X- and Gamma-Ray Astronomy

The IAU Symposium No. 55 on 'X-Ray and Gamma-Ray Astronomy' has occurred, not entirely by coincidence, at an important moment in the development of these new branches of observational astronomy. In X-ray astronomy the data from the first X-ray observatory UHURU have contributed to a new view of the X-ray sky and a new conception of the nature and properties of galactic and extragalactic X-ray sources. In gamma-ray astronomy the exciting and often controversial nature of the results underlines the importance of the forthcoming launch of SAS-B, the first orbiting y-ray observatory. As Bruno Rossi reminds us (p. I), the Symposium occurred almost exactly ten years after the first detection of the X-ray star Sco X-I. During this time we have moved from the detection of a handful of the nearest and brightest sources to the detailed study of the nature of stellar sources in the farthest reaches of our own galaxy and in external galaxies of the local group. The detection of pulsating X-ray sources in bi­ nary systems permits the measurement of pulsation periods, and orbital parameters with precisions comparable to any yet achieved with traditional observational techniques. The strong indications that most X-ray sources are extremely compact objects give us confidence that X-ray astronomy will playa significant and possibly decisive role in the study of stars near the end point of stellar evolution.