nuclear time scale

outgoing particles are abbreviated:

p	=	proton
n	=	neutron

α= alpha particle

γ= photon (gamma ray)

ν¯e	=	electron anti-neutrino
e−	=	electron
νe	=	electron	neutrino
					2
e+	=	positron
d	=	deuteron				H

Typical examples are discussed under s 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, 13C(p, γ )14N.

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 1010 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.

nuclear winter A strong surface cooling phenomenon 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

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 from10 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

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nutation

special relativity, such a vector has zero length even though its components are non-zero. In psuedo-Reimannian geometry, an element 2 of

a linear space with metric g such that g(2, 2) ≡

gab2a2b = 0.

numerical cosmology (computational cosmology) The technique, and a collection of models of the universe obtained by solving Einstein’s equations numerically. Because they proceed in terms of discretization, these models are approximate, but well-formulated models show a convergence as the discretization is refined. For reasonable resolution in 3-dimensional simulations, large computers are required.

numerical model A set of discretized mathematical equations, solved on a computer, which represent the behavior of a physical system.

numerical relativity The study and solutions of the differential equations of general relativity by discretization of the equations and the computational solution of the resulting finite algebraic problem. In many cases without overall symmetry, analytic methods fail and numerical relativity is the only method to elucidate and understand generic solutions for spacetimes in general relativity. Computational simulation has become a real research tool, for instance, the critical behavior and naked singularity found by Choptuik. Similarly, the naked singularity that may exist in some circumstances at the center of the Lemaître–Tolman cosmological model was first identified in numerical calculations. Numerical investigations have amassed considerable knowledge about problems too complicated for analytical treatment: collisions of rotating black holes, emission of gravitational waves by rotating nonsymmetric bodies or by binary systems, collapse of a binary system to a single body, etc. See binary black holes, critical

phenomena in gravitational collapse, gravitational wave.

Nusselt number A dimensionless number quantifying the efficiency of heat transfer in a convecting system. It is the ratio of total heat flux to the conductive heat flux. For example, the conductive heat flux across a uniform layer of thickness h with differential temperature T is λ T /h. If the total heat flux including convective heat transfer is q, the Nusselt number in this case is

Nu =	q
		.
	λ T /h

nutation This is a variation of the orientation (the obliquity) of the Earth’s axis of rotation associated with the Earth’s forced precession. In the theory of the rapid rotation of a rigid body, it is used to imply an analog of the free (i.e., unforced) precession of a body in the case where there is also a forced precession, which causes the body’s axis with the largest principal moment of inertia to “nod” backwards and forwards even as it is precessing. On Earth, this effect is known as the Chandler wobble, and the term nutation may also refer to motions due to the fact that the Earth is deformable and its layers imperfectly coupled, as well as motions caused by time variations in the torque acting on the Earth. This happens because the couple is produced by the tidal interactions with the moon and sun, which vary because the relative orientations of the Earth, moon and sun change with time in various different ways. There are nutations at various periods, the largest nutation being at 18.6 years. Nutation is one of the contributors to the Milankovich cycles which cause climatic variations. In the case of the Earth, nutation causes the obliquity to vary from 21◦ to 24◦ (the current value of Earth’s obliquity is 23.5◦) over a period of 41,000 years. See precession.

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oblique ionogram

O

obduction In tectonic activity, the process in which part of a subducted plate is pushed up onto the overriding plate. The process responsible for the emplacement of ophiolites.

Oberon Moon of Uranus, also designated UIV. It was discovered by Herschel in 1787. Its orbit has an eccentricity of 0.0008, an inclination of 0.10◦, a semimajor axis of 5.83×105 km, and a precession of 1.4◦ yr−1. Its radius is 762 km, its mass is 3.03 × 1021 kg, and its density is 1.54 g cm−3. Its geometric albedo is 0.24, and it orbits Uranus once every 13.46 Earth days.

objective grating A course grating, often consisting of a parallel array of wires, which is placed in front of the first element of a telescope (the objective) which produces a low dispersion spectrogram of each object in the field; used for rapid survey of the properties of sources in a relatively wide region of sky.

objective prism A prism of narrow apex angle (a few degrees) located at the objective of a telescope, most often of a Schmidt telescope. The objective prism, acting as a disperser, provides the spectrum of each object in the field of the telescope. A large number of low resolution spectra (up to 105) can be recorded on a plate or electronic detector. Surveys based on objective prism spectroscopy have been very efficient in finding objects with peculiar spectral energy distribution, such as galaxies with UV excess, or objects with very strong and broad emission lines, such as quasars. Spectral resolving power(λ/ λ) achieved in common usage are 100. Higher spectral resolving power can be achieved with wider apex angle prism.

oblateness For an oblate spheroid, the ratio of the difference between equatorial and polar radii to the equatorial radius. In general, the oblateness of a rotating inhomogeneous spheroid with north-south symmetry depends on

the rotation parameter m = 2R3/GM and the zonal harmonic coefficients Jl(l = 2, 4, 6, ...) in a complicated manner. is the rotational angular speed, R is the equatorial radius, G is the gravitational constant, M is the total mass of the spheroid, and Jl = −(1/M) dr(r/R)lρ(r, θ) Pl(cos θ) where the mass density of the spheroid ρ depends only on the polar radius r and the polar angle θ measured from the north pole, and Pl is the l-th order Legendre polynomial. In planetary physics, analyses of velocity variations during the spacecraft fly-bys at major gaseous giant planets have permitted us to determine their first few zonal harmonic coefficients to a relatively high accuracy, which has in turn provided us with their dynamical oblatenesses. To the first order in J2 (quadrupole moment, i.e., anisotropy in moments of inertia), there is a simple relation oblateness = (3J2 + m)/2, which shows that the oblateness is a critical indicator of how fast a body is rotating. For a model planet consisting of a small dense core of mass Mc clad with a homogeneous spheroidal envelope of mass Me = M − Mc and equatorial radius R, J2/m = (1 − δc)/(2 + 3δc) and hence we find /m = 5/(4 + 6δc) where δc = Mc/M is the fraction of core mass. Thus, with the total mass M, the equatorial radius R, and the rotation rate all being equal, a homogeneous spheroid without a core is more oblate than an inhomogeneous spheroid containing a core.

oblique ionogram The conventional display obtained from an oblique ionosonde, containing information about the ionosphere. The synchronized transmitter and receiver for an oblique ionosonde are separated each at the terminal of an HF link. An oblique ionogram is constructed by displaying the received signal as a function of frequency (on the horizontal axis) and time delay (on the vertical axis). The structure of the oblique ionogram is highly dependent on the path length between the receiver and transmitter. For short paths, less than 100 km, the oblique ionogram will look similar to a vertical ionogram. However, as the path length increases, the F modes will gradually become more obvious. For frequencies below the maximum usable frequency (MUF), for a single mode, the shape of an oblique ionogram shows that two paths are possible. These two paths are for a high and

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oblique-slip fault

low elevation angle. The high elevation path is called the Pederson ray, or the high ray. The high ray penetrates deeper into the ionosphere and suffers greater retardation than for the low ray, which is therefore the preferred propagation path because time dispersion is lower. However, for very long paths the low ray elevation angle may be low enough for it to be obscured, leaving only the high ray to use. At the MUF, the high and low rays are coincident. Fewer characteristic parameters are measured from oblique ionograms than from vertical ionograms. The most important is the maximum operating frequency (MOF) for the link although ideally, an MOF should be measured for each identifiable mode. On longer paths, the MOF will usually be an F mode. Oblique ionograms contain much useful information about a single HF link and this information may contribute to effective link management. See ionospheric radio propagation path.

oblique-slip fault A fault that combines the motions of a dip-slip fault (such as a normal or reverse fault) and a strike-slip fault. Motion thus occurs in both vertical and horizontal directions along an oblique-slip fault.

obliquity The measure of the tilt of a planet’s rotation axis. It is measured from the perpendicular of the planet’s orbital plane. Obliquity can change due to the gravitational effects of other bodies (nutation), which in turn can affect the body’s climate (see Milankovitch Cycles). Planetary obliquities range from 0◦ (for Mercury) to 177◦ (for Venus). The term inclination is sometimes used as a synonym.

obliquity factor See basic MUF.

Occam’s razor A maxim attributed to William of Occam, a 14th century English logician, which, literally translated, states that “Plurality should not be posited without necessity”. However, in modern science this maxim has been strengthened to a form that states that for any two competing interpretations (or theories) the one with the least assumptions is the better one. This corrupted form has been, more appropriately, called a “law of parsimony” or “rule of simplicity”.

occluded front A complex weather frontal system occurring when a cold front overtakes a warm front. Because of the greater density of cold air, the warm air is lifted. Such a situation is likely to produce rainfall as water vapor suspended in the warm air condenses out at the interface between the two. Also known as an occlusion.

occultation The temporary diminution of the light from a celestial body O1 (e.g., a star) caused by its passage behind another object O2 (e.g., the moon) located between the observer and O1. This is called the occultation of O1 by O2. On August 26, 1981 during the encounter of Voyager 2 with Saturn, its ultraviolet spectrometer and photopolarimeter recorded the brightness variation of the star δ Scorpii (O1), at 1300 Å and 2640 Å, respectively, as it was occulted by the C, B, and then A rings (O2) located within Saturn’s shadow. As shown in this example, the observer or receiver is not necessarily Earth bound. Occultation is one of the most powerful techniques to probe the structure and composition of planetary rings and atmospheres. Stellar occultation by the moon is useful in determining the lunar position with high precision. Occultations of radio sources of small angular sizes lead us to determine their precise positions and spatial distributions of radio intensity. In 1977, Uranian rings were serendipitously discovered during measurements of stellar occultation by the central planet, which spurred the subsequent discovery of Neptunian arc rings by similar but ground-based stellar occultations in 1984. Stellar occultations by asteroids have also helped determine the asteroids’ sizes and shapes.

ocean The great interconnected mass of salt water (average salinity about 3.5% by weight), covering about 71% of the surface of the Earth, or any of the five main subdivisions of this great ocean: Pacific, Atlantic, Indian, and Arctic; the southern portions of the first three, which converge around Antarctica, are known collectively as the Antarctic Ocean. Area of about 361,000,000 km2, average depth about 3,730 m, and a total volume of about 1,347,000,000 km3. Current theory holds that ocean water originated through release at seafloor spreading sites.

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