the nature of these lights cannot so easily be determined.

Some new stars are identifiable by their diffraction spikes in NIRCam imagery.

One of the telltale signatures of a star in JWST imagery is the “nightmare snowflake” pattern, reminiscent of a series of hexagonal spikes with additional features and distortions, imprinted on point sources due to the optical geometry of the telescope. The object at the lower-left of this image is clearly stellar in nature.

Other “lights” revealed inside, however, correspond to highly luminous background galaxies.

The objects circled (in yellow) here, despite being located along the same line-of-sight as the neutral gas found in the Horsehead Nebula, can easily be identified as background galaxies due to their extended shapes, still resolvable by JWST’s NIRCam instrument despite the presence of that foreground material. Many of the other lights seen inside may yet be galaxies as well, but a visual inspection is insufficient to reveal their nature.

One includes a potentially active supermassive black hole.

This portion of the JWST MIRI view of the Horsehead Nebula does not have a NIRCam counterpart, as the field-of-view is centered differently. However, the spike-riddled object at the bottom is likely not a point-like star, but rather an active supermassive black hole in a very distant background galaxy: something that the MIRI instrument is outstanding at picking up. Many previously unknown supermassive black holes have been discovered thanks to the power of MIRI.

Just 1300 light-years away, infrared views reveal the internal workings within the Horsehead.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.