RADIAL VELOCITY - Speed of an object in the direction towards or away from the observer. Small period changes in a stars radial velocity have been used to detect exoplanets. In an expanding universe a galaxy with a larger radial velocity generally lies further from the observer than one with a smaller radial velocity.
RADIAN - A radian is an angular measurement. The angular circumference of a circle is 2π radians, or 360°; thus, 1 radian = 360/2π degrees = 57.29578° (57° 17' 44.8").
RADIANT - Point on the celestial sphere from which meteors of a given shower appear to radiate. The effect is due to perspective.
RADIANT FLUX DENSITY - Amount of radiant flux (or power) crossing a unit of area, and is measured in W/m2; also known as intensity or irradiance.
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.
RADIATION PRESSURE - Minute pressure exerted by electromagnetic radiation on everything it encounters. This can be thought of as the transfer of momentum from photons as they strike the surface of the object. In the environs of stars such pressure can become important given the vast quantities of photons emitted. Under the essentially blackbody conditions that exist inside a star, radiation pressure is proportional to the fourth power of temperature via the equation:
where T = temperature, σ = Stefan-Boltzmann constant, and c = speed of light. Consequently, a small increase in temperature results in a large increase in the radiation pressure.
Most main sequence stars, which have internal temperatures of millions of degrees, are primarily supported against gravity by gas pressure although radiation pressure does contribute a few percent. The internal temperatures of massive stars are hundreds of times higher and, at these extreme conditions, radiation pressure begins to dominate. In the most massive stars, the mass of the star is supported against gravity primarily by radiation pressure, which ultimately sets the upper limit for how massive a star can become.
Other astronomical objects are also influenced by radiation pressure. The pressure from solar photons creates the dust tails of comets within our Solar System. Radiation pressure plays a vital role in the formation of planetary nebulae: as the dying star contracts into a white dwarf, it releases vast amounts of heat. This radiation pressure is so strong that the outer layers of the star are pushed out to form the surrounding gaseous nebula. Similarly, a giant star ejects material and gas into the interstellar medium through radiation pressure.
RADIATIVE ZONE - Portion of a star where the primary transport of energy is by photons (electromagnetic radiation).
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:
RADIONUCLIDE – Atomic nuclide that decays radioactively.
RADIUS RATIO – Ratio of the radius of the central ion in a coordination polyhedron to that of the ions surrounding it:
The radius ratio determines the coordination number (CN) as shown in the table. Note that, in this analysis, ions are considered spherical with shapes unaffected by neighboring ions.
RARE EARTH ELEMENTS – Elements with atomic numbers 57–71 (La, Ce, Pr, Nd, etc. to Lu); often abbreviated “REE.” Some reference sources do not include La in this series. For all of these elements the orbital being filled is 4f.
REACTION RATE – Amount (in moles or mass units) per unit time per unit volume that is formed or removed.
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.
REFLECTION NEBULA - Type of nebula created when light from a star is scattered or reflected off a nearby dust cloud. The scattered light is slightly polarized and has a spectrum similar to that of the illuminating star, only bluer. This shift in color arises because the sizes of the dust grains in the cloud are comparable to the wavelength of blue light and, thus, blue light is scattered more efficiently than longer, red wavelengths. Reflection nebulae are usually less dense than dark nebulae, and their sizes are defined by the area over which their brightness remains above the point of detection. The nebulosity surrounding the stars in the Pleiades is perhaps the most well known example of a reflection nebula (below).
REFRACTORY ELEMENTS - Any chemical element that would (or condense) from gas at high temperature (Tc = 1850-1400 K) in the solar nebula. The opposite of refractory is volatile. The lithophile refractory elements are: Al, Ca, Ti, Be, Sc, V, Sr, Y, Zr, Nb, Ba, REE, Hf, Ta, Th, and U. The siderophile and chalcophile refractory elements are: Re, Os, W, Mo, Ru, Rh, Ir, Pt, and Rh.
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.
REGIO - Large planetary region distinguished from nearby ones on brightness or color (pl. regiones).
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.
RESISTANCE (R) - Ratio between the voltage, V, applied to a device and the electric current, I, that flows through it:
Ohmic materials have resistance that is independent of voltage and current. Typically, the word "resistor" refers to an ohmic device in a circuit.
RESISTIVITY (ρ) - The 'part' of the resistance of an object independent of the geometry of the object. Resistivity depends on the type of material used and its temperature. For a homogeneous solid, resistivity, r, is related to resistance, R, in the following manner:
here A is the cross-sectional area of the solid, and, L, is its length, as shown to the right. Resistivity is the inverse of conductivity, σ:
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.
REVERSE BIAS - Voltage applied to a diode in a direction that does NOT produce significant electric current. When a diode is reverse-biased, the positive terminal of a battery is connected to the n-side of the diode, and the negative terminal to the p-side.
REVOLUTION - Orbital motion of a body around its primary.
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). At low Reynolds numbers (Re < 1100), fluid behavior depends mostly on its viscosity and the flow is laminar. At 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).
RIBONUCLEIC ACID (RNA) - Nucleic acid composed of a single polynucleotide strand. RNA nucleotides contain ribose rings and uracil unlike deoxyribonucleic acid (DNA), which contains deoxyribose and thymine. It is transcribed from DNA by enzymes called RNA polymerases and further processed by other enzymes. RNA serves as the template for translation of genes into proteins, transferring amino acids to the ribosome to form proteins, and also translating the transcript into proteins. The basic unit, the nucleotide, consists of a molecule of ribose sugar, a phosphate group, and one of four nitrogenous bases: adenine (A), guanine (G), uracil (U), and cytosine (C).
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 downwards into older rocks. Hadley Rille, visited by the Apollo 15 astronauts, is shown below.
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.
The Roche limit depends on the rigidity and density of the satellite. A rigid satellite will maintain its shape until tidal forces break it apart; in contrast, a fluid satellite gradually deforms leading to increased tidal forces, and eventually breaks apart. When calculating the “rigid body” Roche limit for a spherical satellite, the cause of the rigidity is neglected as are effects such as tidal deformation of the primary, rotation of the satellite, and its irregular shape. It is assumed that the body does not deform from its spherical shape while being held together only by its own self-gravity. Where, R is the primary's radius, ρM is the primary's density and ρm is the satellite's density, the rigid-body Roche limit, d, is:
Note that if the satellite is more than twice as dense as the primary (easily the case for a rocky moon orbiting a gas giant) then the Roche limit will be inside the primary and hence not relevant. For a fluid satellite, tidal forces cause the satellite to elongate, further compounding the tidal forces and causing it to break apart more readily:
The fluid case is closer to reality for most planetary bodies. Consider an icy body (ρ = 1000 kg/m3) orbiting Saturn (ρ = 687.3 kg/m3). The Roche limit is ~2.14R. All Saturn's major rings occur within this limit (assuming a reasonable error of ± 10%). The only exceptions are the E and G rings at ~R and ~2.8R, respectively. The E ring is concentrated at the orbit of Enceladus, and particles from cryovolcanism on this moon are probably the source of the E ring material. The other planetary rings in the solar system are also mainly within the Roche limits of their respective planets.
ROCHE LOBE - Volume of space within which any matter is controlled by the gravity field of a single object, named after the French mathematician Edouard Roche (1820–1883). The Roche lobe is said to be bound by an “equipotential surface” in which the gravitational force exerted by the star is equal at all points. For example, within the region surrounding a binary star system there are two competing gravity fields. The Roche lobes of each component, which in isolation would be spheroidal, are drawn out into cone-like extensions meeting at a point between the two stars known as the Lagrangian point, where the gravitational attractions of the two components are exactly equal. The position of this point along the line joining the centers of the two stars depends upon their relative masses.
Roche-lobe overflow occurs in a binary system when a star fills its Roche-lobe. Any material that passes beyond the Roche-lobe of the star will flow onto the binary companion, often by way of an accretion disk. This occurs through the inner Lagrangian point where the gravity of the two stars cancels, and is responsible for a number of astronomical phenomena including cataclysmic variables, Type Ia supernovae, and many X-ray binary systems.
ROTATION1 - Symmetry operation entailing rotations in 3-dimensional space. The object appears identical if rotated about an axis by a = 360/n = 2π/n degrees. The only allowed n-fold axes for crystal lattices are n = 1, 2, 3, 4, and 6, because lattices must be space filling.
ROTATION2 - Turning of an astronomical object on its axis.
ROTATION CURVE - Plot showing how orbital velocity (V) varies with distance from the centre of an object (R). Curves can be determined for any rotating object, and in astronomy are generally used to show how mass is distributed in the Solar System (Keplerian rotation curves) or in spiral galaxies.
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 b 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 b 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-instability 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, horizontal branch stars with periods ranging from a few hours to 2 days, and optical brightnesses that typically vary between 0.3 and 2 magnitudes. They lie in the instability strip of the Hertzsprung-Russell diagram and suffer instabilities that cause their size to periodically change. This change in size also changes the temperature of the star giving rise to their variability. RR Lyrae are low metallicity (population II) stars that begin their lives with a mass and size similar to that of our Sun. They become RR Lyrae stars during the red giant phase, late in the evolution of the star, and so have typical ages of around 10 billion years. For this reason, they are generally found in globular clusters, as well as the bulge and halo of the Milky Way. RR Lyrae stars exhibit a period-luminosity relation similar to that of Cepheid variable stars.
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.
RUPES - Term applied to scarps on planetary surfaces; many scarps are thought to be the surface expression of faults within the crust of the planetary object.