Introduction

The distance to a celestial object is often crucial to determining its size or luminosity. Unlike the angular position of a star or galaxy on the celestial sphere, distances often can be obtained only indirectly. A significant fraction of astronomical history has been dedicated to the determination of distances. Even now, new techniques to determine the distances of the most distant galaxies are an important aspect of cosmological studies.

The values and ranges of the distances, sizes, masses and luminosities of celestial objects tend to be on the large side; this is astrophysics, not atomic physics. It is important to have a feeling for these quantities, but this can be difficult in view of the large values. It is similar to the difficulty in gaining an intuitive feeling for a national debt. It is helpful to concentrate on the nearest power of 10 (the order of magnitude) rather than the exact number. For example, you might memorize that there are 101 persons in the class, 106 in greater Boston, and 109 on the planet earth. Then if someone tells you there are ~ 104 students at MIT, you can put that number roughly in context. Don't visualize people, visualize exponents; think logarithmically!

Nevertheless, the above should not lull the reader into being casual about learning more precise values of some quantities. Astrophysicists use again and again certain values such as the mass, radius, and distance to the sun, and relate other quantities to them. It behooves the reader to learn these to about two-digit accuracy.

In this chapter we discuss the role of luminosities and fluxes, masses, temperatures and finally sizes and distances in astronomy. We present the ranges of values these objects have and outline some of the methods by which these quantities are determined. The largest portion of the chapter deals with the determination of distances.

Telescopes Mastery

Telescopes Mastery

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