A newly identified separate star cluster in front of the Orion Nebula Cluster
The Orion nebula is one of the great wonders of the night sky. Its discovery goes back 400 years ago when it was first described as "fog" in the observing reports of French astronomer Nicolas-Claude Fabri de Peiresc (1610). The discovery of the Orion nebula is intimately associated with the early development of telescopes but only in the last 60 years have we come to realize the true astrophysical importance of this glamorous object: the nebula, like so many in the Milky Way and in other galaxies, form new stars. Inside the Orion nebula, astronomers have found over the years a wide range of young stellar and stellar-like objects, from massive ionizing stars tens of times more massive than the Sun down to objects known as brown dwarfs, which are not massive enough to burn hydrogen and become stars. Of all the giant nurseries in our Galaxy, the Orion nebula is the closest to Earth, only 1,500 light-years away. This makes this region very special, offering astronomers the best chance to understand how laws of physics lead to the transformation of molecular clouds of very diffuse gas into hydrogen burning stars, failed stars, as well as planets.
Not surprisingly, astronomers see the Orion nebula as the benchmark for star formation studies, a true golden standard, and most of the established measurements of how stars form have been derived from this important region. For example, the distribution of stellar and brown dwarfs masses at birth, their relative age, their spatial distribution, and the properties of the planet forming circumstellar disks surrounding the young stars.
But as it turns out, reality is more complicated. Recent observations of the Orion nebula from the Canada-France-Hawaii Telescope (CFHT) with the 340 Mpx MegaCam camera coupled to previous observations with ESA's Herschel and XMM-Newton, NASA's Spitzer and WISE, as well as 2MASS and Calar Alto, revealed the cluster known as NGC 1980 as being a clearly distinct massive cluster of slightly older stars in front of the nebula. Although astronomers knew of the presence of a foreground stellar population since the 1960s, the new CFHT observations revealed that this population is more massive than first thought, and it is not uniformly distributed, clustering around the star iota Ori at the southern tip of Orion's sword.
The importance of this discovery is two fold: first, the cluster identified as a separate entity is only a slightly older sibling of the Trapezium cluster at the heart of the Orion nebula, and second, what astronomers have been calling the Orion Nebula Cluster (ONC) is actually a complicated mix of these two clusters.
Hervé Bouy, from the European Space Astronomy Centre in Madrid, one of the two authors of this work, explains that "we need refine what we thought were the most robust star and cluster formation observables." He points out the need for a long follow-up work on Orion where "we must 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 for 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 article published in the Astronomy & Astrophysics journal (November 2012 issue) can be consulted here.