Variable stars

All stars display variations of brightness and colour in the course of their passage through subsequent stages of stellar evolution. As a rule, however, a star is called variable when its brightness or colour variations are detectible on time scales of the order of the mean life time of man. The variations may be periodic, semi-periodic or irregular, with time scales ranging from a couple of minutes to over a century. It is this kind of variable star which is the topic of this book. The typical time scale, the amplitude of the brightness variations, and the shape of the light curve can be deduced from photometric observation, and those quantities place the star in the appropriate class. For example, a star of the UV Ceti type typically has brightness variations (the so-called flares) of several magnitudes in an interval of time as short as a few minutes, whereas a Cepheid shows periodic variations of about one magnitude in a time span of several days. However, spectral type, luminosity class and chemical composition are complementary important spectroscopic parameters that are needed for classifying variable stars according to the origin of their variations.

At the very beginning of the space age, spacecraft designers learned that the effects of the space environment on a spacecraft's systems would be vital factors in spacecraft design and operation. Since those early years, the topic of spacecraft–environment interactions has developed into a multidisciplinary field involving engineers and scientists from all over the world. Traditionally, engineers have been interested in spacecraft design and operational issues, and scientists have concentrated on the fundamental physics and chemistry associated with the interactions. These diverse interests have led to numerous books and conferences. The field has grown substantially in the past decade with the advent of the Shuttle and the ability to perform repeatable, in-situ experiments. The authors therefore concluded that, with the growth of the field and the expanding interest in it, it was timely to prepare a comprehensive book summarizing the many recent discoveries. In particular, since the field has evolved in a way that has been driven by mission and spacecraft requirements rather than as a specific discipline, a book would be a valuable step in integrating the field intellectually. Such a book would also serve as an introduction to the discipline for graduate students and professionals. For specific applications, these individuals could then turn to one of the handbooks or collections of conference papers referenced throughout the book.

This book is the direct outgrowth of courses that the authors have taught.

In Chapter 1, the four basic environmental interactions were introduced. To help in understanding the physics of these interactions with a spacecraft, the nondimensional physical parameters that determine the interactions are described in Chapter 2. An understanding of the magnitudes of these nondimensional parameters simplifies the complex physics describing the interactions. This is analogous to the idea that, for a fluid, the definition of a Reynolds number allows the physics of the fluid behavior to be divided into two regimes. When the Reynolds number is small compared to unity, the physics of the fluid is dominated by viscous effects. When the Reynolds number is large compared to unity, the flow is inviscid. Likewise, for a compressible gas, the Mach number allows the physics of the flow to be divided into subsonic physics and supersonic physics. This idea of fundamental scales, like the Reynolds and Mach numbers, is exploited in this and subsequent chapters as a means of characterizing the effects and importance of the fundamental interactions under varying environmental constraints.

The classification of X-ray binaries is somewhat ambiguous. Some authors consider X-ray binaries to be any kind of interacting close binary with a compact degenerate object - that is, a white dwarf, a neutron star, or a black hole. A more specific definition is that X-ray binaries are only those interacting close binary systems which contain a neutron star or a black hole. In this chapter we shall restrict ourselves to the latter definition; interacting close binaries with a white dwarf are usually called cataclysmic variables which are described in Chapter 5 of this book.

The main (empirical) difference between the cataclysmic variable and the X-ray binaries as defined above is the X-ray luminosity: whereas X-ray binaries have X-ray luminosities of 1O∧35 - 10 ∧38 erg s which corresponds to 25 to 25000 times the total solar luminosity, cataclysmic variables have Lx ≦ 1034 erg s. Hence, X-ray binaries are discovered on the basis of their strong X-ray emission. The basic model of X-ray binaries is a close binary system with a ‘normal’ star (main sequence or giant, in exceptional cases a degenerate star too) filling its Roche lobe and transferring matter to the compact object, a neutron star or a black hole. Such a system is called a ‘semi-detached’ system. Due to the orbital angular momentum the matter cannot directly fall onto the compact object, and it forms an accretion disc around the latter (see also Section 5.4). Due to internal friction in the accretion disc (also called viscosity) the matter spirals inward until it eventually falls onto the compact object.

Supernovae

A supernova explosion is a rare type of stellar explosion which dramatically changes the structure of a star in an irreversible way. Large amounts of matter (one to several solar masses) are expelled at high velocities (several tens of thousands km s-1). The light curve in the declining part is powered by thermalized quanta, released by the radioactive decay of elements produced in the stellar collapse, mainly 56Co and 56Ni. The ejected shell interacts with the interstellar medium and forms a SN remnant, which can be observed long after the explosion in the radio, optical and X-ray regions.

Supernovae can be divided into two classes (and several subclasses), viz. SN I and SN II.

SN I have fairly similar light curves (see, for example, Fig. 5.1) and display a small spread in absolute magnitudes. Spectra around maximum show absorption lines of Ca II, Si II and He I, but lack lines of hydrogen. They occur in intermediate and old stellar populations. Their progenitor stars are not clearly identified, but massive white dwarfs (WDs) that accrete matter from a close companion and are pushed over the Chandrasekhar limit are good candidates. Another possibility is the hypothesis of the fusion of a binary consisting of two WDs. The collapse of the white dwarf leads in both cases to an explosive burning of its carbon, and the released energy is sufficient to trigger a disintegration of the complete object.