Number 181, June 3, 1994 by Phillip F. Schewe and Ben Stein
ASYMMETRIC SUPERNOVA EXPLOSIONS can impart a "kick" to the
neutron star remnants born during the explosions. After a new reassessment,
the proper motions (the motions across the sky) of 86 pulsars were found
to have a mean velocity of 450 km/sec, which exceeds the escape velocities
for a number of celestial systems, such as binary stars, globular clusters,
and even for our galaxy. The University of Manchester (UK) astronomers
who performed the study conclude that more than half of all pulsars will
escape from the Milky Way and that those that stay will assume a larger
and more spherical distribution than was previously thought. Therefore,
the astronomers assert, this population of old, high-velocity pulsars might
be responsible for more of the gamma bursts seen by the Gamma Ray Observatory
than previously expected. (A.G. Lyne and D.R. Lorimer, Nature, 12 May 1994.)
IN A QUANTUM CASCADE (QC) LASER electrons emit light in a succession
of steps as they traverse numerous thin quantum wells stacked together
by epitaxial techniques. The wavelength of the output laser light depends
on the relative thickness and spacings of the layers rather than on the
specific semiconductor composition of the device. Federico Capasso and
colleagues at AT&T Bell Labs, who developed the 500-layer QC laser,
believe that the ability to tailor the output wavelength will make the
laser useful for a variety of applications, such as detecting levels of
atmospheric pollution and for inter-chip telecommunications. (Jerome Faist
et al., Science, 22 April 1994.)
TWO LARGE RINGS LOOMING MYSTERIOUSLY NEAR , but not exactly around, Supernova
1987A have been imaged by the Hubble Space Telescope. One ring stands in
front and the other in back of the supernova in the new pictures. Together
with a much-studied inner ring centered on the supernova, the three-ring
structure resembles a gigantic hourglass. A settled explanation for this
bizarre configuration never seen before around a supernova will come only
with additional observations. (Science News, 28 May 1994.)
ACOUSTIC COLOR is the name for the process by which sound waves scatter
from an object at characteristic frequencies ("colors" in analogy
with optical color) depending on the object's composition, shape, and other
properties. Nicholas Makris at the Naval Research Laboratory (202-767-9037)
has demonstrated an acoustic-color imaging technique that can map 200 square
kilometers of ocean floor at a time from a single point at sea. Using a
trio of ships with underwater instruments, Makris and his colleagues first
obtain a "black and white" image of a region of the Mid Atlantic
Ridge, by radiating a powerful, single-frequency sound wave that "illuminates"
a vast region of the ocean floor. The echoes received from different parts
of the ocean floor are detected by a 300-meter-long string of underwater
sensors, yielding a rough topographical map. Then, a more detailed "color"
image is obtained by radiating sound waves over a spectrum of frequencies
from 200-300 Hz. Whereas conventional surveying techniques take a month
to map 200 square kilometers of ocean floor, the acoustic color technique
can map an equivalent area in minutes. (Paper to be presented at an upcoming
meeting of the Acoustical Society of America in Cambridge, MA.)
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