Galaxy

The Galaxy. Schematic views of the structure of our Galaxy (excluding the outer corona), face-on (left) and edge-on (right). 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 of the order of 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 much of its content is concentrated in a thin layer only 2,000 light years thick towards its outer edges, though stars are distributed through a somewhat thicker disc. The central bulge has a radius of about 15,000 light years. Studies of the dynamics of stars and interstellar material suggest that the luminous material we can see accounts for as little as 10 per cent of the total mass of the Galaxy. The rest is so-called dark matter, in a form not yet identified. 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 ionized 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. 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. Centred on the nucleus is a sparsely populated, roughly spherical region, with a radius of at least 50,000 light years, known as the galactic halo. The halo contains globular clusters and, in general, the oldest stars in the Galaxy. There is very little luminous matter in the halo compared with the disc and central bulge, though gravitational studies suggest that the invisible component of the Galaxy's mass is probably distributed in a sphere around the Galaxy rather than concentrated in the disc. This dark matter is thought to extend up to 300,000 light years, far beyond the halo defined by visible objects, into what is sometimes termed the galactic corona. 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 tightly packed sphere of stars and a black hole. The likely mass of the black hole is disputed, some astronomers suggesting it could be as little as 100 solar masses, others saying it could be 3 million solar masses.