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The Gould Belt.

Reputedly the largest galactic structure in the Solar region, the Gould Belt is an apparently ringlike assembly of molecular clouds, star forming regions, and young stars, star clusters and OB associations encircling the Sun at a radius of about 300 parsecs.

The Belt was first noticed by John Herschel in 1847, while observing in South Africa, as a corridor of unusually bright stars lying across the Milky Way band. It was later described and inventoried by Benjamin Gould as "a belt or stream of bright stars [that] appears to girdle the heavens very nearly in a great circle" in the text accompanying his Uranometria Argentina (1879, the "Flamsteed catalog" of southern skies).

The appearance of the Belt is striking during southern spring nights, when the Milky Way is spangled with bright stars and OB concentrations, as the figure (below left) illustrates. The opposite view of the Milky Way during northern fall evenings is less impressive (figure, below right): although adorned with scattered bright stars, the Milky Way is faint and patchy from Cygnus to Canis Major. The tilt of the Belt away from the Galactic equator (green line) is obvious in the southern view, but substantially obscured in the northern view.


The very different appearance of the Belt in the northern and southern hemispheres arises from the position of the Local Arm. The northern view from Cygnus to Taurus is covered by the dark clouds of several active star forming regions and, toward the galactic center, the visually large areas of nearby dark molecular clouds such as the Cygnus Coal Sack and the Aquila Rift. In contrast, the southern view looks toward the products of star formation: new massive stars, dissolving star clusters and OB associations with dispersing molecular clouds and supernova shells.

The form, dimensions and dynamics of the Gould Belt depend on the features used to define it, and different features seem to require different astrophysical explanations for its origin. Hough & Halm (1910) identified peculiar proper motions among the Gould Belt stars, and Edwin Hubble (1922) added nearby H II regions and molecular clouds. The diagram (below) summarizes the features currently associated with the Gould Belt, in galactic coordinates.


There is quibbling in the literature as to whether the gould Belt is a ring, a disk, a cloud — or a sausage (Clube, 1967) — and there is research to suggest that it is more likely a "2D projection effect, and not a physical ring" (Buoy & Alves, 2015). A basic problem in all these studies is distinguishing Gould Belt objects from "background" galactic disk features — the problem of circular reasoning, or confirming your assumptions. If proper motion is used to distinguish Gould Belt stars from unrelated stars, then it is not surprising to discover that Gould Belt stars have unusual proper motions.

Its inner border is usually rendered as an elliptical ring about 360 pc long and 230 pc wide, but the disk of stars around this border extends out to about 600 pc with a scale height of 60 to 100 pc. The figure (below) shows that the disk is tilted from the Galactic plane by ~18° with the long axis approximately toward l = 45° and the ascending node at l = ~285° (it tips farthest above the galactic plane in Scorpius and farthest below in Orion). The Sun is displaced from the ellipse center toward the Scorpius-Centaurus OB association.


The age of the Gould Belt is estimated to be 30 to 60 million years. Over 60% of OB stars within 600 pc and younger than 60 Myr appear to belong to the Belt, and the youngest (< 30 Myr) stars within ~400 pc in Quadrants III & IV are receding from the disk center. However, the kinematics of Gould Belt stars (as peculiar motions in relation to the local standard of rest) are highly disordered, due to the separate trajectories of the many different OB associations involved (cf. Torra & alia, 2000).

Theories to account for the recent origin, tilt and kinematics of the Gould Belt include the collision of a ~107M⊙ molecular cloud with the Local Arm, or the gamma ray burst from a hypernova. Viewing the Gould Belt as a large, active star forming region is consistent with the theory that the assembly resulted from a series of about 20 supernova explosions over the past 60 million years — a rate that is about 5 times the Galactic average. These explosions are linked to the many radio loops, nova remnants and neutron stars of varying ages within the region, and are invoked to explain the tilt of the disk to the galactic plane.

Somewhat contradicting this "origin story" explanation for the Gould Belt the remarkable contrast between the Scorpius–Centaurus association and the Orion star forming complex. In the Scorpius-Centuarus region, an epoch of prolonged and spatially extended star formation, which produced no young star clusters or H II regions, has passed its peak: the extended Scorpius-Centaurus OB associations have dispersed their natal gas and are slowly expanding (see Elias & alia, 2009). In Orion, rapid, dense and centralized star formation seems to be nearing its climax: the area contains several dark clouds and H II regions, including the Great Orion Nebula; a dozen very young star clusters; three compact OB associations; and a burst in stellar radiation that apparently formed the Eridanus bubble. There does not seem a simple way to relate these very different structures to a single initiating event.

It's also instructive to compare the Gould Belt with the concentration of H II regions and OB associations connected with the Cygnus Superbubble, about 2000 parsecs distant at l = 82° and about 600 by 450 parsecs large with a roughly horseshoe profile. This feature is also the center of extensive star formation and is characterized by a belt of molecular material, tilted to the Galactic plane by about 30° (see Uyaniker et al., 2001)…

From our lucky vantage, the active star forming regions around us are consistent with the pattern observed near spiral arm shock waves and spurs. Dense molecular clouds and active star forming regions are predominately located "ahead" of us in Galactic rotation, within the area defined by the Local Arm; mature or completed star formation regions and many supernova remnants are located "behind" us in the span between the Sco-Cen and Ori associations.