Astro Bob: Meet the Horsehead Nebula, a darkly beautiful creature of winter nights

Horsehead Nebula and vicinity — The Horsehead Nebula, a dark cloud of cosmic dust resembling a horse's head, is silhouetted against a veil of bright nebulosity named IC 434. More glowing gas and dust clouds are visible to the left of the Horsehead and near the bright star Alnitak (Zeta Orionis), located in Orion's Belt. This photo was taken through a 10-inch telescope.

By Bob King

February 22, 2022 at 2:42 PM

Orion is a proverbial candy store. Open the door, look around, and you'll find just about anything on its starry shelves. In previous blogs, we've encountered the Orion Nebula, the erratic variable star Betelgeuse and learned how to use Orion's Belt as a stepping stone to other constellations.

I just found yet another reason to return. My friend Chris Schur, an astrophotographer living in Payson, Arizona, sent me a photo of the Horsehead Nebula this week. I've seen this enigmatic blob of cosmic dust in previous photos and even with my own eyes through the telescope. But his image, which features the nebula in a spectacular setting, struck me with its raw beauty. I wanted you to see it, too.

Orion's Belt and Horsehead Nebula — This photo helps to put the Horsehead Nebula in context. It's located just south of the bright star Alnitak in Orion's Belt, appearing as a dark notch along the left side of the nebula IC 443. Ultraviolet light from Alnitak excites the atoms in both IC 443 and the Flame Nebula, which causes them to emit a pink glow.

It's easy to understand how the Horsehead got its name — the face, the neck, the mane. Practically a spitting image. Unlike the Orion Nebula, which glows brightly from stars tucked within its cloudy confines, the Horsehead is a dark nebula. Dark nebulae are made of the same materials as bright ones — gas and dust. But they typically contain a higher concentration of dust and also lack stars that would illuminate them from within.

Two views of the Horsehead — This image shows two different views of the Horsehead Nebula. On the right we see it in visible light, taken using the European Southern Observatory’s Very Large Telescope in Chile. Notice how dark and opaque the nebula appears. On the left we see it in infrared light through the Hubble Space Telescope. Infrared can penetrate dust to reveal far more detail including two recently-born stars along the horse's brow.

The same way you block a light with your hand, these dusty clots block the light of background stars and nebulosity and appear in silhouette. We see the dark Horsehead thanks to the glowing veil of bright nebulosity known as IC 434 located just behind it. Gas in IC 434 fluoresces pink because hydrogen atoms in the nebula are excited by the intense ultraviolet light emitted by the star Alnilam, located nearby.

If you've seen Orion's Belt, you've seen Alnilam. It's the leftmost (easternmost) of that bright trio and a hot, blue supergiant 10,000 times brighter than the sun. One day it will start to expand and come to resemble its neighbor, Betelgeuse. Both stars will soon enough run out of fuel (and the energy it generates) to keep gravity at bay. Without an energy source, the star collapses and then explodes in a blast bright enough to see across millions of light-years.

Dark nebula LDN 1768 — Although these look like empty spaces they're all part of the dark nebula LDN 1768 in the constellation Ophiuchus the Serpent-bearer. Inside this dark nurseries are hundreds of protostars still coalescing out of the clouds' gas and dust. They won't become visible to the eye until they heat up further and produce strong winds (typical of young stars) to literally clear the dust.

Before its annihilation, the star releases gobs of gas into space in an ever-expanding envelope. The explosion sends a shock wave through those gases, compressing the material and causing it to condense into dust. Gravity, sometimes with the help of additional shock waves generated by future supernova blasts, gathers the material into the clouds or nebulae. Within denser pockets of those nebulae, gas and dust coalesce into stars and planets. Many elements and compounds essential to life arose through the death of long-forgotten stars.

Deep within the Horsehead, new stars are forming as gravity draws dust into ever-denser pockets. Compression generates heat which ultimately ignites nuclear fusion . Fusing hydrogen atoms release energy, and a baby star is born. One day, parts of the nebula will glow in the light of newborn suns similar to the Orion Nebula, a much larger stellar nursery, does today.

Orion complex — In this deep image, the constellation Orion is nearly lost in a tangle of nebulae. They all belong to a vast, rich, stellar nursery called the Orion Molecular Cloud Complex. The three belt stars are visible at center. Just below the leftmost star in the belt you can make out the pink strip of IC 434 and a dark notch along its length — the Horsehead Nebula.

The Horsehead Nebula lies 1,375 light-years from Earth and is part of a dense jungle of milky nebulosity called the Orion Molecular Cloud Complex . This prodigious "birthing room" dominates the entire constellation of Orion. Dust that masks stars and clusters for us will one day congeal into fresh, stellar gems that our future descendants will see.

Horsehead in the far infrared — In this image, taken by the European Space Agency's Herschel Space Telescope in far infrared light (a deeper infrared than the Hubble photo above), we can better see how the Horsehead is just a tiny piece of a much larger complex of interstellar clouds in the constellation Orion.

While photographs of the Horsehead reveal incredible detail and contrast, it's actually quite faint to the eye even in a moderately large telescope like an 8-inch or 10-inch reflector. I've observed it in my 15-inch scope from country skies as a dark, ghostly lump. But no matter how dim something appears through the eyepiece, I always get a kick out of seeing the real thing. I hope you'll also get a chance to see it someday. While you're waiting enjoy the photos!