H GROUP - Ordinary chondrites with a high content of free Ni-Fe metal (15-19 vol. %) and attracted easily to a magnet. Their main minerals are olivine (Fa16-20) and the orthopyroxene bronzite (Fs14.5-18.5), earning them their older name of bronzite chondrites. Chondrules average ~0.3 mm in diameter. Comparison of the reflectance spectra of the H chondrites to the spectra of several main belt asteroids has yielded a probable parent body in the asteroid 6 Hebe. However, Hebe might not be the direct source of the H chondrites; most likely, Hebe collided with another asteroid at some point in its history and larger parts of it were dislodged into an elliptical near-earth orbit, from which fragments eventually came to Earth.
HABITABLE ZONE - Zone around a star in which water is in the liquid form (273-373 K). This zone will be farther from hotter stars. Whether a planets is in the habitable zone, in part, depends upon planetary albedo and atmospheric greenhouse effects.
HADRON - Subatomic particles that are composed of quarks and which are acted on by the strong nuclear force. Hadrons may be subdivided into mesons and baryons. Mesons consist of quark-antiquark pairs, whereas, baryon are made of three quarks. Most hadrons, with the exception of protons and neutrons (which are baryons), have very short lifetimes. Only a miniscule part of the mass of a hadron is derived from its quarks, most mass comes from a hadron's kinetic and potential energy. These energies are converted into the mass as described by Einstein's equation that relates energy and mass, E = mc2.
HALF-LIFE - Length of time required for half of the atoms of a radioactive nuclide in a sample to decay. After two half-lives, ¼ (= ½ x ½) of the original radioactive nuclide will remain, etc.
HALOGENS - Reactive nonmetals that are in Group 17 (VIIA) of the periodic table: F, Cl, Br, I and At. All of these elements are strongly electronegative.
HAWKING RADIATION - Theory that black holes emit radiation like any other hot body proposed by Stephen W. Hawking in 1973. Virtual particle-antiparticle pairs are constantly being created in supposedly empty space and occasionally, a pair will be created just outside the event horizon of a black hole. At this juncture there are three possibilities: (1) both particles may be captured by the black hole; (2) both particles may escape the black hole; and (3) one particle may escape while the other is captured. In the first two cases, the virtual particle-antiparticle pair recombine and vanish. In the third case, the escaped particle becomes a real particle and can now be detected by distant observers. However, the captured particle is still virtual and has to restore conservation of energy by assigning itself a negative mass-energy. Since the hole has absorbed it, the black hole loses mass and thus appears to shrink. From a distance, it appears as if the hole has emitted a particle and reduced in mass.
The rate of power emission is proportional to the inverse square of the black hole's mass; thus, the smaller a hole gets, the faster and faster it emits Hawking radiation – a runaway process. What happens when the hole gets very small is unclear.
HAXONITE - Fe-Ni carbide, (Fe,Ni)23C6, found in iron meteorites.
HEAT - Form of energy related to random motions of particles (atoms, molecules, etc.), making up an object.
HEAT CAPACITY - Ability of matter to store heat. The heat capacity is the quantity of heat (J) required to increase a material’s temperature by 1 K. The SI unit for heat capacity is J/K. The heat capacity of a material differs at constant pressure, Cp, and constant volume, Cv.
HEAT CONDUCTION - Flow of internal energy (heat) from a region of higher temperature to one of lower temperature by the interaction of the adjacent particles (atoms, molecules, ions, electrons, etc.) in the intervening space. The factors that affect the rate of heat transfer by conduction are the temperature difference (ΔT), length (l), cross-sectional area (A), and thermal conductivity of the material (k):
Conductivities are usually given in units of W/m·K. They are highest in metallic solids, lower in nonmetallic solids, very low in liquids, and extremely low in gases. The best ordinary metallic conductors are silver (429 W/m·K), copper (401), gold (317), aluminum (237), and tungsten (174). Among nonmetallic solids, diamond is best (895); most silicate minerals have low values (forsterite, 4.6; almandine, 3.6; diopside, 4.2; hornblende, 2.0; feldspar, 2.3–2.7; calcite, ~3). Some minerals show strong dependence of conductivity with crystal axes. For example, the conductivity of quartz is 6.2 ⊥c and 10.2 ||c at 30 °C. The material with the greatest thermal conductivity (105) is the super fluid helium II, which only exists at temperatures below 2.17 K.
HEAT OF CONDENSATION - Equivalent to heat of vaporization, and latent heat of a substance. It is the amount of heat that must be removed from one gram of a vapor at its condensation point to condense the vapor with no change in temperature.
HEAT OF CRYSTALLIZATION - Equivalent to heat of melting (fusion) and latent heat of a substance. It is the amount of heat that must be removed from one gram of a liquid at its freezing point to freeze it with no change in temperature.
HEAT OF REACTION - Enthalpy which is given off or absorbed during a reaction. If the pressure of the reaction remains constant, you can think of enthalpy as simply being heat.
HEAT OF VAPORIZATION - Heat required to vaporize one mole of a substance at its boiling point under standard pressure (101325 Pa). The heat of vaporization is expressed in kJ/mol.
HED GROUP - Group of meteorites comprising three closely related classes of achondrites (howardites, eucrites, and diogenites) whose members all resemble terrestrial igneous rocks. HED meteorites are thought to have from the same parent body, the third-largest asteroid, 4 Vesta. Close similarities in oxygen isotopes and other chemical similarities point to a common heritage. Crystallization ages of 4.43–4.55 Ga are consistent with a large and differentiated parent body that became geologically inactive after a brief but intense igneous history. 4 Vesta fits this bill and has a reflectance spectrum very similar to HED group members.
HEISENBERG UNCERTAINTY PRINCIPLE - Conclusion that it is impossible to know simultaneously the absolute exact position and the absolute exact speed of a particle such as an electron. The more precisely the position is known of a particle at a moment in time, the less precisely can its speed be determined. Conversely the more precisely the exact speed of a particle is known, the less precisely its position can be known. Werner P. Heisenberg, a German physicist, developed this principle from his work with quantum mechanics.
HELIOSPHERE - Region of space containing plasma and magnetic fields of solar origin; a cavity carved in the interstellar medium by the flow of the solar wind.
HELIUM (He) - Second lightest and second most abundant element (after H) in the universe. The most abundant isotope is 4He (99.9998%), 3He is very rare. Helium comprises ~8% of the atoms (25% of the mass) of all directly observed matter in the universe. Helium is produced by hydrogen burning inside stars, but this process cannot, by itself, account for the amount of helium actually observed. Production of He in the Big Bang (when temperatures were ~109 K) can account for the deficit, an observation strongly favoring the Big Bang theory.
HELIUM BURNING - When the temperature in the core of a star reaches ~107 K, three colliding helium nuclei can fuse to form a carbon nucleus - the triple alpha process. This is the main source of energy production in red giants and red supergiants.
HELIUM CAPTURE - Formation of heavy elements by the capture of helium nuclei. For example, at ~6 x 108 K, carbon can form heavier elements by fusion with other carbon nuclei:
However, fusion with is more likely to occur by helium capture, which requires less energy and lower temperatures (~2 x 108 K):
Similarly, 16O may fuse with other 16O nuclei at ~109 K to form 32S, but it is much more probable that 16O will capture a 4He nucleus (if one is available) to form 20Ne. Again, 4He capture is more likely because it requires a lower temperature. Elements with nuclear masses of 4, 12, 16, 20, 24, and 28 have much higher abundances than other isotopes.
HELIUM PRECIPITATION - Mechanism responsible for the low abundance of helium of Saturn's atmosphere. Helium condenses in the upper layers to form a mist, which "rains" down toward Saturn's interior, just as water vapor forms into rain in the atmosphere of Earth. Also called "helium rain."
HEMATITE - Fe-oxide mineral (Fe2O3) that may be the major cause of the red color on Mars. Courser-grained gray hematite has the same chemical formula as the red variety, but a different crystalline structure. Deposits of gray hematite found in the Terra Meridiani region of Mars may suggest that water once circulated through the region's rock layers.
HENRY’S LAW - Law formulated by the English chemist J. William Henry in 1800, stating that the concentration of a dissolved gas is directly proportional to the partial pressure of the gas above the solution:
where, p = partial pressure of the gas, c = its molar concentration, and K = constant. Henry’s Law also applies to solid-melt equilibrium for trace elements, with the constant of proportionality being called the partition coefficient.
HERBIG-HARO OBJECT - Small bright nebula in a star-forming region, named after the American astronomer George Herbig and Guillermo Haro who discovered the first three such objects in 1946-1947 in images of the nebula NGC 1999 in Orion. All Herbig-Haro objects have been found within the boundaries of dark clouds and are strong sources of infrared radiation. HH objects are created when fast-moving jets of material from a newborn star collide with the interstellar medium. As the bipolar flow from a young star plows into the surrounding gas, it generates strong shock waves that heat and ionize the gas. In the cooling gas behind the shock front, electrons and ions recombine to give an emission spectrum characteristic of Herbig-Haro objects.
HERCYNITE - Fe-Mg spinel, (Fe,Mg)Al2O4, found in many meteorite classes.
HERTZSPRUNG-RUSSELL (HR) DIAGRAM - Plot of luminosity against temperature (or spectral class) for a group of stars.
HETEROGENEOUS EQUILIBRIA - Equilibria involving species in more than one phase. Compare homogeneous equilibria.
HEXAGONAL CLOSEST PACKING (HCP) - Way in which atoms (considered as hard spheres) pack together to fill space. In cubic closest-packing, there are three alternating hexagonal layers, a, b, and c, offset from one another so that the spheres in one layer sit in the small triangular depressions of neighboring layers. This system of packing is subtly different than cubic closest packing. Each sphere is touched by 12 neighbors, 6 in the same layer, 3 in the layer above, and 3 in the layer below.
HEXAHEDRITE - One of the main types of iron meteorites composed almost entirely of kamacite and named for its cubic (hexahedral) cleavage of α-Fe-Ni crystals. Upon etching, hexahedrites do not display a Widmanstätten pattern, but do often exhibit fine, parallel lines called Neumann lines for their discoverer, Franz Neumann, who first studied them in 1848. These lines represent shock-induced, structural deformation of the kamacite plates, and evince an impact history for the hexahedrite parent body, at least for the hexahedrites related to chemical group IAB.
HIBONITE - Ca-aluminate, CaAl12O19, that occurs in terrestrial metamorphic rocks and in CAIs of many chondrites.
HIGH-PRESSURE MINERAL PHASES - Mineral forms stable only at the extremely high pressures typical of Earth's deep interior, but not its surface. Such pressures also are generated instantaneously during meteorite impact. For example, coesite and stishovite are high-pressure polymorphs of SiO2 (silica) and diamond is a high-pressure modification of graphite (carbon).
The minerals in mantle rocks undergo phase changes with depth, transforming into denser forms. Particularly notable are the transformations that occur near 400 and 700 km depth. The former corresponds to pyroxene transforming into a garnet structure.
HII REGION - Volume of space where H in the interstellar medium is ionized rather than in a neutral state. These are regions where hot, blue OB stars are pouring large amounts of ultraviolet radiation into the surrounding cloud from which they recently formed. OB stars can ionize all H (and other atoms) for dozens or even hundreds of light-years in every direction, producing a Stromgren sphere.
Ultraviolet light strips electrons from H in a process known as photoionization. Electrons recombine with protons (H nuclei) and emit a characteristic Lyman series of emission lines as they cascade through the energy levels of the atom. The visible radiation in these lines imparts a beautiful colored glow to HII regions. HII regions have typical kinetic temperatures of 10,000 to 20,000 K, and densities ~10 atoms/cm3. The most famous HII region is the Orion Nebula (Messier 42); photograph shows the Strömgren sphere of the Rosette Nebula.
HILL SPHERE - Spherical volume that approximates the gravitational of influence of one astronomical body in the face of perturbations from another heavier body around which it orbits. It was defined by the American astronomer, George William Hill. It is also called the Roche sphere because the French astronomer Édouard Roche independently described it. If the mass of the smaller body is m, and it orbits a heavier body of mass M at a distance a, the radius r of the Hill sphere of the smaller body is:
For example, the Earth (5.97 × 1024 kg) orbits the Sun (1.99 × 1030 kg) at 149.6 × 109 m. The Hill sphere for Earth thus extends out to about 1.5 × 109 m (0.01 AU). The Moon's orbit, at a distance of 0.370 × 109 m from Earth, is well within the gravitational sphere of influence of Earth and is, therefore, not at risk of being pulled into an independent orbit around the Sun. The planet with the largest Hill sphere is Neptune, with r = 116 × 109 m (0.775 AU). Its great distance from the Sun compensates for its small mass relative to Jupiter (r = 53 × 109 m). Hill spheres for Main Belt asteroids can reach 220 × 106 m (1 Ceres), and diminish rapidly with its mass.
The Hill sphere is an approximation, and other forces (e.g., radiation pressure) can make an object deviate from within the sphere. Orbits at or just within the Hill sphere are not stable in the long-term; results of numerical simulations indicate that stable satellite orbits must be < 0.33–0.5 of the Hill radius (with retrograde orbits more stable than prograde orbits).
HOLE - Essentially the absence of an electron in an otherwise filled energy band, a hole can be treated as a positively-charged particle moving through the valence band of a solid. Its effective mass can be experimentally determined. A hole in solid silicon has an effective mass of about 81% of the rest mass of the electron, or 7.38 x 10-31 kg.
HOMOGENEOUS EQUILIBRIA - Equilibria involving only one species in a single phase. For example, all gases, all liquids or all solids.
HOOKE’S LAW - Law applicable to the situation in materials in which the stress, ε, applied to any solid is proportional to the strain, σ, it produces within the elastic limit for that solid. The constant of that proportionality is the Young modulus of elasticity, E, for that substance:
HORIZONTAL BRANCH - Region of the Hertzsprung-Russell diagram where post-main sequence stars again reach hydrostatic equilibrium. At this point, the star is burning helium in its core, and hydrogen in a shell surrounding the core.
HOT DARK MATTER - Particle whose energy is high at the time it decouples from other matter early in the Big Bang. Such a particle could be responsible for the origin of large-scale structure through a top-down structure formation process.
HOWARDITE (HOW) - One type of meteorite in the HED achondrite group. Howardites are named after the English chemist Edward Howard (1774-1816), one of the pioneers of meteoritics. Consisting mostly of eucritic and diogenitic clasts and fragments, howardites are polymict breccias. However, they also contain dark clasts of carbonaceous chondritic matter, other xenolithic inclusions, and impact melt clasts, indicating a regolith origin. Howardites probably represent regolith breccia from the surface of 4 Vesta, consisting of eucritic and diogenitic debris excavated by the large impact that created the enormous crater near 4 Vesta's south pole. These fragments mixed with parts of the chondritic impactor, and the mixture was subsequently pulverized and metamorphosed by smaller impacts and the solar wind to form a regolith. Similar regoliths cover the surface of the Moon and as is the case with howardites, lunar regolith breccias display high abundances of noble gases that have been implanted into the rock by the solar wind.
HUBBLE CLASSIFICATION SCHEME - Classification of galaxies according to shape and appearance (left). Hubble defined three main classes, elliptical galaxies, spiral galaxies, and irregular galaxies, with subgroups in all categories.
The ratio of molecular to atomic hydrogen varies for different Hubble types of spiral galaxies. The histograms below are presented in order of declining bulge to disk ratios, with the S0/Sa galaxies having the largest bulges. The increasing fraction of dense molecular clouds towards earlier Hubble types may reflect a higher cloud formation rate in the presence of a stronger gravitational field.
HUBBLE CONSTANT (H0) - Constant in the linear relationship between the distance to a galaxy (R) and the velocity with which that galaxy is receding from us (v) due to the overall expansion of the universe:
H0 is the constant of proportionality known as Hubble's constant. The present "best" value of the Hubble constant is 71 +0.04/-0.03 (km/s) / Mpc.
HUBBLE DIAGRAM - Plot of apparent fluxes (usually expressed as magnitude) of objects at cosmological distances, against their redshift values. It is used as a tool to measure the global geometry of the universe, and as a probe of galactic evolution.
The diagram above shows curves for different values of omegas (Ω); the best fit (not shown) is for ΩM = 0.28 (baryons), ΩΛ = 0.72 (dark matter).
HUBBLE SPACE TELESCOPE - A 2.4-meter reflecting telescope, which was deployed in low-Earth orbit (600 kilometers) by the crew of the space shuttle Discovery (STS-31) on 25 April 1990. The Hubble Space Telescope is a cooperative program of the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA). HST's current complement of science instruments includes three cameras, two spectrographs, and fine guidance sensors (primarily used for astrometric observations). These instruments produce high resolution images of astronomical objects. Ground-based telescopes seldom provide resolution better than 1.0 arc-seconds; HST's resolution is about 10 times better, or 0.1 arc-seconds.
HAYASHI TRACK - A complicated path on the Hertzsprung-Russell diagram showing the evolution of a protostar onto the main sequence where fusion can begin. The lower figure indicates the times required to reach the main sequence for different starting masses.
HYBRIDIZATION - Process where atomic orbitals of different type but similar energies are combined to form a set of equivalent hybrid orbitals. These hybrid orbitals do not exist in the atoms but only in the formation of molecular orbitals by combining atomic orbitals from different atoms. Groups 1, 5, 6 and 7 in the period table do not hybridize. The hybrid orbital model (HOM) was developed by Linus Pauling to explain observed shapes and bond orders in molecules. The fundamental tenet of the HOM is that atomic orbitals of differing energies will mix to form a new set of hybrid orbitals with energies determined by a weighted average of the atomic orbitals that combined. The number of hybrid orbitals formed must equal the initial number of atomic orbitals combined. For example, hybridization of one 2s and three 2p orbitals (below) yields four sp3 orbitals with 75% p character and 25% s character.
Sigma bonds (s) between C hybrid 2sp3 orbitals produce the structure of diamond. The silicon tetrahedra, SiO44-, the building block of silicate minerals, results from silicon hybrid 3sp3 orbitals bonded to oxygens.
HYDRATION - Reaction of a substance with water.
HYDROCARBON - Organic compound containing only carbon and hydrogen.
HYDRODYNAMICS - Study of fluid in motion
HYDROSPHERE - Portion of a planet that is water, including liquid water, ice, and water vapor on the surface, underground, or in the atmosphere.
HYDROGEN (H) - Lightest and most common element in the universe (~92% by atoms; ~75% by mass). Isotopes are: 1H (99.9885 %) and 2H (0.0115 %); the latter is called deuterium. Tritium, 3H, a radioactive (t½ = 12.32 y) by-product of atmospheric thermonuclear tests occurs in Earth's hydrosphere and atmosphere. Interstallar regions of neutral hydrogen are referred to as HI regions. Regions of ionized hydrogen are HII regions.
HYDROGEN BOND - Type of attractive intermolecular force that exists between two partial electric charges of opposite polarity. Although stronger than most other intermolecular forces, the typical hydrogen bond is much weaker than both the ionic bond and the covalent bond. When H bonds with a strongly electronegative element (such as O) the electronegative element attracts the electron cloud from around the hydrogen nucleus and, by decentralizing the cloud, leaves the atom with a positive partial charge. The resulting charge, though only partial, nevertheless represents a large positive charge density that is attracted to sites of negative charge. The hydrogen bond is not a simple attraction between point charges, but possesses some degree of orientation preference. Consequently, it has some of the characteristics of a covalent bond. This covalency tends to be more extreme when acceptors bind H from more electronegative donors.
Hydrogen bonds form the crystal structure of ice, bond layers in some phyllosilicates, connect the two strands of deoxyribonucleic acid (DNA) by joining attached amino acids (below), and play a major role in the folding of proteins.
HYDROGEN BURNING - Processes by which hydrogen (1H) is fused into helium (4He) with in a star. The five possible fusion paths can be divided into two sets of processes: the Proton-Proton (PP) process, which depends only on the amount of H and He in the star, and the CNO cycle (carbon-nitrogen-oxygen), which depends on the amount of these elements in addition to the amount of H and He in the star. There are three branches to the PP process of convert hydrogen (1H) into helium (4He). The first branch does the conversion without creating any nuclei heavier than He. The remaining two branches go through a step that creates Be. The core temperature determines which of these branches is dominant. The first PP process branch (shown) dominates in the production of He for core temperatures below roughly 15 million degrees (1.3 keV).The other two PP processes occur at higher core temperatures (15–25+ million degrees).
The core temperature of a star rises with its mass, so the PP process dominates at low masses, and the CNO cycle dominates at high masses. For main-sequence stars with elemental abundances similar to the Sun, the conversion of H into He is equal for the two processes when a star is ~2 Msun. Below ~1.2 Msun, the contribution to the energy production from the CNO cycle is insignificant; this means that the Sun is powered only by the PP process. Above ~3 Msun, virtually all of the energy generated in a star comes from the CNO cycle. The minimum mass of a star is ~0.075 Msun. Below this mass, the core temperature never rises high enough for hydrogen fusion to begin.
HYDROGEN SERIES - Sequences of lines in the hydrogen spectrum corresponding to atomic transitions, each ending or beginning with the same atomic state in H. For example, the visible light Balmer Series involves transitions starting (for absorption) or ending (for emission) with the first excited state of hydrogen, while the UV Lyman Series involves transitions that start or end with the ground state of hydrogen.
HYDROLYSIS - Reaction of a substance with water or its ions.
HYDROSTATIC EQUILIBRIUM - Balance between gravity and gas pressure. In the case of a star, gravity originates in mutual gravitational attraction of the entire mass of the star. Gas pressure is produced by nuclear reactions that heat the star's gas. Any change in the internal temperature of a star results in a change in its diameter: if a star cools, it will contract; if it grows hotter, it will expand. The same concepts apply to the gaseous atmospheres of planets.
HYDROXYL - Refers to the OH- ion.
HYSTERESIS - Tendency of a magnetic substance to remain in a certain magnetic condition.