With a capital initial G, by convention, the galaxy or family of stars to which the Sun and solar system belong, visible in the night sky as the Milky Way.
Our Galaxy is a spiral, possibly a mildly barred spiral galaxy, containing two hundred billion stars as well as much interstellar matter, both dark and luminous. It is disc-shaped with an almost spherical bulge at the centre. The disc is 100,000 light years across but only 1,000 light years thick towards its outer edges. The central bulge has a radius of about 10,000 light years.
The spiral arms are concentrations of stars and interstellar material appearing to wind outwards from the edge of the bulge. Regions of star formation and onized hydrogen are concentrated in the arms. In the space between the arms, the average density of matter is a factor of two or three lower than within the arms.
Centred on the nucleus is a sparsely populated, roughly spherical region, with a radius of about 50,000 light years, known as the galactic halo. The halo contains globular cluster and, in general, the oldest stars in the Galaxy. There is very little matter in the halo compared with the disc and central bulge.
The innermost nucleus, lying in the direction of the constellation Sagittarius, is concealed from direct optical observation by dense opaque dust. However, observations in the infrared and radio regions of the spectrum, and at gamma-ray and X-ray wavelengths, suggest that the core contains a sphere of stars and perhaps a black hole of about 100 solar masses.
The Sun is located about 28,000 light years from the galactic centre, within the disc, near the inner edge of a spiral arm.
The whole Galaxy is in rotation, but not as a rigid body, so it is constantly deforming. The Sun takes about 220 million years to complete a circuit, but stars nearer the centre take shorter times.
