The mystery of star formation in the Horsehead Nebula

The Horsehead Nebula
The Horsehead Nebula

Two articles, one published in the journal “The Astronomical Journal” and one in the journal “Astronomy and Astrophysics”, describe as many researches on the Horsehead Nebula, iconic for its particular shape. Two teams of researchers used NASA’s SOFIA flying observatory to map the distribution of gas and dust connected to the star formation activity within it.

The Horsehead Nebula, also known as Barnard 33, is part of the giant Orion B molecular cloud, which in turn is part of the Orion Molecular Cloud Complex, a large region of star formation among the most active. In astronomical terms it’s close, with a distance estimated between 1,500 and 1,600 light years from Earth, and is not covered by dense clouds, so it’s the subject of a considerable amount of studies.

A team led by John Bally of the Center for Astrophysics and Space Astronomy at the University of Colorado at Boulder, author of the article published on “The Astronomical Journal”, aimed at understanding if the intense radiation from nearby stars was strong enough to compress the gas inside the Horsehead Nebula to trigger the formation of new stars. To do this, the researchers combined the data collected with SOFIA with those of other observatories.

It turned out that the radiation from neighboring stars generates hot plasma that compresses the cold gas within the Horsehead Nebula. The pressure exerted, however, is insufficient to trigger the birth of new stars so more research will be needed to understand exactly what’s happening.

The researchers still managed to understand that the shape of the Horsehead Nebula is due to an ionization wave caused by the stellar radiation that hit the great cloud. The densest area stopped that wave, which instead kept on advancing in less dense areas.

Another team led by Cornelia Pabst of the University of Leiden, in the Netherlands, author of the article in “Astronomy and Astrophysics”, analyzed the structure and brightness of the gas within cold and dark regions in the Horsehead Nebula and around it. It’s a dark nebula referred to as L1630 where there’s a very limited star formation compared to that of Orion B giant molecular cloud or that of the Orion Nebula, also known as M42.

These researchers obtained only partial results too, concluding that current models are unable to provide a complete explanation for the shape, structure and brightness of the gas in the Horsehead Nebula. The situation even in a small part of the molecular cloud is more complex than expected.

The Horsehead Nebula map (NASA/SOFIA / J. Bally et al) created by both teams shows the Nebula in red and green with the ionized regions close to the molecular cloud in blue. The red areas are made of carbon monoxide molecules, while the green areas are made of carbon atoms and ions affected by radiation emitted by nearby stars.

The SOFIA (Stratospheric Observatory for Infrared Astronomy) flying observatory made it possible to create that map thanks to the upgraded GREAT (German Receiver at Terahertz Frequencies) tool. That allowed to create the map much more quickly using 14 detectors at the same time instead of just one. SOFIA is installed on a modified Boeing 747SP so the instruments can be upgraded between flights. At the altitudes at which it flies the atmosphere doesn’t filter infrareds, helping astronomical research such as the one on the Horsehead Nebula.

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