The Orion Nebula hosts a well studied star cluster, the gold standard by which astronomers measure all other clusters. New research suggests that this benchmark might need to be revised.
To amateur astronomers, the Great Orion Nebula (also known as M42) is one of the sky’s most accessible and rewarding targets. It’s barely visible to the unaided eye in a dark sky as a fuzzy patch in Orion’s Sword. With a small telescope you can travel deep into the nebula’s core, set alight by four dominant blue-white stars called the Trapezium.
To researchers, M42 is one of the most thoroughly studied objects at all wavelengths, with observational records dating from the invention of the telescope 400 years ago. At a distance of 1,400 light-years, it’s the closest site of energetic massive-star formation. Now, with the help of telescopes like the Canada-France-Hawaii Telescope (CFHT), Calar Alto Observatory, and the Spitzer satellite, João Alves (University of Vienna, Austria) and Hervé Bouy (Centro de Astrobiología, Spain) have traveled even deeper to take an accurate census of all the nebula’s stars, numbering in the thousands.
M42 contains several star clusters burning away at the thick veils of gas and dust. The central Orion Nebula Cluster is the most familiar and includes the Trapezium. It has set the benchmark for astronomers’ understanding of how stars form in massive clusters.
But it turns out that membership in the ONC has been overestimated. Alves and Bouy have published a study in Astronomy & Astrophysics showing that as many as 20% of its assumed members actually belong to the closer NGC 1980 cluster, which is centered around Sigma Orionis ½° to the south.
Contamination by foreground stars “has long been a concern,” says Charles Lada (Harvard-Smithsonian Center for Astrophysics), who was not involved in the study. “It has been known for almost half a century that there must be some overlap between the young cluster population and young foreground stars.”
Dust as Decontaminant
The Orion stellar nursery has been churning out stars for the last 10 million years in bursts lasting a couple million years apiece. As a result, many overlapping groups of young stars punctuate the Orion Nebula’s region, and the uncertainties in their distances mean that astronomers have a hard time deciphering their three-dimensional positions.
Alves and Bouy overcame that difficulty using dust. Dust blocks visible light in a biased way, scattering blue light while letting red light through. (That’s why sunsets are red and skies are blue.) So when Alves and Bouy sorted stars near the Orion Nebula by reddening, they were also sorting by distance — more distant stars tend to be behind more dust.
Using dust reddening as a proxy for distance, Alves and Bouy found a distinct population of stars lying in front of the ONC. Their properties, such as their spread in ages and brightnesses, set them apart..
The authors suggest that the measured properties of the ONC, a benchmark of star-formation regions, will have to be revised. Once the ONC catalog is “cleaned” of false members, Lada says, adjustments may have to be made to the details of star-formation theories. Other results, such as the recent simulation that grew a massive black hole at the center of the nebula, are only marginally affected by the contamination.
For more, see the researchers’ paper: "Orion Revisited: The Massive Cluster in Front of the Orion Nebula Cluster".