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When a galaxy forms, its material typically develops into a relatively flat disc that extends out from the galaxy's nucleus, and the resulting structure is referred to as a disc galaxy. Interstellar matter within the disc tends to be concentrated by gravitational forces into arms that curve out from the central regions, giving rise to a spiral galaxy. In galaxies where this material has been consumed by stellar formation or other effects, the spiral arms are absent, and instead the disc is largely featureless, though may still show lanes of dark dust. This latter form of disc galaxy is known as a lenticular galaxy.

A galactic disc consists of clouds of interstellar gas, primarily hydrogen, within which stars are formed. The gas and stars follow orbits around the galaxy's nucleus that are close to circular in shape. These orbits are also closely confined to a single plane, giving the disc its narrow, flat shape. The dimensions of galactic discs vary from galaxy to galaxy, but in the Milky Way the disc averages some one thousand light years in thickness.

Galactic discs are not amorphous, but instead show a distinct nested structure, and are effectively formed from three discs, one within the other. The innermost of these is the thin disc where younger Population I stars are being formed, and this is where most of the gas and the stars of the disc are found. The thin disc is subdivided into two elements, the innermost young thin disc where star formation is most active, is about 300 light years thick, and is surrounded by the old thin disc, which is some three times thicker. The thin disc is enveloped by the considerably wider thick disc, which is populated by older, metal-poor Population II stars, similar to those surrounding a galaxy's nucleus.

The Solar System lies deep inside the thin disc of the Milky Way Galaxy and so, from Earth, the galactic disc is seen from within. The result is a thin band of light encircling the entire sky, the Milky Way itself, where the concentrated stars and gases of the disc are most prevalent.


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