This is a book about X-ray astronomy. We take a historical perspective because this is how we saw it happen and because this gives a feeling for the observable universe. In a table listing all members of a class of objects, the brightest source does not stand out, but in the first observation, it is a splendid object and remembered fondly by those involved in the discovery.

Some 50 years ago X-rays from stars other than our Sun were unknown and unexpected by all but a few pioneering scientists. Since the discovery of cosmic X-rays in 1962 the field has grown at an astonishing rate. Since the first edition of this book, published in 1995 and including results from the first X-ray telescopes, the sensitivity of X-ray observations has increased dramatically. In 1999 the Chandra and XMM X-ray observatories were launched and, in 10 years of operation, have produced X-ray images of comparable angular resolution to those obtained by the largest ground-based observatories. More importantly, X-ray spectroscopy of sufficient resolution to allow comparison with spectra at other wavelengths has become possible. Technical improvements in dispersive spectroscopy mean that high resolution X-ray spectra of faint sources have become available for the first time. This has helped propel X-ray astronomy to its rightful place as a sub-discipline of astronomy, where a knowledge of truly multiwavelength results is necessary for the study of any class of objects. This book, however, is about X-rays.