According to one current theory, a spiral galaxy begins as a giant, roughly spherical rotating mass of gas and dust, which gradually flattens out at the edges to create the disk. The original spherical shape lives on in the outermost region of a galaxy, known as the galactic halo, and, to a lesser extent, in the bulge. However, this view is challenged by observations of some bulges. The bulge of the Triangulum Galaxy (M33), for example, contains young- and intermediate-age stars, and has a star distribution that suggests the disk goes all the way to the center. This raises questions about how M33 as a whole formed and what triggered the birth of the relatively youthful stars in its bulge.
Related category GALAXIES
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