Star-forming regions of Auriga

The star-forming regions in Auriga include numerous molecular clouds and extensive H II regions, visible in the direction of the constellation Auriga. They are located at various distances and appear one behind the other in this direction, appearing close together only due to a perspective effect. These objects all lie within a galactic longitude between 167° and 180°, i.e. in the direction opposite the Galactic Center, as seen from the Solar System. Among them, the nebular complexes located on the Perseus Arm dominate, which in this direction presents a notable concentration rich in young hot and massive stars gathered in several OB associations, while objects located on the Orion Arm are scarce. The Cygnus Arm, the outermost one, is rather sparse and discontinuous in the direction of Auriga, but according to the most recent studies, it is possible to distinguish a significant concentration within it, consisting of a bright OB association linked to the large nebular complex of IC 410. The study of the galactic plane in the direction of Auriga, although difficult, has proven useful for understanding various morphological features of the outer spiral arms in the direction opposite the galactic center, such as the highly irregular distribution of nebular and stellar concentrations.

Observation

The star-forming regions of Auriga lie in the direction opposite the Galactic Center, in a section that is partly heavily obscured and partly, on the contrary, very rich in star fields; however, despite the large expanse of sky involved, with the naked eye or with the aid of small instruments, little is visible beyond the aforementioned star fields and a few open clusters, among which the three major objects of Auriga, cataloged by Charles Messier, stand out: M36, M37 and M38. In addition to these, numerous other star clusters can be observed with large-diameter instruments; in photographs, however, large nebular systems are very clearly evident. The northern part of Auriga, on the other hand, appears rather poor in objects and the Milky Way appears thin and highly obscured. Being at a northern declination, between 50°N and 25°N, the constellation Auriga (and with it its nebular regions) dominates much of the northern nights and appears circumpolar from regions further north; Auriga is one of the most classic figures of the autumn and winter evenings in the Northern Hemisphere, when it is at the zenith over much of North America and Europe, making it clearly visible until well into spring. From the Southern Hemisphere, however, the view is penalized and for most of its regions it remains rather low above the northern horizon.

In precessional epochs

Due to the phenomenon known as the precession of the equinoxes, the celestial coordinates of stars and constellations can vary significantly, depending on their distance from the north and south poles of the ecliptic.

General overview

The nebular regions visible in Auriga, when viewed from Earth's perspective, align around 167°–180° of the galactic equator, where 0° corresponds to the direction of the Galactic Center; this means that they lie in a galactic region farther out than Earth and thus farther from the galactic center. This direction is often called the Galactic anticenter. In this direction, stars and nebular regions belonging to three different spiral structures of the Milky Way align: the closest to the Solar System are those belonging to the Orion Arm; since the Sun is located near the inner edge of the arm, much of it extends outward, so that even its outer objects fall in this direction. Beyond the boundaries of the Orion Arm, starting at a distance of at least 1500 parsecs, lies the Perseus Arm, one of the two main spiral structures of the Milky Way; at this point falls most of the observable objects in the direction of Auriga, including the major star formation areas. Beyond 4000 parsecs finally lies the Cygnus Arm, a secondary arm which in this direction tends to dissolve, having almost reached the end of its length. It is interesting to note that while around 170°–180° of the galactic equator numerous nebular complexes and stellar concentrations are observable aligned at various distances, at the galactic longitude of 150°–170°, included in the northern part of Auriga, neither significant nebular complexes nor massive young stars capable of ionizing the gases in this direction are observed; it appears that in this direction the Perseus Arm and the Cygnus Arm behind it do not present a continuous and regular structure as instead occurs in the direction of Cassiopeia and Perseus; even in the inner Orion Arm there are few significant structures, opening a sort of window towards the outside of the Milky Way. In the southern part of Auriga, where the nebular concentrations are located, two extensive OB associations have traditionally been identified, also located one behind the other, to which the designations Auriga OB1 and Auriga OB2 were assigned; estimates of their distance, as well as those of the nebulae associated with them, have always been affected by uncertainties and the values indicated have often been contradictory among different publications, until it was realized that the two identified associations are actually both composed of two physically distinct groups, located at different distances. This is of fundamental importance for understanding this sector of the Milky Way.

Auriga OB1

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