Starlight found to be a key ingredient in forming the stuff of life

The dusty side of the Sword of Orion is illuminated in this infrared image from the European Space Agency's Hershel Space Observatory(Credit: ESA/NASA/JPL-Caltech)

To have life, you first need organic molecules, but where did these come from? It's a big question that isn't easy to answer, but data from ESA's Herschel Space Observatory indicates that ultraviolet light from stars may be a key factor in turning interstellar gases into complex molecules. According to the Jet Propulsion Laboratory (JPL) in Pasadena, California, infrared observations of the Orion Nebula show that starlight could be what drives the formation of precursor chemicals that become the building blocks of life.

Lying some 1,344 light years from Earth, the Orion Nebula is a vast cloud of gas and dust 12 light years wide in the sword of Orion. Long recognized as a stellar nursery where massive stars are born, it's relative proximity to our solar system makes it a handy laboratory for seeing how simple primordial ingredients become not only stars, but also the ingredients of life.

By using spectral analysis, usually as the molecules absorbed light passing through the Nebula, astronomers can look for the distinct spectra of various molecules, such as methylidyne (CH) and its ionized version (CH+), which were the first organic molecules detected in interstellar space in 1937.

These and other more complex organic molecules were believed to be produced by shock waves generated by exploding supernovae or young stars spitting out jets of gas. It was thought these shock waves would cause atoms to collide, making them ionize and then bond together to form molecules.

This seemed logical and even likely until Herschel mapped the amount, temperature, and movements of Methylidyne and CH+ in the Orion Nebula. The unmanned probe found there was CH+ in large quantities, but it was emitting light rather than absorbing it and was at a higher temperature than the surrounding gas. This was unexpected because CH+ requires energy to form and is very active, so the hydrogen in the vicinity should have destroyed it. This indicated that something is creating more CH+ on a regular basis.

JPL says Herschel was designed to look at far infrared regions of the spectrum associated with cold objects that no other space telescope could reach. This allowed the probe to look at the Orion Nebula as a whole rather than just areas where nearby stars shine through, and could do so with such sensitivity that it could track gas cloud movements. The problem was that what it showed revealed that the old hypothesis didn't hold because it didn't correlate with the shock waves and the levels of CH+.

Models based on the Herschel data indicates the CH+ is more likely created by the ultraviolet emission of very young stars in the Orion Nebula. When hydrogen molecules are struck by photons of UV light, they vibrate and rotate as they absorb more energy until they can bond with other molecules. In the large amounts of gas in the Orion Nebula, the hydrogen heats up and starts to form hydrocarbons – especially CH+, which then captures an electron to form neutral CH.

According to the researchers, this mechanism could have wider implications because it could explain complex organic molecule formations in other galaxies with similarly dense regions dominated by ultraviolet light.

"This is the initiation of the whole carbon chemistry," says researcher John Pearson JPL. "If you want to form anything more complicated, it goes through that pathway."

Launched on May 14, 2009 from the Guiana Space Centre in French Guiana atop an Ariane 5 rocket, the Herschel Space Observatory carried out a four-year mission to study galaxy and star formations, the atmospheres of bodies in the Solar System, and interstellar gases. It was deactivated in June 2013 after its supply of liquid helium used to supercool the telescope ran out.

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