Images made from 21 cm observations, showing the large scale structure, projected in a galactic coordinate system, with the plane of the Milky Way acting as the equator. The map shows the locations of the largest HI shells, called 'supershells'.The larger circles indicate larger shells.To give an idea of galactic scale, these are superimposed on estimates of where pieces of spiral arms may lie.We will talk more about the spiral structure of the galaxy in Chapter 16. [Naomi McClure-Griffiths,Australia Tel. National Facility/McClure-Griffiths, N. M. et al., Astrophys. J., 578, 189,2002, Fig. 18]
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