ISSN:0031-9228    

DOI:10.1063/1.2810062

出版商:AIP    

年代:1991

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2810064

出版商:AIP    

年代:1991

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2810065

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

At a number of construction sites around the world, crews are laboring in and around large circular tracks to install the accelerators and storage rings that will form the basis of new synchrotron radiation facilities. Most of the budding facilities are designed to produce synchrotron radiation, in the soft‐ or hard‐x‐ray regions, that is far brighter than available sources can provide. They are sometimes described as the “third generation.” According to this rough categorization, the first generation consists of circular accelerators originally intended for other purposes: They provide synchrotron radiation to parasitic experiments or, in some cases, they have become partially dedicated to such uses. The second generation comprises facilities specifically designed to support synchrotron radiation experiments, with the radiation produced primarily as electrons or positrons curved in the field of the machines’ bending magnets. The third‐generation machines, by contrast, are designed to optimize the radiation that is produced as the electrons or positrons traverse devices known as wigglers and undulators. The new machines will complement existing facilities, and they will provide more opportunities for the growing user community.

ISSN:0031-9228    

DOI:10.1063/1.2810066

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The US has built and operated many large facilities such as the synchrotron radiation facilities for the benefit of public users. These users are not charged for their time on the machines, provided they publish the results of their experiments in the scientific or technical literature. But Congress is now looking at the large expenditures on such user facilities and wondering if more of the costs should be shouldered by users. Thus a provision in the Budget Reconciliation Act instructs the Department of Energy to explore ways “to reasonably increase revenues to the United States through users fees” and report back this spring. The study is to include facilities of all agencies, not just DOE.

ISSN:0031-9228    

DOI:10.1063/1.2810067

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

A sudden eruption on Comet Halley enveloped it in a cloud of dust that is 300 times brighter and about 20 000 times larger than its nucleus. (The width of the photograph above corresponds to 71 arcseconds, or about 700 000 kilometers at the distance of Halley.) Previously the comet had behaved as expected, reappearing in our solar system right on schedule. At that time, our visitor had also generated a cloud of dust, but such a phenomenon was expected when the comet was close to the Sun: The solar energy sublimated the water ice on this “dirty snowball,” and the outward‐moving water vapor carried the dirt particles off the surface. But as the comet gradually receded from the Sun, its surface temperature fell back and the dust cloud virtually disappeared. Then, on 12 February, as Comet Halley was between Saturn and Uranus, some two billion km from the Sun, comet trackers Olivier Hainaut and Alain Smette of the European Southern Observatory in Chile found the comet to be greatly enlarged, as shown in the above photo. Three days later, another Halley watcher, Karen Meech (University of Hawaii) independently saw the vast cloud of dust. Smette subsequently made spectral measurements and determined that the comet's light was consistent with sunlight reflecting from dirt particles. Richard West of ESO reports that through 17 March the comet was still extremely active, with the shape of the cloud changing from night to night. This behavior just deepens the mystery, West feels.

ISSN:0031-9228    

DOI:10.1063/1.2810068

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Astronomers and astrophysicists have learned that the climates and weather patterns of planets in the solar system are driven by many of the same physical processes that create the Earth's environment; that stars form out of clouds of gas and eventually die either in quiet solitude or spectacular explosions; that most of the common chemical elements are created in explosions of stars; that stars group together in isolated galaxies; that galaxies and clusters of galaxies stretch in sheets and filaments as far as the largest telescopes can see; and that the universe itself was born in a violent explosion some15×109years ago. Most amazingly, we have learned that the laws of nature on Earth apply to the farthest reaches of the observable universe.

ISSN:0031-9228    

DOI:10.1063/1.881284

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The enormous technological and scientific advances of the 1980s give us an unparalleled opportunity to address fundamental issues in astrophysics through observations at infrared wavelengths. The recommendations of the Astronomy and Astrophysics Survey Committee reflect these opportunities for research in the 1–1000‐micron wavelength range. The highest priority recommendations for large new projects in this part of the spectrum are for a Space Infrared Telescope Facility, orSIRTF, and an 8‐meter ground‐based Infrared Optimized Telescope, or IRO. A Stratospheric Observatory for Infrared Astronomy, orSOFIA, is a highly recommended medium‐sized initiative, and there are strong recommendations for several other infrared initiatives.

ISSN:0031-9228    

DOI:10.1063/1.881285

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

In the years immediately following World War II, radioastronomers concentrated on following up on the accidental wartime discovery by British and American radar operators of radio bursts from the Sun. However, the continued push toward shorter and shorter radio wavelengths, combined with greatly increased sensitivity and angular resolution, quickly led to a series of remarkable and unexpected discoveries. Radio galaxies, quasars, pulsars, interstellar masers, gravitational lenses and the microwave background radiation—all now familiar topics of the astronomical and popular literature—were discovered serendipitously because of their radio emissions. A new generation of radiotelescopes to be built in the 1990s, together with major improvements in existing instruments, will give even further large gains in sensitivity, angular resolution and image quality, especially in the newly opened millimeter and submillimeter regions of the electromagnetic spectrum.

ISSN:0031-9228    

DOI:10.1063/1.881286

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

During the last 350 years, astronomers have improved the sensitivity of their observations by a factor of almost a billion by using larger telescopes and better detectors. At optical wavelengths, however, there has been little improvement in angular resolution since Galileo's time. Until very recently, atmospheric turbulence limited the resolution of astronomical observations with even the largest telescopes at the best sites to around 0.5”. Astronomy is on the verge of realizing the promise of two new techniques that will improve the limiting resolution of optical and infrared observations by factors of tens or even thousands.

ISSN:0031-9228    

DOI:10.1063/1.881287

出版商:AIP    

年代:1991

数据来源: AIP