Galaxy Fundamentals

What is a spiral galaxy?

A spiral galaxy is a galaxy that appears to have spiral-shaped structures, or arms, that contain bright stars. As it turns out, those spiral arms are not solid structures, but rather spiral density waves: ridges in a disk of spinning dust and gas that are denser than the regions surrounding them. Spiral galaxies have star-filled, ellipsoid-shaped bulges at their centers; a star-filled, thin disk of spinning gas surrounding the bulge; and a sparsely populated stellar halo that envelops both the disk and the bulge.


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