A. Galaxies 

Together, stars, nebulae and clusters occur in gravitationally linked groupings called galaxies. They vary greatly in size and shape, and range up to really large galaxies that include hundreds of billions of stars. The two galactic shapes that are most readily apparent are elliptical (more or less oval blobs) and spiral (giant pinwheels of stars and gases). Others, generally small and asymmetric, show little pattern to their shape and are called irregular galaxies. 

Spiral galaxies come in two main varieties, normal spirals, where the spiral arms extend directly from the nucleus, and barred spirals where the spiral arms come off a bar of stars that extends from the nucleus. The Milky Way is a normal spiral (type Sb). 
 

Elliptical galaxies are systems without spiral arms. They vary in shape from spherical to football-shaped to almost flat, lens-shaped. Some are dwarf galaxies, some are giants larger than the Milky Way. We generally think that elliptical galaxies evolved from the collision of spiral galaxies and one interpretation for irregular galaxies is that they are pieces of spiral arms that got separated during collisions between galaxies. 

Our own galaxy, the Milky Way, is a spiral galaxy, majestically rotating in space. Over 100,000 light years across it is made up of some 150 billion stars. The sun takes some 225 million years to revolve around the center of this galaxy even at its speed of 220 km/s. Hidden from sight behind these spiral arms is a nearly spherical central nucleus where stars are more dense than in other parts of the galaxy and at whose center lies a black hole packing several million solar masses in a volume smaller than the solar system. Spiral arms radiate outward from this nucleus to form a flattened disk. Our star, the sun, lies about two thirds out from the galactic center (some 30,000 LY) in one of these spiral arms (the Orion arm) and travels among the other stars at 19 km/s (nearly 12 mi/s). Scattered between the stars that are light years apart in in the spiral arms are gases (mostly hydrogen and helium) and dust. Extensive clouds of these materials make up the nebulae. Lying outside the nucleus and the disk are a few hundred globular clusters.