RADIOACTIVE DECAY - Process in which an element's nucleus changes ("decays") to produce a new element. The original atom is called the "parent" and the resulting atom, the "daughter." There are three modes of radioactive decay: 1) emission of an a particle (He nucleus), which decrease the atomic number (Z) by 2 and the atomic weight (A) by 4 mass units; 2) emission of a b particle (electron), which increases Z by 1 and does not change A; and 3) K-electron capture, which decreases Z by 1 and does not change A. The later two processes entail the reaction:
RADIATION - Process of emission of energy or particles. Various forms of radiation may be distinguished, depending on the type of the emitted energy or matter, the type of the emission source, properties and purposes of the emission, etc. For example, electromagnetic radiation is a stream of photons.
RADIATION DARKENING - Effect of chemical reactions that result when high-energy particles strike the icy surfaces of objects in the outer solar system. The reactions lead to a build-up of a dark layer of material.
RADIATIVE ZONE - Portion of a star where the primary transport of energy is by photons (electromagnetic radiation).
RADIONUCLIDE - Atomic nuclide that decays radioactively.
RED DWARF - Small, dim, low-mass main sequence star. Red dwarf stars are hard to detect because they are so dim. In principle, they could constitute a major mass constituent of the universe, if their production is heavily favored in the star formation process. In that case they could constitute a significant source of dark matter. They could also be an example of a MACHO.
RED GIANT - Giant star in the later stages of stellar evolution after it has left the main sequence. These stars are found on the upper-right hand side of the Hertzsprung-Russell diagram (high luminosity, temperature ~2000-3000 K, diameter 10-100 Rsun). The Sun will become a red giant in ~5 billion years.
REDSHIFT - Shift in the frequency of a photon toward lower energy, or longer wavelength. The redshift is defined as:
Note that positive values of z correspond to increased wavelengths (redshifts). Different types of redshifts have different causes: Doppler shift, gravitational redshift, and cosmological redshift.
REE - Rare-earth elements; those with atomic numbers 57 to 71; La, Ce, Pr, Nd, etc. to Lu.
REFRACTORY ELEMENTS - Any chemical element that would be vaporized at high temperatures (or would condense from gas at high temperatures) in the solar nebula; e.g., Al, Ca, Ti, REE, W and U. The opposite of refractory is volatile.
REFRACTORY INCLUSION - Inclusions made of the minerals spinel and melilite and hibonite, which are rich in the refractory elements Ca-, Al-, and Ti. These inclusions are often referred to as Ca-, Al-rich inclusions, or "CAIs." Amoeboid olivine aggregates are also considered refractory inclusions.
REGMAGLYPT - Depressions resembling (and often called) thumbprints produced on the surface of some meteorites during atmospheric transit by ablation.
REGOLITH - Mixture of unconsolidated rocky fragments covering the surface of an asteroid or planet, the product of "gardening" by repeated meteorite impacts.
RELICT GRAIN - Crystal in a chondrule that survived the melting event that formed the chondrule; i.e., it did not crystallize in situ.
REYNOLDS NUMBER (Re) - Dimensionless ratio of inertial resistance to viscous resistance for a flowing fluid.
where, ρ = density, v = velocity, l = thickness of the layer, and η = viscosity. The number is named after the British physicist and engineer Osborne Reynolds (1842-1912). For low Reynolds numbers (Re < 1100), fluid behavior depends mostly on its viscosity and the flow is laminar. For high Reynolds numbers (Re > 2100), the momentum of the fluid determines its behavior more than the viscosity and the flow is turbulent. For intermediate Reynolds numbers, the nature of the flow is transitional (partly laminar and partly turbulent).
REST ENERGY - Energy corresponding to the rest mass according to E = moc2.
REST MASS - Mass of an object measured in its own rest frame. An important invariant quantity.
RILLE - Long narrow depression on the surface of the Moon; also called "sinuous rilles". Lunar rilles usually flow away from small pit structures and probably mark lava channels or collapsed lava tubes that formed during mare volcanism. In some cases, the lunar flows may have melted their way down into older rocks.
RINGWOODITE - High pressure polymorph of olivine with a γ-spinel structure. Found in highly shocked meteorites (above ~50 GPa) and Earth's mantle (~13 GPa); the difference in pressure is a result of kinetic effects. Under high pressure in the mantle (~24 GPa), ringwoodite decomposes into perovskite, (MgVI,FeVI)SiVIO3, and magnesiowüstite, (Mg,Fe)O, whose properties are completely different. This transformation explains the observed discontinuity between Earth's upper and lower mantle. At lower pressure, ringwoodite transforms into wadsleyite, another olivine polymorph.
ROCHE LIMIT - Often called the tidal stability limit, the Roche limit gives the distance from a planet at which the tidal force, due to the planet, between adjacent objects exceeds their mutual attraction. Objects within this limit are unlikely to accumulate into larger objects. The rings of Saturn occupy the region within Saturn's Roche limit.
RP-PROCESS - Rapid proton capture (hence "rp") process is very similar to the r-process, except it goes by successive proton absorption and β+ decay; thus, it tracks somewhere between the valley of stability and the "proton drip line".
R-PROCESS - Rapid (hence "r") absorption of neutrons by atomic during a supernova explosion when the neutron flux is very high (~1022 neutrons per cm2 per second). In the r-process, neutron capture is very rapid, with the time between captures much shorter than the average β decay half-live (on the order of seconds). Capture moves the nucleus toward "neutron drip line" where the probability for absorbing a new neutron is overwhelmed by the probability that a neutron will be knocked off by photodisintegration. This balance point defines the (n, γ) <--> (γ, n) equilibrium. The path of nucleosynthesis moves up along a line somewhere between the valley of stability and the neutron drip line (the offset depending on conditions such as temperature, neutron flux, and photon flux) until finally fission blocks the chain in the actinide region. Nuclei with "magic" neutron numbers serve as bottlenecks to nuclei climbing the r-process path.
For example, 130Cd is an isotope with the A = 82 "magic" number, but the heaviest stable isotope of cadmium is 116Cd with 14 fewer neutrons. If the neutron source only lasts for a short time, highly unstable nuclei will be left on the r-process path, with many stuck at the "magic" bottlenecks. These undergo β decay back to the line of stability. In our example, 130Cd would eventually decay to 130Te, the most abundant isotope of tellurium. Since β decay reduces the number of neutrons, abundance peaks show up at lower neutron number than the s-process peaks. In some cases, the r-process may be fast enough to break through the region of α-instabilit beyond 208Pb. The stable actinides may be produced directly from a neutron-rich precursor, or from α-decay of even heavier elements.
RR LYRAE STAR - Variable star whose luminosity changes in a characteristic way. All RR Lyrae stars have more or less the same period.
RUMURUTIITE - Member of a rare group of chondrites, formerly named the Carlisle Lakes group, after a meteorite found in Australia in 1977. It is now named for the type specimen Rumuruti that fell in Kenya, Africa, in 1934. Rumuruti is the only witnessed fall of this group and only one small individual has been preserved in the collection of the Humboldt Museum Berlin, Germany, since 1938. Rumuruti was considered an anomalous chondrite until it was reclassified in 1993 and the R group was formed. The R chondrites are quite different from ordinary chondrites and are opposite the E chondrites when it comes to mineralogy and oxidation state. R chondrites are highly oxidized, containing high amounts of Fe-rich olivine. They contain almost no free metal (most of the Fe is either oxidized in silicates) or in the form of Fe sulfides. The Fe-rich olivine and oxidized nature of the Fe, give most R chondrites a typical red appearance. The meteorites of this group contain fewer chondrules than do ordinary or E chondrites, but they often contain xenoliths that are samples of asteroid regolith. Another indicator for a regolith origin is that most R group meteorites contain high concentrations of noble gases implanted by the solar wind. The parent body of the R chondrites has yet to be found, but must have undergone many impact events during its history to yield the high degree of brecciation shown by most R group members.
