New observations of the Orion B nebula reveal the anatomy of a star-forming reservoir

Using the IRAM 30 meter radio-telescope at the Pico Veleta (Spain), an international team of astronomers has obtained the most complete radio-observations of the Orion B star-forming region, famous for hosting the iconic Horsehead and Flame nebulae. Taking advantage of the fact that the cold molecular gas shines at radio wavelengths, the team revealed the hidden anatomy of Orion B. Through a careful dissection of the cloud into regions of different molecular composition, this work sheds new light on how the coldest and densest molecular clouds give birth to new stars.

Stars are born in cold and dense condensations that develop in the interior of interstellar clouds. The strong winds and ultraviolet radiation from newly born stars then erodes and disrupts the parental cloud.

Using the IRAM 30 meter radio-telescope in Pico Veleta (Spain), an international scientific program led by Jérôme Pety (IRAM & Observatoire de Paris, France), has achieved the most complete observation of the Orion B cloud in the radio domain. This region is a huge reservoir of interstellar matter (mostly gaseous molecules and dust grains), containing about 70,000 times the mass of the Sun.

Emeric Bron and Javier Goicoechea, both from ICMM-CSIC (Spain) and members of the team, explain: “We have been studying the famous Horsehead nebula for years. The new instrumentation at the IRAM 30m telescope now allows mapping much larger areas of the sky and detecting the emission produced by different molecules simultaneously.  The obtained maps are equivalent to about 160,000 images of 325×435 pixels, enough to make a movie of 1h50m at 24 frames per second. We have created images of the emission produced by molecules such as carbon monoxide, carbon monosulfide, cyanides, methanol, and small hydrocarbons. Detecting these molecules is crucial, since molecular hydrogen, which makes up about 75% of interstellar gas, is invisible in cold molecular clouds. Detecting the radio emission from those species is thus not only important to understand the chemistry of these regions, but it also provides the best radiography of cold molecular clouds that are otherwise invisible to the naked eye (see Fig. 1).”

This project provides unprecedented images of a region of the sky that is only seen as a dark region in the visible. Moreover, the wealth of data opens the possibility to characterize the structure and the physical and chemical properties of molecular clouds in unprecedented detail. It is truly a dive into the inner anatomy of the Orion B cloud. “Diffuse gas, filaments, and dense condensations could be the equivalent of muscles, bones, and vital organs, respectively. And the images of different molecular lines enable to radiography different parts of Giant Molecular Clouds, just like Magnetic Resonance Imaging (MRI) enables to reveal the interior of the human body” says Pety.

In the series of 3 articles accepted by Astronomy & Astrophysics, the team explains how the emission from different molecules is a very sensitive diagnostic tool of the gas physical conditions, its density, temperature, turbulence and the properties of the ultraviolet light arising from massive stars in the region. The team shows how the sites for future star-formation, the coldest and densest condensations, are only revealed by the emission from certain molecules such as the diazenylium ion (N2H+).

Another important result is the discovery of a clear relationship between the kind of turbulent motions (shocks or vortices) and the local star formation activity. This pioneering work, involving the simultaneous statistical analysis of many molecular tracers will provide the needed tools to characterize star formation in the interstellar medium. It brings radio-astronomy into the era of big data!

Star Wars inspiration

Astronomy is very inspiring and also very cinematographic. Our perception is determined by our experiences. This is why it is not surprising that, while observing the sky, we are able to find wonderful shapes and even characters (the origin of constellations names).

This is what happened to Audrey Pety, pursuing design studies, who says: “I immediately saw the skeleton of a Jedi knight when first discovering this image. This reminded me that constellations are human interpretation of the stellar patterns on the sky and that Orion is a hunter. And I ended up drawing my own modern constellation on this image (Fig. 2).”


  • Emeric Bron –
  • Javier R. Goicoechea –
  • Jerome Pety –


Figure 1: Orion B molecular seen in the optical (lower panel: Image credit & copyright Sergi Verdugo Martínez) and as a composition of three radio emission lines observed in this work (12CO, 13CO and C18O isotopologues, upper panel). The ionized hydrogen gas shines in bright red in the optical, whereas radio observations reveal the intrinsic structure of the dense molecular gas (Image credit & copyright J. Pety, the ORION-B Collaboration & IRAM).

Figure 2: Artistic view of a Jedi knight overlaid on an image of the molecular gas from Orion B (Image Credit & Copyright: Audrey Pety).

Additional information

Project history

The ORION-B project results from 10 years of research. It was made possible by the advent of a new generation of  wide bandwidth receivers combined with high resolution spectrometers at the IRAM 30 meter telescope, and builds on long experience in radio-astronomy by the team members. Jérôme Pety explains: “Using the IRAM 30 meter telescope, we undertook in 2011 a systematic survey of the chemical content of the Horsehead’s mane, during a project named Horsehead WHISPER. This allowed us to discover in 2012 a new molecule in the interstellar medium, the propynylidyne ion (C3H+) that is a member of the hydrocarbon family. Building on this success, we decided to generalize these observations, i.e., to observe one hundred thousand positions in Orion B instead of a single one towards the Horsehead nebula, each direction delivering information at 160,000 different frequencies!” Harvey Liszt, astronomer at NRAO, adds: “It’s flabbergasting to see how this field has grown tremendously. During my PhD, I tuned a precursor millimeter receiver to obtain the first noisy detection of carbon monoxide towards one direction in Orion B, together with Nobel prize winner Bob Wilson. And only 45 years later, we easily gets such wide maps for so many molecules over the whole cloud!” No doubt that the future will offer even faster and wider observations for radio-astronomers. When that happens, the pioneering statistical approach of the ORION-B project will provide the needed tools and experience to handle bigger and bigger datasets.

Scientific papers

  1. The anatomy of the Orion B Giant Molecular Cloud: A local template for studies of nearby galaxies”, by Jérôme Pety, Viviana V. Guzmán, Jan H. Orkisz, Harvey S. Liszt, Maryvonne Gerin, Emeric Bron, Sébastien Bardeau, Javier R. Goicoechea, Pierre Gratier, Franck Le Petit, François Levrier, Karin I. Oberg, Evelyne Roueff, Albrecht Sievers.
  2. Dissecting the molecular structure of the Orion B cloud: Insight from Principal Component Analysis, by Pierre Gratier, Emeric Bron, Maryvonne Gerin, Jérôme Pety, Viviana V. Guzman, Jan Orkisz, Sébastien Bardeau, Javier R. Goicoechea, Franck Le Petit, Harvey Liszt, Karin Öberg, Nicolas Peretto, Evelyne Roueff, Albrecht Sievers, Pascal Tremblin.
  3. Turbulence  and star formation efficiency in molecular clouds: solenoidal versus compressive motions in Orion B, by Jan H. Orkisz, Jérôme Pety, Maryvonne Gerin, Emeric Bron, Viviana V. Guzmán, Sébastien Bardeau, Javier R. Goicoechea, Pierre Gratier,  Franck Le Petit, François Levrier, Harvey Liszt, Karin Öberg, Nicolas Peretto, Evelyne Roueff, Albrecht Sievers, Pascal Tremblin.


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