In astronomy, the bulge is the name given to the spherical mass of old stars found at the centers of most disk galaxies. The spatial extent and mass of a galactic bulge is generally a function of the Hubble-type of a given spiral disk galaxy, with early-type Sa galaxies having the largest, and late-type Sc/d having the smallest. In the Milky Way, the Bulge contains roughly 5 percent of the mass of the galaxy (or 10,000,000,000 solar masses). (The total mass of the Milky Way is about 200 thousand million solar masses.)

When we look at other spiral galaxies outside our own, we typically see a central concentration of older, red- and yellow-giant stars at the center of the galaxy, surrounded by spiral arms of younger, bluish stars. Bulges are composed mostly of older, evolved stars because they formed very early in the lives of galaxies. As primordial gas fell into the gravitational potential of the early galaxy, very vigorous star formation occurred there. Thus, most of the gas in the galactic center was depleted relatively quickly, and stars formed over a short period of time. Most of the massive, blue stars have since evolved and become supernovae or white dwarfs, leaving only the cooler, less massive stars behind. Gas left over in the outer regions of the galaxy then collapsed onto a disk along the plane of rotation, and began a period of more quiescent star formation which continues to the present time.

Despite being very old stars, which are normally poor in metals (called Population II stars), stars in galactic bulges are metal-rich (Population I stars). The primordial, metal-poor gas falling in to form bulges was rapidly enriched in metals by supernovae; a first generation of metal-poor stars would form, explode in supernovae, and transfer the metals they synthesized in their cores back into the gas. Thus the next generation of stars had a higher metal content than the previous one, and so on. And although bulges of galaxies are generally old, most studies of their stellar populations show they are usually composed of multiple, independent populations of stars. These extra stars may have come from accreted globular clusters, or from satellite galaxies (like the Magellanic Clouds) which fell into the Milky Way during its lifetime.

Bulges of galaxies are generally spherical, though some are ellipsoidal or triaxial in shape, sometimes having the shape of a bar through the center of the galaxy. Individual stars within a bulge do not rotate within the plane of the spiral disk, but orbit in a spherical cloud around the central gravitational potential. The density distribution of these stars usually follows a de Vaucouleurs or "R1/4-law," where the density of stars in space falls off as an exponential function of the radius to the one-fourth power.

Bulges are generally considered a part of the spheroid of spiral galaxies, which is made up of the bulge, the halo stars, and the globular cluster system. The gravitational potential of the bulge is important for the dynamical behavior of the inner regions of the galaxy, but further out, the mass of the disk and the dark matter dominate, producing flat galactic rotation curves. Bulges are considered to be analogs of elliptical galaxies, in that their stars are distributed spherically or triaxially around the central potential, rather than in a disk. It is likely that they formed in the same way -- by irregular accretion of matter clumps by a central gravitational well.

Despite the lack of continuous star formation, the bulges of galaxies can be busy places. Some galaxies funnel gas into the centers of galaxies, where it can collapse and vigorously form millions of stars simultaneously, in events called starbursts. These starbursts can outshine the rest of the galaxy at some wavelengths. The centers of spiral galaxies often contain supermassive black holes, and thus this gas may also fall into the black hole, resulting in a Seyfert galaxy.

J. Mould, "Galactic Bulge" Encyclopedia of Astronomy and Astrophysics, Inst. of Physics Publishing, 2001, v.1
Gilmore, King, and van der Kruit, The Milky Way As A Galaxy, University Science Books, 1989
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