Originally, studies of HI emission offered the opportunity to map the structure of the Milky Way (Oort, Kerr & Westerhout 1958). HI emission on longitude-velocity plots clearly demonstrated that there were some coherent, large-scale structures, which were probably spiral arms in the Milky Way. However, determining accurate distances to HI clouds proved problematic and this made the task of turning longitude-velocity data into a true plan-view of the Milky Way very uncertain (Burton 1988). Later, millimeter-wave observations of molecules, such as CO, also revealed coherent, large-scale structures with higher contrast than seen in HI (e.g., Dame et al. 1986). But, again, uncertain distances to molecular clouds precluded making a true map of the Milky Way with sufficient accuracy map the spiral structure of the Milky Way.
Georgelin and Georgelin (1976), hereafter GG76, published the results of a very large project that resulted in a plan-view model of the spiral structure of the Milky Way (see Fig.~1). The GG76 approach involved combining optical observations of young stars and radio data of HI cloud and HII region emissions. Luminosity distances to nearby stars were used where available and kinematic distances elsewhere, mostly for more distant HII regions. While subject to very significant uncertainties from kinematic distances, the GG76 model has remained the "standard" model of the spiral structure of the Milky Way for nearly 30 years.
|Fig. 1: Georgelin and Georgelin (1976) spiral model of the Milky Way. Considerable controversy exists as to the accuracy of this model, largely because many of the distances used are very uncertain. Researchers in the field even disagree on the number of spiral arms.|
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