The Horsehead Nebula (its official name is Barnard 33) is one of the most famous objects in the sky. In pictures taken even using amateur telescopes, she looks very impressive. What is this object like and does it always look like in the usual photographs in the optical range?
Where does the space horse live?
The Horsehead Nebula is located in the constellation Orion - the richest region of the sky for interesting objects - just below the bright star Alnitak (the left star of the Orion 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.
Nevertheless, it is not easy to observe it with amateur telescopes, although photographing is possible, 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 - emission, intensively radiating precisely in this band of the spectrum. Using this filter, the photo of the Horsehead 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 is as if selected from a giant dark cloud illuminated by stars. This magnificent sight is able to shock and fascinate a person, especially if you remember that the “neck” and “head” of the space horse occupy an area of space with a diameter of about 3.5 light years.
The huge formation, which they are part of as a small part, in turn, is just an element of an even more grandiose structure with a length of hundreds of light years. This structure includes large interstellar clouds of dust and gas, bright diffuse nebulae, dark globules - isolated gas and dust clouds, young and emerging 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 it shields the light from the emission nebula IC 434 located behind it.
Such objects are relatively dense (by interstellar standards), a very long cloud of gas and dust. They are characterized by very irregular and fuzzy boundaries and often have a complex irregular shape.
These clouds are cold, their temperature does not exceed several tens, sometimes even units, kelvin. Gas exists in molecular form there, and interstellar dust is 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 clouds can be seen with the naked eye, for example, the Coal Sack in the constellation of the Southern Cross or the Great Dip in the constellation Cygnus.
Infrared portrait
The development of all-wave astronomy made it possible to see the world in a wide range of its manifestations. After all, physical objects are capable of emitting 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 cosmic emissions.
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 occurring in it. Dust is not an obstacle to these rays.
In 2013, using the space telescope them. Hubble received one of the most remarkable images of the Horsehead Nebula. The photo was taken at wavelengths of 1.1 microns (blue color applied) and 1.6 microns (orange color); north to the left. But she’s not so much like a horse anymore.
What is inside?
Infrared images seem to remove the dust curtain from the nebula, as a result of which the Barnard 33 cloud structure becomes visible to the eye. The dynamism of its outer regions is clearly visible: gas outflows there under the influence of hard radiation from young hot stars. One of such stars is in the upper part of the cloud.
The destruction of the cloud also occurs as a result of the ionizing radiation of the emission nebula IC 434. If we now look at the optical image, the glow along the edge of Barnard 33, the ionization front, where energetic photons meet the outer layers of the cloud, is striking. All these emissions, ionizing the gas, literally "blow off" it. Accelerating in a strong magnetic field, it leaves the cloud. Thus, the Horsehead is gradually melting, and in a few million years it will completely disappear.
The image obtained in infrared rays with long wavelengths shows a different structure in the composition of the nebula: the gas arch is clearly visible where in optics we find the usual silhouette of a horse.
Chemistry of a gas and dust cloud
Since dark nebulae are extremely cold, their own radiation is in the long-wavelength part of the spectrum. Therefore, the chemical composition of such clouds is studied by analyzing the peaks of microwave and radio spectra - the so-called signatures, spectral signatures of various molecules. Infrared dust radiation is also being investigated.
The main component of any nebula is, of course, hydrogen - about 70% of it. Helium - approximately 28%; the rest is other substances. It should be noted that their concentrations in different nebulae may vary. Signatures of water, carbon monoxide, ammonia, hydrogen cyanide, neutral carbon, and other substances common to interstellar clouds were found in the spectra of the Horsehead. There are organic compounds: ethanol, formaldehyde, formic acid. However, there was a certain unidentified line.
In 2012, a message appeared that, finally, a molecule was found responsible for this mysterious signature. It turned out to be a simple hydrocarbon compound C 3 H + . Interestingly, in terrestrial conditions such a molecular ion would not be stable, and in an interstellar nebula, where the substance is extremely rarefied, nothing prevents it from existing.
Star Crib
Cold and dense molecular clouds are a 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 in dark nebulae at different stages of development, as well as very young stars, was proved with the help of a large amount of observational data.
The Horsehead in the constellation Orion is no exception. In general, the entire giant molecular cloud of Orion is characterized by active star formation. And in the dense regions of Barnard 33, the processes of birth of stars are underway. For example, a bright object almost at its very top is a young star that has not yet left its "manger" from dust and gas. There are similar objects in the area where the nebula joins a large cloud. So the "star nurseries" in the Horsehead work, and, in the end, they will lead to the destruction of this spectacular cosmic structure.