galactic bulge

The galactic bulge is the bulge of stars surrounding the Milky Way's galactic center. It is challenging to study from our position within the Milky Way, with extinction due to clouds in and around the bulge, and the description below is subject to debate and future correction. The Milky Way bulge is a bar, the center of which is about 8 kpc from Earth (the diameter of the starry-region of the Milky Way is on the order of 30 kpc). Dimension-determinations of the bar vary between 3-4.5 kpc length, with other dimensions on the order of a kpc. Some have determined it to be a pseudobulge (a flat bulge, i.e., little or no projection from the galactic disk). There is structure within the bulge: for example, it is suggested that there are two bar structures of different stellar populations, i.e., different age and metallicity. The bars cross each other like an X, though at a small angle, with one bar shorter than the other. The specific characteristics of the Milky Way bulge are not rare; other observed spiral galaxies show similar bulge characteristics.

Roughly in the center of the bulge (the galactic center) is the Milky Way's central supermassive black hole (SMBH), Sag A*, surrounded by a cluster of millions of stars (a compact stellar nucleus) within a parsec of the SMBH, a region roughly as wide as the distance between the Sun and Alpha Centauri.

The term galactic bulge is sometimes used for the bulges of other galaxies: the term galactic is somewhat ambiguous regarding whether it refers to galaxies in general or to the Milky Way in particular: with "the", e.g., the galactic bulge, typically means that of the Milky Way unless the context shows otherwise. Also, the Milky Way's is sometimes indicated by capitalization, i.e., "the Galactic Bulge" or "the Galactic bulge".