Number 270, May 9, 1996 by Phillip F. Schewe and Ben Stein
TWO EFFORTS TO MEASURE THE HUBBLE CONSTANT are converging somewhat.
Wendy Freedman of the Carnegie Institution reported at a NASA press conference
today that she and her colleagues were finding that values for the Hubble
constant (H), a measure of the expansion of the universe, hovered in the
range 68 to 78 km/sec/Mpc. (In 1994, they reported a preliminary value
of 80.) A separate group led by Allan Sandage, also of Carnegie, recently
reported a Hubble constant of 57. Freedman's team is midway through a 3-year
program of measuring the distance to 20 distant galaxies by observing Cepheid
variable stars, whose intrinsic brightness is related to the rate at which
their luminosity varies. These observations in turn can be used to calibrate
other means for determining distances to objects at even larger scales
where local gravitational interactions have a lesser impact on a calculation
of H. The secondary yardstick methods include the determination of the
peak brightness of type-Ia supernovas and the use of the Tully-Fisher relation,
according to which a galaxy's luminosity is related to its rotation rate.
The latest entry in Freedman's inventory is galaxy NGC1365 in the Fornax
cluster, at a distance of 60 million light years. (NASA press release,
8 May 1996.)
THE OLDEST STARS IN THE MILKY WAY ARE 15 BILLION YEARS OLD. An important
adjunct to the debate over the Hubble constant is the notion that the universe
cannot be younger than its oldest stars, which appear to be those in globular
clusters, spherical clumps of hundreds of thousands or millions of stars
found near and around our galaxy. Don VandenBerg of the University of Victoria
(davb@uvvm.uvic.ca, 614-721-7739) uses the Canada-France-Hawaii telescope
to view the ancient, metal-poor stars (they largely lack the elements heavier
than helium which many younger stars inherit from earlier supernova explosions
) in globular clusters. By plotting the stars' luminosities versus their
colors, and by employing the standard model for stellar evolution, the
age of the stars can be calculated. VandenBerg, speaking at last week's
meeting of the American Physical Society in Indianapolis, said the oldest
reliably dated stars, in globular cluster M92, were most likely 15 billion
years old. Uncertainties in the determination of the distances to the clusters
(affecting calculations of the stars' luminosities) might permit an age
of 13 or even 12 billion years. But VandenBerg asserted that the ages could
not be much younger than that. New observations of his in globular cluster
M13 did not alter this assessment.
RECORD HIGH LASER INTENSITY . The advent of tabletop terawatt lasers has
promoted the study of new nonlinear optical effects. Donald Umstadter of
the University of Michigan sends a powerful laser pulse into a sample of
argon gas. The leading edge of the pulse rips electrons from the argon
atoms. The rest of the pulse interacts with the ensuing plasma, setting
up a self-focusing process which results in a laser intensity as high as
10**20 W/cm**2. Furthermore, the laser beam clears a micron-sized path
for itself through the plasma. In the act of excluding plasma electrons
from this region, pressures probably exceeding 1 giga-bar (higher than
any other manmade pressure) are created. Umstadter, who spoke at the APS
meeting, said that the high laser intensities might open new possibilities
for the study of nonlinear optics. Another goal is the development of a
tabletop accelerator for generating electrons with GeV energies (see Update
260).
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