Number 147, October 13, 1993 by Phillip F. Schewe and Ben Stein
THE PHYSICS NOBEL PRIZE GOES TO JOSEPH TAYLOR AND RUSSELL HULSE , both
of Princeton University, for their discovery of the first binary pulsar
and for subsequent studies leading to a verification of the theory of general
relativity for a system outside our solar system. Using the 300-m Arecibo
radio telescope in Puerto Rico, Taylor and Hulse in 1974 monitored the
beacon-like emissions of the pulsar PSR 1913+16 and inferred that the pulsar---believed
to be a rapidly spinning neutron star---was accompanied by a nearby comparably-massive
(1.5 solar masses) and compact (20-km diameter) companion object. A great
deal has been learned from the pulsar's radio bursts, which arrive at Earth
about 17 times a second with a regularity that rivals that of the best
atomic clocks. For example, a Doppler effect evident in the pulse sequence
provides the information needed to work out the orbit parameters for the
system. Furthermore, by recording the pulses over a multi-year period,
general-relativistic properties of the binary system could be extracted.
In particular, a very slight inward spiralling of the two partners causes
their mutual orbit to speed up and close in. According to Taylor, this
phenomenon, which shows up as a decrease in the orbital period of about
75 msec per year, comes about because the system is losing energy (about
10**32 ergs/sec) via the emission of gravitational waves. (Equivalently,
the advance of the system's periastron is 4.2 degrees per year; by contrast
the advance of the periastron for the planet Mercury is only 43 arc-sec
per century.) Because the observed decrease in the period so closely matches
the value predicted by Einstein's theory of general relativity, many astronomers
regard these observations as being important (albeit indirect) evidence
for the existence of gravitational waves. (Taylor, 609-452-4368; Hulse,
609-243-2418. See the Oct. 1981 issue of Scientific American.)
AN ACTIVE GALAXY'S CENTER AND ITS NUCLEUS do not always coincide. E.
Mediavilla and S. Arribas, astronomers working in the Canary Islands, report
that two-dimensional spectra of the Seyfert galaxy NGC3227 reveal the galaxy's
nucleus---identified in the spectral map as being that area with intense
broad emission lines, indicative of high-velocity gas moving under the
presumed influence of a supermassive black hole---is offset from the galactic
center by about 750 light years. The astronomers offer several explanations:
the displacement may have resulted from the merger of two galaxies or the
black hole, if one is there at all, may be orbiting some concentration
of dark matter. (Nature, 30 Sept. 1993.)
PROTON HALOES have been observed around several nuclei. Analogous to
the neutron haloes around nuclei such as lithium-11 (Update 115), a proton
halo consists of weakly attached protons lying outside a more densely-grouped
core nucleus. Scientists at the University of Osaka and the University
of Tokyo found boron-8 to be such a nucleus; measurements of the nuclear
quadrupole moment suggested that a two-proton halo lay at an average radius
of 2.9 fm outside a core nucleus (consisting of three protons and three
neutrons) with a radius of 2.2 fm. Scientists using the ISOLDE facility
at CERN have studied the properties of proton haloes in neon-17. In a related
development, a Russian-Korean-Japanese team at the RIKEN lab in Japan has
observed evidence for the existence of helium-10 which, with two protons
and eight neutrons, should have a prominent neutron halo. (New Scientist,
2 Oct.)
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