The massive star cluster before the Orion Nebula

The Orion Nebula (also known as M42) is one of the great wonders of the night sky. Its discovery dates back 400 years, when it was first described as “fog” in the observation notebooks of the French astronomer Nicolas-Claude Fabri de Peiresc (1610). The discovery of the Orion nebula is closely associated with the earliest development of the telescope, but it is only sixty years ago that we have ascertained the true astrophysical importance of this very attractive object: the nebula, like so many others in our Galaxy and in outer galaxies. , it is a nursery for stellar babies. Inside the Orion Nebula, a very wide range of young objects of a stellar or star-like nature have been detected, from massive and ionizing stars, dozens of times heavier than the Sun, to objects known as brown dwarfs, which they don’t have enough mass to burn hydrogen and become stars in their own right. Of all the massive stellar nurseries in the Galaxy, the Orion nebula represents the closest to Earth. This circumstance makes that cosmic region special, because it offers the best opportunities to understand how nature turns diffuse gas clouds into fiery suns, failed stars, or even planets.

It is not surprising to learn that the Orion Nebula provides astronomy the test bed, the touchstone, for studies of star formation, and that many of the strongest measurements of the mechanisms of star birth come from this very region. important such as, for example, the mass distribution of stars and newborn brown dwarfs, the relative ages of these stars, their distribution in space, or the properties of the circumstellar disks in which planets form around young stars in the Orion Nebula.

But the reality is a bit more complex. Recent observations of the Orion Nebula made at the Calar Alto Observatory (CAHA), with the Canada-France-Hawaii Telescope (CHFT) and the Sloan Sounding (Sloan Digital Sky Survey, SDSS) by H. Bouy (CAB) and J Alves (University of Vienna) have shown that there is a second massive star cluster, with slightly older stars, located in front of the nebula. Although it was already known since the 1960s that there is a preceding stellar population, observations have revealed that these stars are more massive than previously believed, and that their distribution is not uniform, but rather forms a cluster around a star that With the naked eye you can see iota Orionis, the one that marks the southern end of the sword of Orion. The importance of this finding is twofold. On the one hand, the newly identified cluster is but a somewhat older brother of the Trapezium cluster, located in the heart of the nebula. On the other hand, what until now had been called “the Orion cluster” turns out to be, in fact, made up of a complex mixture of these two clusters, with the addition of some preceding stars, unrelated to them.