Our sun resides in an unremarkable part of a vast stellar system, the Milky Way Galaxy. Shaped like a great circular disk, our galaxy has a breadth nearly 10 times greater than its thickness. Within the plane of the disk, long "arms"—dense concentrations of bright, young stars, dust and giant gaseous clouds—spiral outward from its center.
Figure 1. Infrared images of the Milky Way help astronomers identify some of the major structures in our galaxy. Here starlight-heated dust shines brightly in the far-infrared wavelengths (100 micrometers). The concentration of dust (left) is greatest in the galactic midplane (near 0 degrees latitude), but fine filaments of dust, called cirrus, rise high above and below the plane. The distribution of nearby cirrus (above) can be emphasized by compensating for the overwhelming brightness of the midplane dust. Here major collections of dust are evident in several giant molecular clouds, and a bright bulge in Cygnus represents a superposition of dust clouds along a segment of the local spiral arm. Also visible are satellite galaxies of the Milky Way: the Large and Small Magellanic Clouds. These Mollweide-projection maps of the sky were made in the early 1990s by the Diffuse Infrared Background Experiment (DIRBE), which was part of the Cosmic Background Explorer satellite (COBE).
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