PHYSICS NEWS UPDATE A digest of physics news items prepared by Phillip F. Schewe, AIP Publ

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PHYSICS NEWS UPDATE A digest of physics news items prepared by Phillip F. Schewe, AIP Public Information Number 160 January 14, 1994 A REJUVENATED HUBBLE SPACE TELESCOPE can now do what it was built to do---glimpse faint objects 12 billion light years away as well as provide unprecedentedly sharp views of nearer objects such as individual stars in certain galaxies. This is essential since to better establish the existence of black holes it is necessary to observe the motions and not just the density of stars near the hypothetical black hole. Also the observation of single Cepheid variable stars in galaxies 50 to 100 million light years away will improve the calculation of astrophysical distances and consequently the determination of the Hubble constant. Hubble scientists spoke at this week's meeting of the American Astronomical Society (AAS) in Virginia. They recounted the flawless repair mission carried out in December by Space Shuttle astronauts, including the installation of a corrective-optics package for the main mirror and a new wide field planetary camera and showed "before" and "after" pictures of various celestial objects, thus showcasing Hubble's crisp new vision. Optical tests are nearly complete, after which scientific observations will resume. GALAXY M81 LACKS DARK MATTER. The very idea of dark matter arose partly to explain the velocity profile of matter swirling around spiral galaxies. In many such galaxies the velocity of objects (determined by the doppler shift of their light emissions) seemed to be nearly constant as a function of the distance out from the center of the galaxy. Such a distribution would not occur, many scientists believed, unless a considerable amount of nonluminous matter were present in or near the galaxy. But new measurements of the neutral hydrogen in galaxy M81, made with the Very Large Array radio telescope, indicate that the velocity of hydrogen falls off with radial distance from the galactic center, a distribution suggesting a lack of dark matter. David Adler of the National Radio Astronomical Observatory and David Westpfahl of the New Mexico Institute of Mining and Technology said that these results, announced at the AAS meeting, demonstrated that dark matter is not distributed in uniform amounts among galaxies. GAMMA-RAY FLASHES IN EARTH'S ATMOSPHERE have been observed by the Compton Gamma Ray Observatory (GRO), according to Neil Gehrels of NASA Goddard, who spoke at the AAS meeting. The GRO looks for gammas from across the sky out to the furthest corners of the universe; scientists had not expected to see them at our home planet, Gehrels said. He suggested that the flashes might be occurring over intense storms and may result from upward-going lightning. THE SOLAR CHROMOSPHERE IS COLDER than we thought. The chromosphere is the region between the photosphere (the sun's surface, at a temperature of about 6000 K) and the corona (whose temperature is 1 million K or more). Previously scientists had figured that the chromosphere temperature was relatively cool---an estimated 4300 K at an altitude of 500 km above the sun's surface--- but new measurements made at Kitt Peak show that the chromosphere is colder than this. High-resolution infrared observations of carbon monoxide molecules at the limb of the sun provide a new minimum temperature of 3500 K which, furthermore, seems to occur at a higher altitude, 1100 km. Robert Noyes of Harvard Smithsonian says that carbon monoxide clouds may be a transitory phenomenon in the solar atmosphere. (Science, 7 Jan. 1994, Science News, 8 Jan.)

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