Extreme Ultraviolet Astronomy , Martin A. Barstow and Jay B. Holberg Cambridge U. Press, New York, 2003. $100.00, (390 pp.). ISBN 0-521-58058-7
Before it became trendy to prefix practically any endeavor with “extreme,” astronomers were challenging nature and pushing the limits of optics to observe radiation lying between 100 Å and 1000 Å—the shortest wavelengths of UV light. Due to the quirks of atomic physics, few materials either reflect well or transmit easily in that extreme UV region. In addition, the abundant neutral hydrogen in interstellar space naturally limits astronomers’ ability to peer very far into the universe. Absorption at wavelengths shorter than the 912-Å Lyman limit of neutral hydrogen makes the interstellar medium practically opaque at distances beyond a few hundred light years, about 1/100 of the way to the center of our galaxy.
As Martin Barstow and Jay Holberg explain in the introductory chapters of Extreme Ultraviolet Astronomy, the pioneers of EUV astronomy challenged the accepted wisdom that EUV radiation was really unobservable and went on to open yet another window for astronomical discovery. Peering through the window, they discovered that local interstellar space is more transparent than previously thought. Their observations of active stars and nearby white dwarfs have provided new insights into stellar evolution and the ultimate fate of stars like our Sun. Both Barstow and Holberg played key roles in the development of EUV astronomy through their involvement with the wide field camera (WFC) on the ROSAT satellite and the UV spectrometer on the Voyager spacecraft.
With a readable blend of history, experimental technique, and observational results in chapters 1 and 2, the authors provide an in-depth introduction to the field, suitable for an advanced graduate student or professional astronomer. Chapters 3 and 4 cover major results from the ROSAT/WFC and Extreme Ultraviolet Explorer (EUVE) all-sky surveys, and subsequent chapters are devoted to EUVE spectroscopic observations of stellar winds, stellar coronae, white dwarfs, cataclysmic variables, active galactic nuclei, and the local interstellar medium. The book’s final chapter gives Barstow and Hol-berg’s vision of the future of EUV astronomy. The authors highlight—unintentionally—the youth and difficulty of EUV astronomy by including a slim appendix that contains all 1100 sources of EUV radiation so far cataloged.
Most EUV radiation sources detected with ROSAT and EUVE lie within the irregularly shaped, highly ionized “local bubble” that surrounds our solar system. Perhaps the major accomplishment of EUV astronomy has been to discern the structure of that bubble, understand the processes by which the gas within it is ionized, and by extension, improve our understanding of the rest of the interstellar medium in the Milky Way. EUVE observations of objects inside the bubble enabled astronomers to measure the surface composition of white dwarfs and provided the first accurate temperature measurements of the hottest of them. In chapters 4–10, Barstow and Holberg provide a wealth of illustrations, tables, and references covering all facets of those EUVE results. A major shortcoming in the presentation, however, is the authors’ narrow and nearly exclusive focus on observational details. None of the chapters makes more than a perfunctory attempt to establish the larger meaning of the results in the wider astronomical arena. For example, the temperatures of white dwarfs and the composition of their atmospheres provide a key link in our understanding of the late stages of stellar evolution, but little more than a few pages is devoted to discussing that link. Broader views would make the volume more interesting and attractive to new students and nonspecialists.
In 2001, EUVE ended its operations, and a space observatory dedicated to EUV observations no longer exists. However, important portions of the EUV band can be accessed with the Far Ultraviolet Spectroscopic Explorer and the Chandra X-ray Observatory . The authors mention neither, although both were launched in 1999 and the current capabilities of those satellites enable many of the same studies of white dwarfs, stellar coronae, cataclysmic variables, and active galactic nuclei as were done with EUVE.
When the authors consider the future of EUV astronomy, their vision suffers from the same narrow focus as their discussion of the past. Rather than describing a broad-based scientific case for a future EUV observatory, they emphasize the usual technical requirements of greater sensitivity and higher spectral resolution. Nevertheless, I recommend Extreme Ultraviolet Astronomy as a solid introduction to the history and practice of the field.
