Galaxies

Abstract

Galaxies, like stars, come in a range of sizes. Like stars too, there are not so many big galaxies, but plenty of small ones; and again, like stars, the big ones are called “giants” and the small ones “dwarfs.” But photographs of galaxies are much more interesting than photographs of stars because galaxies have such a variety of shapes. The main division is between spiral galaxies like our own and elliptical galaxies. Spirals come in two basic forms, depending on the nature of their central bulge. Ordinary spirals have a fairly symmetrical central bulge; barred spirals have an elongated central bulge. The relative size of the bulge and the spiral arms varies, as does the tightness with which the arms are wrapped round the bulge. Given all these possibilities, it is hardly surprising that spiral galaxies can differ considerably from each other in appearance. Elliptical galaxies differ less. They look more like the central bulge of a spiral galaxy, but less flattened and without the spiral arms. As their name suggests, they are more or less elongated in cross-section. Besides these main types, some galaxies are labeled “irregular”—that they do not have an obvious shape—while others are “peculiar”—meaning that they may have a definite shape, but there is something odd about them.