Number 115, February 25, 1993 by Phillip F. Schewe and Ben Stein
THE HARVARD-SMITHSONIAN REDSHIFT SURVEY , which measures the Doppler
shift of spectra from galaxies out to a distance of about 500 million light
years from Earth, has been expanded. The earlier survey, featuring numerous
voids and clumps of galaxies, including a manlike stick-figure pattern
of galaxies, covered a slice of sky 6 degrees thick and 135 degrees around.
The newer survey again shows large congregations of galaxies (but none
larger than the "Great Wall" of galaxies previously reported)
for an enlarged annular volume 36 degrees thick and 360 degrees around.
By 1995, the Harvard astronomers---Margaret Geller, John Huchra, and Ron
Marzke---expect to have a complete survey of northern-hemisphere galaxies
brighter than 15th magnitude. (Astronomy, April 1993.)
THE LITHIUM-11 NUCLEUS consists of nine nucleons surrounded at some
distance by a pair of neutrons constituting a weakly-bound halo. The existence
of the halo was first suspected when, five years ago, nuclear scattering
experiments showed that the reaction cross section for Li-11 was significantly
larger than expected; this suggested that the nuclear size was larger than
normal. A new experiment has now actually measured the size of the halo
in the act of breaking up (Nature, 11 Feb. 1993). At the National Superconducting
Cyclotron Lab at Michigan State, a beam of radioactive Li-11 is created
by sending oxygen-18 ions into a thin lithium foil. The Li-11 is then scattered
from a target of lead nuclei, whose electric fields cause the relatively
gentle breakup of the Li-11 into Li-9 and a pair of neutrons (I. Ieki et
al., 8 Feb. 1993 Physical Review Letters). When Li-11 is scattered instead
from light nuclei, such as Be-9, fragmentation occurs through the agency
of the strong nuclear force (N.A. Orr et al., 5 Oct. 1992 Phys. Rev. Lett.).
Both types of scattering suggest that the Li-11 halo is five times larger
than the size of the Li-9 nucleus. Since the halo represents a sort of
"neutron matter," experiments at MSU and several other labs,
such as GANIL and SACLAY in France and RIKEN in Japan, hope to examine
the interactions between the halo neutrons and to seek out other nuclei
with halos ("Physics News in 1992," in the March 1993 APS News).
THE CLOSEST KNOWN PULSAR, PSR J0437-4715 (with a rotation period of
5.75 msec), is only about 400 light years from Earth, several times closer
than any other known millisecond pulsar. It was discovered by astronomers
in Australia (with the Parkes 64-m radio telescope) making a southern-sky
search for msec pulsars. They estimated (very roughly) the distance to
the pulsar by measuring the dispersion of its radio pulses caused by the
waves' passage through the intervening ionized interstellar medium; the
dispersion was the smallest for any known pulsar. PSR J0437-4715 is close
enough, however, that its distance might be computable independently by
parallax techniques, in which case the local interstellar electron density
could be measured. Rosat and the Gamma Ray Observatory will be searching
for the object at x-ray and gamma wavelengths, respectively. (Simon Johnson
et al., Nature, 18 Feb. 1993.)
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