From Black Clouds to Black Holes

The following sections are included:

• Preface

• Contents

As the bright star Alpha Centauri faded into the background the three occupants of the tiny spaceship woke up from their mechanically induced sleep. Time for action was fast approaching…

A casual glance at the sky on a clear night may give the false impression that all stars are alike. This impression is soon corrected when we look at individual stars more carefully. We then find that some stars are brighter than others. We may even be able to notice subtleties of colour: the overall golden appearance may have tinges of red in some cases and even of blue in some others. We might also be able to notice the differences in size…

The last chapter indicates how light from a distant and physically inaccessible source can tell us, in many different ways, something interesting about that source. In Chapter 1 we saw how the physical characteristics of a human population vary from member to member and this variation tells us something about the life of a human being. In our present task of putting together a star’s biography, we will next see how the important characteristics of stars as conveyed by their light vary from one star to another in a stellar population. The key role in setting out these vital statistics of stars is played by a diagram which is somewhat analogous to the height-weight diagram for human beings shown in Figure 1.1. This diagram was first introduced into astronomy by E. Hertzsprung (in 1911) and H. N. Russell (in 1913) for different populations of stars. It is commonly known as the Hertzsprung-Russell diagram or simply the H-R diagram. We will refer to it by the shorter name.

To understand what the H-R diagram is about we have to introduce some technical jargon.

More than two thousand years ago, the most advanced astronomy in the world was practised in Greece. The Greeks had a simple view of the Universe. They assumed that the Earth is at the centre of the Universe, occupying a fixed and rigid position. The Sun and the stars rise and set periodically because they are attached to the celestial sphere which revolves round the Earth (see Figure 4.1)…

We now take up the story of a star’s life, beginning with its birth. According to the current theories of star formation, this story begins in a dark interstellar cloud of gas and dust.

Figure 5.1 describes the Orion Nebula. It is a cloud of gas in which stars are being born. The bright part of the cloud is lit up by the light from newly born stars. There is, however, more to this cloud than what meets the eye. Considerable information is hidden from us if we just rely on the light from the cloud coming through the visual window. As the techniques of microwave and infrared astronomy improved in the 1960’s and 1970’s the interstellar clouds began to reveal much of their hidden contents.

By the mid 1920’s astrophysicists had a reasonably accurate picture of what a typical star is made of. Regardless of the considerations of the previous chapter which give a scenario about how a star is born, much of which came to be understood later, the astrophysicists of the 1920’s took the star as a ready-made object and investigated the conditions under which it could maintain its shape and appearance for hundreds of millions of years. Pioneering work in this field was done by Sir Arthur Stanley Eddington at Cambridge. Even today astrophysicists working on stellar interiors start with the equations set up by Eddington (1882-1944).

Without going into technical details we begin with the ideas behind Eddington’s equations.

In the previous chapter we found that the source of a star’s radiant energy lies at its centre and that it has the form of a nuclear fusion reactor. We will now see how, as the star ages, this nuclear reactor also changes and how this change in turn affects the overall makeup and size of the star.

By applying the laws of physics discovered in the man-made laboratory we have been able to figure out how and why the stars shine, why they change their appearance from main sequence to the giant branch and how their internal composition changes. But we have not yet reached the end of the story. For, some very dramatic events are in store for a star which has become a red giant. Before we describe them, let us look at a tale from three continents, which contains factual accounts of the most catastrophic stage in a star’s life.

The Sun has a mass of 2 × 10³⁰ kg and its radius is 7 × 10⁸ metres. If it were a homogeneous ball of matter its average density would be 1.4 × 10³ kg per cubic metre or about 40% more than the density of water. Of course the Sun is not homogeneous and its density varies from the very low value of less than ten millionth part of the density of water at the outer surface (the photosphere) to about a hundred times the density of water at the centre…

Right from its birth the star’s life is a succession of struggles to maintain its internal balance against heavy odds. The odds of course are brought upon by the star’s own weight — by the force of contraction of its own gravity. Whether by nuclear fusion or by degenerate pressures the star does its best to counter this force, changing wherever necessary its internal composition and overall size. The different parts of the H-R diagram with the main sequence stars, the red giants, the white dwarfs and the invisible neutron stars indicate the extent to which the stars have to adapt themselves to changing circumstances. We saw in the preceding chapter that provided the final mass of the star does not exceed certain critical values it can continue to exist as a white dwarf or as a neutron star…

We have reached the end of our stellar odyssey. By adapting Purna’s technique for the study of humans in Chapter 1, we have managed to build a reasonably coherent picture of how stars form, why and how long they shine, why they change their outward appearance, and how they reach the end of their lives. Nuclear physics and gravity hold the key to our understanding of stars…

The following sections are included:

• Appendix: Powers of Ten and Logarithms

• Index