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