Galactic coordinates are a system of celestial coordinates which use the orientation of the plane of the Milky Way as the reference frame. The coordinates are centered on Earth as with equatorial coordinates, but the origin of galactic longitude and latitude are coincident with the center of the Galaxy, the longitudinal direction is along the galactic plane, and the latitudinal direction is along the line perpendicular to the plane of the galactic disk passing through the Earth. The longitude, measured in degrees, increases eastward (i.e. in the direction of Ophiuchus and Cygnus) and runs from 0° to 360°.
Galactic coordinates are commonly used as a matter of convenience in various fields of
galactic astronomy, particularly when discussing things such as the distribution of the interstellar medium or globular clusters within the galaxy, and the positions of its spiral arms. Sometimes the coordinates of galaxies outside our Milky Way are given in galactic coordinates because the galactic latitude gives the position relative to the Milky Way's zone of avoidance. This is a region of strong dust extinction within the plane of the Galaxy which blocks many wavelengths of light from reaching us.
Galactic coordinates are expressed in terms of galactic latitude, bII, and galactic longitude, lII.
The zero points of the galactic coordinate system expressed in the equatorial coordinate system are: α 17h 45.6m, δ -28° 56'. The powerful radio source Sgr A* is located close to galactic coordinates (0,0). It is believed to be a massive black hole at the gravitational center of the Milky Way, containing the mass of over a million suns.
Coordinates from simbad.harvard.edu. Helpful hints from NASA's "Multiwavelength Milky Way" poster.