DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

nebular hypothesis
nebular hypothesis The hypothesis that the
solar system, and typical systems around other
stars, formed by condensation of material in a
molecular cloud in which the star first formed.
Because there will be some initial angular momentum
in such a cloud, cooling will lead to
the formation of a disk. Planetary formation
in the environment of a flattened disk gives an
explanation for the general uniformity of rotational
behavior among the planets, and for the
fact that the planetary orbits all lie close to the
same plane, the ecliptic. The composition of
the planets, with rocky planets near the sun, and
gas giants at larger distances can be explained
by early strong radiation from the forming sun.
However, several extrasolar planetary systems
are now known, where this arrangement of planetary
bodies does not seem to hold.
nebular lines Forbidden emission lines observedinopticalspectraofgaseousnebulae,typically
planetary nebulae, HI regions, and external
galaxies. The strongest nebular lines are
observed at 495.9, and 500.7 nm, at 654.8 and
658.4 nm, and at 372.7 nm. Nebular lines are
forbidden lines, not observed in laboratory spectra.
Nebular lines were shown (Bowen, 1928) to
be due to forbidden transitions between the lowest
terms of singly and doubly ionized abundant
atomic species, such as oxygen and nitrogen.
See forbidden lines.
needle-probe method A method for measuring
thermal diffusivities of unconsolidated
sediments or other soft materials. A needlelike
probe containing a calibrated electric heat
source and temperature sensors is inserted into
the specimen. The diffusivity of the material is
determined by analyzing the rate of the temperature
rise after the heat source is turned on but
within the time window when the line source of
heat can be assumed to be infinitely long and the
medium to be infinitely large.
neon (From the Greek neos, new.) Noble
gas, Ne, discovered in 1898 by Ramsay and
Travers. Atomic number 10, natural atomic
weight 20.179. Colorless, odorless, inert gas.
Present in the atmosphere as 15 ppm. Melting
point 24.56 K, boiling point 27.07 K.
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The dominant natural isotope is 20 Ne, which
constitutes more than 90% of the abundance.
The other two stable isotopes are 21 Ne (0.25%)
and 22 Ne (9.25%).
neon burning The set of nuclear reactions
that convert neon to heavier elements, including
magnesium and silicon. It occurs in hydrostatic
equilibrium in the evolution of stars of more than
about 10 solar masses (between carbon burning
and oxygen burning). It is not a major energy
source even for them because neon is never very
abundant.
Neptune Giant gas planet, eighth planet from
the sun. (Occasionally, as at present, Pluto is
actually inside the orbit of Neptune.) Outermost
gas planet. Mass 1.02 × 10 26 kg, diameter
49528 km. Average distance from sun
30.06 AU, orbital eccentricity 0.009. Rotation
period 19.1 hours, orbital revolution period
164.8 years. Inclination to orbit 29.6 ◦ . The visible
surface has a composition closely matching
primordial amounts: 74% hydrogen, 25% Helium.
Its surface mean temperature is 48 K. Its
surface albedo is of order 0.5.
Nereid Moon of Neptune, also designated
NII. It was discovered by G. Kuiper in 1949. Its
orbit has an eccentricity of 0.751, an inclination
of 27.6 ◦ , a precession of 0.39 ◦ yr −1 , and a semimajor
axis of 5.51 × 10 6 km. The orbit, which
has the highest eccentricity of any satellite in the
solar system, indicates that Nereid is a captured
Kuiper Belt object. Its radius is 170 km, and its
mass is not known, but has been estimated to be
about 2 × 10 19 g based on an assumed density
of1gcm −3 . It has a geometric albedo of 0.155,
and orbits Neptune once every 360.1 Earth days.
net photosynthetic rate The total rate of
photosynthetic CO2 fixation minus the rate of
loss of CO2 in respiration.
network In solar physics, general name given
to the distribution of photospheric magnetic field
outside of sunspots and faculae. The network
pattern consists of magnetic fields with characteristic
cell dimensions of 20,000 to 40,000 km,
covering the quiet photosphere. There is a cor-