The Horsehead Nebula (its official name is Barnard 33) is one of the most famous objects in the sky. In pictures taken even with the use of amateur telescopes, it looks very impressive. What is this object and does it always look like in the usual photographs in the optical range?
Where the space horse lives
The Horsehead Nebula is located in the constellation of Orion - the region of the sky richest in interesting objects - just below the bright star Alnitak (the left star of Orion's Belt). The distance to it is approximately 1600 light years (about 490 parsecs). It's not too far; by galactic standards, she is our neighbor.
However, it is not easy to observe it with amateur telescopes, although it is possible to photograph it, especially if you put a special filter on the lens that transmits only one of the spectral bands of light emitted by ionized hydrogen. The fact is that Barnard 33 is visible to us against the background of another nebula - an emission nebula that intensely radiates precisely in this bandspectrum. With this filter applied, the Horse Head photo looks like this (see below).
Coming out of the cloud
If you look closely at the photograph of the nebula, you can see that it seems to be emerging from a giant dark cloud illuminated by stars. This majestic sight can shock and fascinate a person, especially if you remember that the "neck" and "head" of the space horse occupy a region of space with a diameter of about 3.5 light years.
The huge formation of which they are a small part, in turn, is just an element of an even grander structure hundreds of light-years long. This structure includes large interstellar clouds of dust and gas, bright diffuse nebulae, dark globules - isolated clouds of gas and dust, young and forming stars. This whole complex is called the "Orion Molecular Cloud".
Nature of the Dark Horsehead Nebula
The term "dark" means that it absorbs light, but does not emit or scatter it itself, and is visible in the optical range only because its silhouette shields light from the emission nebula IC 434 behind it.
Such objects are relatively dense (by interstellar standards), very long clouds of gas and dust. They are characterized by very irregular and indistinct borders and often have complex irregular shapes.
These cloudscold, their temperature does not exceed several tens, sometimes even units, kelvin. Gas exists there in molecular form, and interstellar dust is also present - solid particles up to 0.2 microns in size. The mass of dust is about 1% of the mass of gas. The concentration of a substance in such a molecular cloud can be from 10-4 to 10-6 particles per cubic centimeter.
The largest of the clouds can be seen with the naked eye, such as the Coal Sack in the constellation of the Southern Cross or the Great Pit in the constellation Cygnus.
Infrared Portrait
The development of all-wave astronomy made it possible to see the world in the widest range of its manifestations. After all, physical objects are capable of radiating not only in the optical range. Moreover, this frequency range - the only one available to our direct perception - is very narrow, and it accounts for only a small fraction of all the radiation from space.
Infrared rays can tell a lot about various space objects. So, in the study of molecular clouds, they are now an indispensable tool. Absorbing light of optical frequencies, the cloud will inevitably re-emit it in the infrared region of the spectrum, and this radiation will carry information about the structure of the nebula and about the processes taking place in it. Dust is no barrier to these rays.
In 2013, with the help of the space telescope. Hubble captured one of the most remarkable images of the Horsehead Nebula. Photo taken at wavelengths of 1.1 µm (blue overlay) and 1.6 µm(Orange color); north on the left. But she doesn't look like a horse anymore.
What's inside?
Infrared images seem to remove the dust curtain from the nebula, as a result of which the cloud structure of Barnard 33 becomes visible. The dynamism of its outer regions is perfectly visible: there is an outflow of gas under the influence of hard radiation from young hot stars. One of these luminaries is located at the top of the cloud.
The collapse of the cloud is also due to ionizing radiation from the emission nebula IC 434. Looking now at the optical image, the glow around the edge of Barnard 33 is striking - the ionization front, where energetic photons meet with the outer layers of the cloud. All these radiations, ionizing the gas, literally “blow off” it. Accelerating in a strong magnetic field, it leaves the cloud. Thus, the Horse Head is gradually melting, and in a few million years it will disappear completely.
The long-wavelength infrared image shows a different structure within the nebula: a gas arc is clearly visible where we see the familiar silhouette of a horse in optics.
Chemistry of the gas and dust cloud
Because dark nebulae are extremely cold, their own radiation falls on the long-wavelength part of the spectrum. Therefore, the chemical composition of such clouds is studied by analyzing the peaks of the microwave and radio spectra - the so-called signatures, the spectral signatures of certain molecules. Infrared radiation from dust is also being investigated.
The main component of any nebula is, of course, hydrogen - about 70% of it. Helium - approximately 28%; the rest is accounted for by other substances. It should be noted that their concentrations in different nebulae may differ. Signatures of water, carbon monoxide, ammonia, hydrogen cyanide, neutral carbon, and other substances common to interstellar clouds were found in the Horsehead spectra. There are also organic compounds: ethanol, formaldehyde, formic acid. However, there was also some unidentified line.
In 2012, it was reported that the molecule responsible for this mysterious signature had finally been found. It turned out to be a simple hydrocarbon compound C3H+. Interestingly, under terrestrial conditions such a molecular ion would not be stable, but in the interstellar nebula, where matter is extremely rarefied, nothing prevents it from existing.
Star Nursery
Cold and dense molecular clouds are the source of star formation, the cradle of future stars and planetary systems. In the theory of star formation, some details of this process are still unclear. But the very fact of the existence of protostellar objects at different stages of development in dark nebulae, as well as very young stars, has been proven using a large amount of observational data.
The Horsehead in the constellation of Orion is no exception. In general, the entire giant molecular Orion Cloud is characterized by an activestar formation. And in the dense regions of Barnard 33, the processes of star birth are going on. For example, a bright object almost at its very “crown” is a young luminary that has not yet left its “nursery” of dust and gas. There are similar objects in the area where the nebula connects to a large cloud. So the 'star nursery' in the Horsehead is working and will eventually lead to the destruction of this spectacular cosmic structure.
