DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

nuclear time scale
outgoing particles are abbreviated:
p = proton
n = neutron
α = alpha particle
γ = photon (gamma ray)
¯νe = electron anti-neutrino
e − = electron
νe = electron neutrino
d = deuteron 2 H
e + = positron
Typical examples are discussed unders process,
pp chain, CNO cycle, and elsewhere.
Nuclear reactions will occur spontaneously if
the products have a total mass less than the sum
of the incoming particles (and if certain quantum
numbers have the right values). The energy corresponding
to the extra mass is liberated, and
nuclear reactions are thus the primary energy
source for all stars. The general format is target
nucleus (incoming particles, outgoing particle)
product nucleus, as, for instance, 13 C(p,γ) 14 N.
nuclear time scale The time scale on which
a star changes when its luminosity is derived
from nuclear reactions. In the case of hydrogen
burning, this is about 10 10 years for the sun,
and proportional to(M/Mo) −2 for stars of other
masses. Main sequence evolution occurs on the
nuclear time scale, as does the evolution of a
binary system in the Algol state, when the mass
donor is the less massive star.
nuclearwinter Astrongsurfacecoolingphenomenon
due to nuclear war. From numerical
simulation results, nuclear war can cause a serious
change of global weather and climate. Nuclear
explosions on the surface can carry a large
amount of dust into atmosphere, and nuclear explosions
in the air can leave heavy smoke and
fog in the atmosphere. They will strongly reduce
the short-wave solar radiation received by
Earth’s surface, cause the outgoing long-wave
radiation from the surface to be larger than the
reaching solar radiation to the surface. Thus, the
surface temperature will decrease rapidly below
the freezing point, and can even reach to −20 ◦
to −25 ◦ C.
nucleation Condensation or aggregation process
onto a “seed” particle. The seed may be of
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the same material as that condensing, or different,
but with a similar crystal structure. Rain
and snow condense onto airborne particles, in
one example of this process.
nucleus In nuclear physics, the central positively
charged massive component of an atom,
composed of a number of protons and neutrons
held together via the nuclear force, even in the
presence of the repulsive force associated with
the accumulated positive charge. In astronomy,
the actual physical body of a comet, a few kilometers
in size, composed of ices and silicates.
The nucleus is embedded in the head of the
comet, which may be hundreds of thousands of
kilometers across. Outgassing and loss of dust
from the cometary nucleus is the source of the
head and the tail(s) of the comment.
In astronomy, the roughly spherical central
region of many spiral galaxies. The popuation
of stars in the nucleus is much more evolved than
that of the spiral arms.
nucleus (of a comet) The solid “body” of
the comet. The nucleus contains water-ice, organic
and silicate dust grains, and other frozen
volatiles such as CO and CO2. These volatiles
may also be trapped in the water–ice matrix as
clathrates. The density of the nucleus of comet
Halley has been found to be roughly 0.5 g cm −3 .
This implies that the nucleus is very porous.
Comet nuclei range in size from several kilometers
to several hundred kilometers. They can
be highly non-spherical and can have low albedos,
indicating that the surface is covered by a
dust layer. Rotation rates of comet nuclei have
been estimated from light curves, and vary from
∼ 10 hours to days.
null infinity In general relativity, a domain
I of a (weakly) asymptotically simple spacetime
which is isometric with a neighborhood of
conformal infinity of Minkowski space-time. In
other words, it encodes the ultimate radiation behavior
of regions of matter and gravitation-free
spaces far from any source. See asymptotically
simple space-time, conformal infinity.
null vector In special relativity (and in general
relativity), the tangent to a light ray. Because
of the indefinite signature (− +++) in