|Milky Way Galaxy
A spiral galaxy, of which the solar system is a small part. It is the second largest in our Local Group of galaxies. The Milky Way is a disk-shaped system, with a diameter of between 80,000 and 100,000 light-years and a thickness of about 2,000 light-years, containing more than 10^11 stars. The stars are divided into two main categories, Population II stars and Population I stars. The core, or nucleus, of the Galaxy is surrounded by an ellipsoidal central bulge that measures some 15,000 light-years in diameter and about 6,000 light-years in the direction perpendicular to the plane of the disk. Surrounding the bulge and extending in a near spherical distribution above and below the Galactic plane is the Galactic halo. The halo contains about 200 globular clusters and an extremely thinly scattered population of individual stars. The Sun is located just over half way out from the center to the edge of the disk at a distance of about 25,000 light-years. In common with other stars, the Sun revolves around the Galactic Center. Its orbital velocity is about 220 km s^-1 and its orbital period is about 225 million years. Overall, the Galaxy exhibits differential rotation, that is stars and gas clouds closer to the center have shorter orbital periods than those that are located further out. The spiral arms of the Milky Way lie within its disk, where bright young stars, H II regions, and molecular clouds of gas and dust are concentrated into curved 'arms' that appear to radiate from the central bulge in a spiral pattern. The Galaxy's spiral pattern consists of several major arms and a number of shorter segments, one of which, the Orion arm, contains the Sun and the Orion star-forming region. Near-infrared observations have shown that the stars in the central bulge are arranged in an elongated galactic bar, about twice as long as it is wide, that is seen nearly end on from the present location of the solar system. The exact center, or nucleus, of the Galaxy coincides with a strong source of radio emission, called Sagittarius A, that is less than 15 astronomical units in diameter. Observations of the speeds at which clouds of ionized gas are revolving round the Galactic center imply that several million solar masses of material are concentrated within a region of about one light-year in radius. Since only about half of this mass can be accounted for by stars, it seems likely that the balance (about 2.5 million solar masses) is contained in a central black hole and that accretion onto this black hole is the underlying source of the energy radiated by Sagittarius A. The Milky Way also has a dark matter component. The Galactic rotation curve indicates that there is a large amount of invisible non-baryonic surrounding the whole Galaxy.