How do refracting and reflecting telescopes differ?

Generally speaking, a telescope† is an instrument that enhances the observation of celestial objects by increasing their apparent size and luminosity. This applies to the entire electromagnetic spectrum, from radio waves to gamma rays, including of course the “optical domain,” which covers visible light and radiation in the infrared and ultraviolet. Optical telescopes can be very diverse, but basically they work just like a photographic camera: they focus the light of a celestial object to form a real image‡ on film or on an electronic detector. In the visual version, an “eyepiece,” which is basically a magnifying glass, is used to observe the image directly.

The terms “refracting” and “reflecting” simply refer to the composition of a telescope's optics. If lenses are used, the instrument is a refracting telescope; if mirrors are used, it is a reflecting telescope.

Which system is better? Well, in general, the larger the telescope – that is, the larger its main mirror or lens – the more light it can collect and the fainter the object it can observe (Q. 205). Reflecting telescopes can be built very large indeed, whereas refracting telescopes have serious size limitations.

Introduction

Since gravity is a long-range attractive force, any star in a galaxy attracts all the other stars in the galaxy all the time. For simplicity, we can regard the stars as point particles. Then a galaxy or a star cluster can be regarded as a collection of particles in which all the particles are attracting each other through an inverse square law of force. The aim of stellar dynamics is to study the dynamics of such a system of self-gravitating particles. We, of course, know that there is also gas between the stars in a galaxy, which can add extra complications. However, it is generally believed that stellar dynamics holds the key to understanding the structure of galaxies or star clusters.

We have discussed our Galaxy in Chapter 6 and shall discuss external galaxies in Chapter 9. Although some galaxies are irregular in appearance, we shall see in §9.2 that most galaxies have very regular shapes. The fundamental question of stellar dynamics is: why do collections of self-gravitating mass particles tend to take certain particular configurations in preference to many other possible configurations? A fully satisfactory answer to this question is still not known. Hence the subject of galactic structure is on a much less firm footing compared to the subject of stellar structure. We know that the gravitational attraction of the stars has to be balanced by their motions, to ensure that the stars do not all fall towards the centre of the stellar system together due to their mutual gravitational attraction.

The shape and size of our Galaxy

When we look around at the night sky, we find that the stars are not distributed very uniformly. There is a faint band of light – the Milky Way – going around the celestial sphere in a great circle. Even a moderate telescope reveals that the Milky Way is a collection of innumerable faint stars. Herschel (1785) offered an explanation of the Milky Way by suggesting that we are near the centre of a flat disk-like stellar system. When we look in the plane of the disk, we see many more stars than what we see in the other directions, thus producing the band of the Milky Way. After the development of photography, it became much easier to record distributions of stars in different directions. In the beginning of the twentieth century, Kapteyn attempted to put Herschel's view on a firm footing, by undertaking a huge programme of counting stars in different directions and measuring their proper motions with a view of estimating distances. From a painstaking statistical analysis of these data, it was inferred that we are at the centre of an oblate stellar disk with a thickness of a few hundreds of pc and a disk radius of about a few kpc (Kapteyn and van Rhijn, 1920; Kapteyn, 1922). This model is usually referred to as the Kapteyn Universe, since it was believed at that time that this was the whole Universe!

Introduction

At the beginning of §2.4, we pointed out the scope of the subject stellar interior. It appears from observational data (to be discussed in detail later) that various quantities pertaining to stars have some relations amongst each other.For example, a more massive star usually has a higher luminosity and also a higher surface temperature. To explain such observed relations theoretically, we have to figure out the equations which should hold inside a star and then solve them to construct models of stellar structure.

The years ≈1920–1940 constituted the golden period of research in this field, when theoretical developments led to elegant explanations of a vast mass of observational data pertaining to stars. Ever since that time, the subject of stellarinterior or stellar structure has remained a cornerstone of modern astrophysics and improved computational powers have ledto more detailed models. This is a subject in which theory and observations are intimately combined together to build up an imposing edifice. While presenting a subject like this, the first question that a teacher or a writer has to face is this: from a purely pedagogical point of view, is it better to start with a discussion of observational data or with a discussion of basic theoretical ideas?

It follows from simple theoretical considerations that there must be objects like stars, provided energy can be generated by some mechanism in the central regions.

What is life?

Are we alone? Does life exist anywhere besides Earth? Modern astronomy can help to answer these fundamental questions, but first we need to know what is meant by life.

The concept is hard to define. It used to be generally accepted that life is organized matter that exhibits seven crucial characteristics: growth, respiration (i.e. exchange of gases), nutrition, excretion, reproduction, reaction to external stimuli, and locomotion (the last is only partly true, since plants and many other organisms are not mobile). Matter in a crystal is organized, and one could argue that a crystal grows by nourishing itself with neighboring atoms, that it reproduces when it branches out, and that it reacts to external stimuli, contracting when subjected to an electrical charge, for example (as in the piezoelectric effect used in quartz watches). Nevertheless, a crystal does not breathe, does not excrete, and does not move. Then what about fire? A fire nourishes itself with combustible material, absorbs oxygen (breathes), grows, moves, can ignite additional fires in new locations (reproduces), excretes heat, and reacts to external stimulations such as wind or a fire extinguisher …

But if it can be argued that fire meets the seven basic criteria for life, living creatures fulfill one additional condition: they can mutate. This allows an organism to adapt to new environments and thus to evolve. Fire cannot evolve into a different form of oxidation, such as rusting, but we know from the fossil records that life transforms itself all the time, resulting in the extraordinary variety of forms around us: from bacteria to watermelons to oysters to elephants.

Introduction

A plasma is a gas in which at least some atoms have been broken into positively charged ions and negatively charged electrons. Most of the matter in the Universe exists in the plasma state. The gases inside stars are ionized because of the high temperature, as can be shown easily with the help of the Saha equation (2.29). We have seen in §6.6.4 that HII regions in the interstellar medium are fully ionized due to energetic photons from very hot stars. Even the HI regions are partially ionized, with some free electrons present in them. Our aim in this chapter is to give an introduction to some dynamical principles as well as some radiation processes involving plasmas, which are of great relevance to astrophysics.

The reader may wonder why this introductory chapter on plasma astrophysics is put exactly in this place of the book. We could, of course, introduce the subject much earlier. However, since we shall illustrate the dynamical principles by applications to stars and the interstellar medium, I felt that a prior acquaintance with these systems will put you in a better position to appreciate the relevance of plasma processes in astrophysics. There is also some justification for introducing this subject before a discussion of extragalactic astronomy. In Chapter 9 we shall discuss some extragalactic systems suchas active galaxies in which plasma processes are extremely important. So it will be helpful to have some knowledge of plasma astrophysics before we launch into a study of extragalactic astronomy.