Like the elliptical galaxies, the central bulges and the globular clusters of the spirals are not provided with big quantities of gas and young stars. They are mainly composed by a very dense amount of population II stars, which are old and possess a weak metallic content (elements heavier than helium). These stars are responsible for the yellowish look displayed by such bodies.
The bulges of some galaxies assume the form of a bar, as it happens in the Milky Way. For the higher metallic content observed at the stars located in the bar itself, it's estimated that this one is younger than other features of the bulge.
The stars and gases of the bulge draw complex orbits inside the barred spiral galaxies, opposing to the circular orbits observed at the regular spiral galaxies. Nevertheless, the bar rotates as a solid structure, collecting materials that are therefore directed into the centre of the bulge.
The Centre of the Bulge
The collected material is retained in a ring of gases located about 1000 light years from the very centre. The accumulation of the gas at this ring prevents that it transits directly until the centre, which would provoke a spectacular light burst at the active core of the Milky Way.
In the centre of the bulge of our galaxy, distributed in a disk that is about 1000 light years wide and about 200 light years thick, one may observe powerful radio waves forming interlaced filaments that are distributed along the magnetic fields. It's estimated that the latter ones are feeded by the magnetic fields of the material that is absorbed by the bar.
Very powerful and dot-shaped radio sources are frequently observed at the heart of the bulges. It's thought that they have their origin in giant black holes and that the production of energy at the various wavelengths follows a process that is analogue to the one that occurring in the quasars or in the radio-galaxies.
The centre of the Milky Way, located at a distance of 28000 light years from us in the direction of Sagittarius, is also conjectured to be host to a giant black hole, with more than 1 million solar masses. However, comparing to the ones placed in the interior of the active galaxies, the giant black hole of the Milky Way has smaller dimensions and fuels less spectacular phenomena.
Sgr A, the radio waves source at the centre of the Milky Way (K.Y. Lo, University Illinois)