Summary

This chapter narrated astronomers' efforts, in the span of two centuries, to piece together the structure of the Galaxy. You have seen that this fitting together of the celestial architecture depended on the measurement of distances, which in turn is based on the fundamental properties of stars and electromagnetic radiation. Star counting-like that of Herschel and Kapteyn-gives only the relative shape of the Galaxy. Parallaxes were first used to set the distance scale. Later the H-R diagram-the spectroscopic distance method-was used to get more distances. But the big breakthrough came with Leavitt's discovery of the period-luminosity relationship for cepheids. Once calibrated in terms of luminosity using statistical parallaxes, this relationship could be used to find the distances to globular clusters. From these, Shapley found that the Galaxy was large, with the sun far from the center.

Three general regions make up the Galaxy: the nucleus (containing the nuclear bulge and core), the disk, and the halo. The stars in these regions are classified in two general groups: Population I and Population II. Population I stars are metal-rich. If young, the most luminous are blue, and associated with regions of gas and dust. But an old Population I can have luminous red giants as its brightest members. Population II stars are red, old, and metal-poor; they are found in regions relatively free of interstellar material.

The stars in the halo, including those in globular clusters, are Population II stars. From infrared observations of the galactic nucleus, and of other galaxies, astronomers infer that most of the stars there are old Population I. In the disk away from the nuclear bulge-such as in the neighborhood of the sun-young Population I stars abound.

Along with the Population I stars in the disk is an interstellar medium that consists of gas and dust. The gas is mostly hydrogen atoms and molecules, with some other molecules mixed in. A large fraction, perhaps as much as half, of the molecules appears to be concentrated in molecular clouds that lie within 4 kpc from the Galaxy's center. The inner part of the disk seems to be tilted 24°. The outer parts are much thicker than the rest and are warped up on one side, down on the other.

Radio, infrared, and x-ray observations of the Galaxy's nucleus have penetrated the dust veil there to find a place very different from the part of the Galaxy where we are located. The nucleus contains old Population I stars, singly and in clusters. It also contains molecular clouds, dust, ionized gas (all moving swiftly), and-surprisingly-a few young О and В stars, all concentrated in a small region of space. And the very heart of the Galaxy may harbor a supermassive black hole.