© CAMBRIDGE UNIVERSITY PRESS 1983, 1993

1.2 Our Galaxy

Figure 1.1 shows a schematic representation of the Milky Way. In Figure 1.1(a) we see it face-on and in Figure 1.1(b) edge-on. The bright parts are made of light from many stars, while the dark parts are the observations produced by absorbing gas and dust clouds. The face-on picture shows the spiral structure of the galaxy, while the edge-on picture demonstrates that it is a disc with a central bulge. The disc is also referred to as the galactic plane.

Although the physicist would prefer the light-year as a unit of astronomical distance, the astronomer (for historical reasons) has grown accustomed to using the parsec (pc), the kiloparsec (kpc), and the megaparsec (Mpc) as distance units. 1 pc 3.26 light-years 3.0856 x 10¹⁸ cm. Using the kiloparsec as the unit for galactic dimensions, the diameter of the disc is estimated to be ~ 30 kpc, and its thickness ~ 1 kpc. The Sun along with all its planets is located ~ 10 kpc from the centre. The galaxy rotates about its polar axis as shown in Figure 1.1, although not as a rigid body. The Sun, for example, takes ~ 200 million years to make one complete orbit. Other stars have highly eccentric orbits that take them out of the galactic plane and also to the galactic centre. The former type of stars (like our Sun) with nearly circular orbits in the disc are called Population I stars, while the latter type of stars are called Population II stars. From the metal contents of the two types of stars and the theory of nucleosynthesis it is possible to argue that Population II stars are older than Population I stars. Astronomers also refer to an even earlier generation of stars called the Population III stars, which were very massive and burnt out quickly.

Fig. 1.1. The Milky Way, seen (a) face-on as a circular system with spiral arms, and (b) edge-on as a disc with a central bulge. We (that is, the Sun and its planets) are located about two-thirds of the way out from the centre. The Galaxy rotates about a central axis, with N and S the galactic North and South poles. C is the centre of the Galaxy.

The mass of our Galaxy is estimated at ~ 1.4 x 10¹¹ M, where mass of the Sun 2 x 10³³ g (a convenient mass unit in astronomy.) It is estimated that there are upwards of 10¹¹ stars in the galaxy. However, stars alone do not make up the whole of the galaxy. The dark lanes in Figure 1.1 show that obscuring matter is also present.

Absorption lines in the spectra of galactic stars show that absorbing gases are present in the interstellar medium. Gas appears in various forms - atomic and molecular, hot and cold. Emission nebulae around stars are made of gas that absorbs the ultraviolet radiation from stars and radiates it as visible light in spectacular colours. The so-called H II regions are hot regions near stars and contain hydrogen gas that has been ionized by the ultraviolet light of the stars. By contrast, the HI regions are cool regions of atomic hydrogen. The 21-cm observations in radio astronomy were largely responsible for detecting neutral hydrogen in the galaxy. Moreover, since the 1960s radio and microwave studies have revealed the existence of several complex molecules in the interstellar gas clouds.

Dark nebulae in the Galaxy are, by contrast, due to the presence of dust (see Figure 1.2). Interstellar dust may exist in several forms, such as graphite, silicates, or solid hydrogen. The effect of dust is to reduce the intensity of light from distant stars in the Galaxy. In the early days astronomers overestimated stellar distances in the Galaxy because they failed to correct for interstellar absorption. (Without correction, the faintness of a star was assumed to be wholly due to its distance from us.) The early astronomers also mistook dark regions for ``holes'' or empty regions in the Galaxy.

Fig. 1.2. The Horsehead Nebula in Orion. The dark shape arises from interstellar dust. (Courtesy of Kitt Peak National Observatory.)