The Horsehead Nebula (optical), a dark nebula in Orion

 
The Horsehead Nebula (optical), a dark nebula in Orion

Image Credit: Canada-France-Hawaii Telescope/Coelum

The Horsehead Nebula (Barnard 33) is a diffuse dark nebula located about 1500 light-years away in the constellation Orion, just to the south of the easternmost star in Orion’s Belt, Alnitak. The dark cloud of dust and gas is part of a region in the vast and complex Orion Nebula where star formation is taking place right now.

It is one of the most identifiable nebulae because of the shape of its swirling cloud of dark dust and gas, which is similar to that of a horse’s head when viewed from Earth. The dark molecular cloud is visible only because its obscuring dust is silhouetted against the bright red emission nebula IC 434. The Horsehead is a fascinating, active, and complex neighborhood.

The red glow originates from hydrogen gas predominantly behind the nebula, ionized by the nearby bright, hot blue 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. The dark gas extends below the more obvious horse’s head into a shapeless blob beneath it.

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. The visible heart of the nebula emerges from the gaseous complex to serve as an active site of the formation of “low-mass” stars.

Stellar nurseries can contain over 100 known organic and inorganic gases as well of dust consisting of large and complex organic molecules. The region of the Orion Nebula containing the Horsehead is a stellar nursery. The darkness of the massive nebula is not explained by the dust and gas, but by the complex blocking the light of stars behind it. The heavy concentrations of dust in the Horsehead Nebula results in alternating sections of nearly complete opacity and transparency.

As a cloud core emerging from its parental cloud, and as an active site of low-mass star formation, the Horsehead is revealing the intricate interrelations between gas, dust, and the light from hot stars. Small red spots in the base of the Horsehead betray the presence of hidden protostars, and red streaks near the yellowish nebula are Herbig-Haro objects, which are jets of material ejected from protostars.

The Horsehead Nebula is rotating.

The radio waves are Doppler shifted as different parts of the nebula move toward or away from us, producing either a blueshift or a redshift. The horse’s nose is turning toward us and part of the mane is turning away–as if the horse were trying to look in our direction. Furthermore, most of the horse’s neck shares the same spin: the left side is approaching and the right side is receding.

Astronomers estimate that most of the neck will take about 4 million years to spin once–if the nebula survives that long. However, radiation from Sigma Orionis, hits and erodes the top of the nebula. As a result, astronomers think the Horsehead, which is about half a million years old, will endure for only another 5 million years. That’s enough time to let the nebula pirouette once before its demise.

The Horsehead’s rotation has sculpted the gas and dust into its unique shape. The rotation has caused a centrifugal force that’s flung what is now the horse’s nose, on the left, and the mane, on the right, away from the neck, thereby giving the nebula its beautiful appearance. Without this rotation, the Horsehead Nebula probably wouldn’t look like a horse.

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