Number 160, January 14, 1994 by Phillip F. Schewe and Ben Stein|
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
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.)