Orion Nebula Lives Behind Cluster Of Older Stars
November 14, 2012

Astronomers Find New Star Cluster In Front Of The Orion Nebula Cluster

April Flowers for redOrbit.com - Your Universe Online

A little over 400 years ago, French astronomer Nicolas-Claude Fabri de Peiresc first discovered the "fog" that is the Orion nebula. Located a mere 1,500 light years from our solar system, the Orion nebula is one of the great wonders of the night sky whose discovery is intimately associated with the early development of telescopes. Scientists have only realized the importance of the nebula in the last 60 years: the Orion nebula, like others in the Milky Way, forms new stars.

Over the years, astronomers have found a wide range of young stellar and stellar-like objects inside the Orion nebula. These objects range from massive ionizing stars tens of times more massive than the Sun down to objects known as brown dwarfs. Brown dwarfs are not massive enough to burn hydrogen and become stars.

The Orion nebula is the closest of the giant nurseries in our Galaxy, making the region very special as it offers astronomers the best chance to understand how physical laws lead to the transformation of molecular clouds of very diffuse gas into a variety of objects including hydrogen-burning stars, failed stars and planets.

The Orion nebula is viewed, unsurprisingly, as a benchmark for star formation studies by astronomers; a true golden standard. Most of the established measurements of how stars form have been derived from observations of the Orion nebula. These include distribution of stellar and brown dwarf masses at birth, their relative age, their spatial distribution, and the properties of the planet forming circumstellar disks surrounding the young stars.

The reality of the Orion nebula, it turns out, is even more complicated. Scientists recently used observations from the Canada-France-Hawaii Telescope (CFHT) with the 340 Mpx MegaCam camera and combined them with previous observations from ESA's Herschel and XMM-Newton, NASA's Spitzer and WISE, as well as 2MASS and Calar Alto. These observations revealed the cluster NGC 1980 as being a distinct massive cluster of slightly older stars in front of the nebula. Astronomers have known of this cluster since the 1960s, however, the new CFHT observations revealed that this population is more massive than was previously thought. It is not uniformly distributed; rather it clusters around the star iota Ori at the southern tip of Orion's sword.

Scientists are finding this discovery important for two major reasons: first, the cluster is only a slightly older sibling of the Trapezium cluster at the heart of the Orion nebula, and second, what they have been calling the Orion Nebula Cluster (ONC) is actually a complicated mix of these two separate clusters.

"We need refine what we thought were the most robust star and cluster formation observables," said Herve Bouy from the European Space Astronomy Centre in Madrid. Bouy pointed out the need for a long follow-up to "untangle these two mixed populations, star by star, if we are to understand the region, and star formation in clusters, and even the early stages of planet formation."

"For me the most intriguing part is that the older sibling, the iota Ori cluster, is so close to the younger cluster still forming stars inside the Orion nebula," says João Alves from the University of Vienna. "It is hard to see how these new observations fit into any existing theoretical model of cluster formation, and that is exciting because it suggests we might be missing something fundamental. Clusters are very likely the favorite mode of star formation in the Universe, but we are still far from understanding why that is exactly."

The findings of these observations have been published in the November issue of Astronomy & Astrophysics.