ABSTRACT
The Universe contains a huge number of stars. Our galaxy has in excess of 200 billion stars, and there are billions of galaxies in the Universe. The density of stars is very high near the center of the Milky Way and decreases as we go outward along the galactic disk. In Figure 6.1 we show a Hubble telescope picture of the Sagittarius Star Cloud, located near the center of the Milky Way. We observe a very high density of stars with varying luminosities and colors. We also observe huge clusters of stars, called globular clusters, in the galactic halo. One example is shown in Figure 6.2. The properties of stars show wide variation in luminosity, size, mass, and spectrum. It is of course not possible to directly measure all these attributes. For most of the stars, we are only able to observe their flux density at different frequencies. Hence we know their apparent magnitudes corresponding to different filters, such as R, B, V, U, etc. Furthermore, their spectral lines can be measured very accurately. For some binary systems, the masses and radii of stars can also be directly measured. We point out that all stars, other than the Sun, appear as pointlike objects. It is not possible to directly measure their angular diameters. For some stars the angular diameter measurement has been made either by interferometry or using occultations. Using flux and spectral measurements, it is possible to deduce other properties, such as their mass, radius, chemical
composition, surface temperature, etc. In this and the next few chapters we study how this is accomplished.
