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helium burning

strated to be 4He. Helium is extracted from natural gas, produced by alpha decay of heavy elements in the rock. It is the fusion of hydrogen to helium which produces the energy of all main sequence stars, like the sun. The fusion of hydrogen into helium provides the energy of the hydrogen bomb.

Helium is widely used in cryogenic research as its boiling point is close to absolute zero and can be controlled by pumping the helium vapor; its use in the study of superconductivity is vital. The speciﬁc heat of helium gas is unusually high. Helium exhibits superﬂuidity at low temperatures. Liquid helium (He4) exists in two forms: He4I and He4II, with a sharp transition point at 2.174 K. He4I (above this temperature) is a normal liquid, but its superﬂuid form He4II (below 2.174 K) is unlike any other known substance. It expands on cooling; its conductivity for heat is enormous; and neither its heat conduction nor viscosity obeys normal rules. It has other peculiar properties.

Helium is the only liquid that cannot be solidiﬁed by lowering the temperature at ordinary pressures. It can be solidiﬁed by increasing the pressure.

helium burning The set of nuclear reactions that converts helium to carbon and oxygen. See triple-alpha process.

helium ﬂash The onset of helium burning in a star of less than about 1.5 solar masses. The event is sufﬁciently explosive to change the structure of the star suddenly from a red giant to a horizontal branch or clump star. The reason for the explosion is that the helium fuel is degenerate when it ignites. This means that energy generation, though it increases the local temperature, does not immediately increase the pressure of the gas, so the core does not expand and cool. Rather, the fusion reaction goes faster and faster until the gas is so hot that it is no longer degenerate and the core of the star expands and cools, slowing down the nuclear reaction rate. See clump star, degeneracy, helium burning, horizontal branch star, red giant.

helmet streamer Structure of closed coronal loops overlaid by open ﬁeld lines which extend outwards to larger radial distances like the

feather on a medieval helmet. Helmet streamers are associated with solar activity: they are much more frequent around solar maximum than during solar minimum, their closed loops are often the anchoring ﬁeld lines of ﬁlaments or protuberances, and reconnection in the tip of a helmet streamer might lead to the expulsion of matter and magnetic ﬂux. See disconnection event.

Helmet streamer.

Helmholtz free energy (free energy) An extensive thermodynamic potential H given by

H = U − ST ,

where U is the internal energy, S is the entropy, and T is the temperature of the system. For a reversible process at constant T and V , work stored as free energy can be recovered completely.

Helmholtz theorem A general continuous and ﬁrst differentiable vector ﬁeld A is the sum of two vectors: A = B+C, where ×B = 0 (B is irrotational, has no vorticity); and · C = 0, C is solenoidal, has no divergence.

Henyey–Greenstein scattering phase function An approximate phase function that is parameterized by only the mean cosine of the scattering angle.

Herbig Ae/Be star The analog of T Tauri stars when the object is about 3 to 10 solar masses. They are hotter, brighter, and shorter lived than the lower-mass T Tauri stars. Characterized also by excess infrared emission from circumstellar dust, perhaps arranged in a disk. See T Tauri star.

high-pressure and high-temperature experiment

Herbig–Haro object An object that is apparently a forming star, surrounded by still accreting matter. These objects have strong stellar winds, and mass loss including jets. Herbig– Haro objects are powerful emiters of infrared radiation; the spectrum consists of strong emission lines presumably produced by internal shocks within protostellar jets or winds.

herringbone burst		See type II radio burst.
Hertz (Hz)	Unit of frequency equal to s−1.
Hertzsprung	Gap	The	region on	a
Hertzsprung–Russell		Diagram	between	the

main sequence and red giant branches in which relatively few stars are found. The reason is that stars evolve across the gap (at roughly constant luminosity) very quickly because the hydrogen fuel in their cores has been exhausted and the hydrogen around the core is not yet hot enough to fuse. Thus, the only energy source is gravitational potential energy from core contraction, some of which is used up in simultaneously expanding the outer layers of the star. The gap is particularly conspicuous in the HR diagram of a young open cluster of stars, where all the stars are the same age. See Hertzsprung–Russell diagram, main sequence star, open cluster, red giant.

Hertzsprung–Russell diagram	See HR di-
agram.

Hesperian Geophysical epoch on the planet Mars, 1.8 to 3.5 Gy BP. Channels on Mars give evidence of large volumes of water ﬂow at the end of the Hesperian and the beginning of the Amazonian epoch.

HI 21-cm line Spectral line emitted in the radio domain, at a wavelength of 21 cm (corresponding to a frequency of 1420 GHz), due to the hyperﬁne transition between two energy states in the ground level of the hydrogen atom, the lower state with electron spin and proton spin anti-parallel, the higher state with the two spins parallel. The HI 21-cm emission line was ﬁrst detected in 1951; since then it has been used to map the distribution of neutral hydrogen within the galaxy, and in external galaxies, with radio

telescopes and interferometers. See forbidden lines.

Higgs mechanism Mechanism responsible for the existence of massive particles in the standard model of quantum ﬁeld theory. The mechanism arises in a spontaneous breakdown of symmetry. For instance, far before a gauge U(1) symmetry the symmetry breakdown (i.e., at high temperatures), we have a complex scalar ﬁeld φ and a massless gauge ﬁeld Aµ (like the photon, for example) with two degrees of freedom (two polarizations). As the temperature lowers, φ selects one possible phase in the range [0 − 2π]. This is called spontaneous symmetry breakdown. After the breakdown, φ represents a massive scalar ﬁeld and Aµ becomes a massive gauge ﬁeld (three degrees of freedom), totaling again exactly four degrees of freedom as before the transition. Such events were presumably important in the very early universe. See Abelian Higgs model, cosmic topological defect, Goldstone model, spontaneous symmetry breaking.

higher derivative theories Theories of gravity in which the equations of motion contain higher than second derivatives of the basic variables (the potentials) of the theory.

highlands, lunar See lunar highlands.

high-pressure and high-temperature experiment An experiment to measure density of rocks and minerals, velocity of elastic waves and elastic modulus, and to investigate materials, phases, and melting, realizing the high-pressure and high-temperature state of the Earth’s interior. There are two kinds of high-pressure apparatuses: a dynamic high-pressure apparatus utilizing shock waves and a static high-pressure apparatus applying constant load. For the former, it is possible to produce pressure of several hundred gigaPascals (GPa) instantaneously, corresponding to the pressure at the Earth’s core. However, because of the short reaction time and inaccuracies of the estimate of temperature, other approaches are being used, such as piston cylinder apparatus which is able to produce pressure and temperature characteristic of the uppermost mantle (up to 6 GPa and 2,800 K), multianvil type apparatus which can produce pres-

HI region

sure and temperature characteristic of the uppermost part of the lower mantle (up to 30 GPa and 2,300 K), and diamond-anvil apparatus which produces pressure up to 300 GPa and temperature over several thousand K. A quench method has been used to identify the phases for phase equilibrium experiments, and in situ observation under high-pressure and high-temperature state is now being done employing X-rays.

HI region A part of the interstellar medium where hydrogen atoms and most other atoms remain neutral. Most HI regions have important concentrations of molecules and dust. The main chemical constituents are hydrogen atoms and molecules. The temperature can vary from 102 to 103 K while the density is of the order of 100 or less particles cm−3. See interstellar medium.

HII region A part of the interstellar medium where all hydrogen is ionized by ultraviolet light from one or many main sequence stars of effective temperature between 3 × 104 to 5 × 104 K. Most other elements are also singly ionized. HII regions can have varying amounts of mass up to several thousand solar masses and temperatures between 6 × 103 to 1.5 × 104 K. The density varies from 10 to 104 particles cm3 , and up to 106 particles cm−3 in ultracompact HII regions. Their shapes are often irregular, and determined by the surrounding medium, and are generally associated with molecular clouds. The mass typically varies from 0.1 to 104 solar masses with sizes from 0.01 to 10 parsecs. They are located mostly in the spiral arms of galaxies. Many of them, associated with particular phases of stellar evolution, are also called supernova remnants, planetary nebulae, and nova remnants. See interstellar medium. Compare with planetary nebula.

Himalia Moon of Jupiter, also designated JVI. Discovered by C. Perrine in 1904, its orbit has an eccentricity of 0.158, an inclination of 27.63◦, and a semimajor axis of 1.148×107 km. Its radius is 93 km, its mass 9.56 × 1018 kg, and its density 2.83 g cm−3. It has a geometric albedo of 0.03, and orbits Jupiter once every 250.6 Earth days.

hindcasting (wave) A procedure of determining wave conditions which existed at a prior time, using observations of atmospheric pressure or wind measurements.

historical climate The accumulation of weather records analyzed for long-term trends over periods of years. Climate records kept beginning about 2000 years ago, until very recently, were over land only.

hoarfrost Frost that crystallizes onto vegetation and surfaces by direct deposition from saturated air during a hard freeze. See hard freeze.

Holmberg radius The length of the semimajor axis of a galaxy, either expressed in angular or linear units, measured from the center to a minimum surface brightness of 26.5 photographic magnitudes per square second of arc (approximately 1.5% the surface brightness of the night sky).

homogeneity A system is homogeneous if all points in it are equivalent with regard to relevant properties.

In cosmology, independence of position in space. The Euclidean space, used as a background for Newtonian physics, is homogeneous even though its contents are not. The spacetime describing a realistic cosmology may be spatially homogeneous, i.e., its geometry may be independent of position in each space of constant cosmological time, but in a correct (general relativistic) description, this requires the matter stress tensor to be homogeneous under the same symmetry that describes the homogeneity of the space. It cannot be fully homogeneous as a space-time because homogeneity in four dimensions would imply independence of time, while there is observational evidence for the expansion of the universe. Whether our actual spacetime is even approximately spatially homogeneous is a question that can, in principle, be answered by observations. However, the observations are difﬁcult and, thus far, inconclusive (see cosmological principle). At small scales (of clusters of galaxies at least) the universe is obviously inhomogeneous. If our spacetime is not spatially homogeneous, even on long scale average, then it must be modeled by inhomogeneous models.

horizontal branch star

A space that is isotropic with respect to all of its points is also homogeneous, but the converse is not true. See isotropy, Killing vector.

homologous ﬂares Solar ﬂares that occur repetitively in the same active region, with essentially the same position and with a common pattern of development.

homologous temperature (T /Tm) The temperature (T ), normalized by the “melting temperature”, Tm.

homothety A one-parameter family of maps of space-times with the property

Lξ gik = σgik .

Here ξ is the generator and gik the metric. L denotes the Lie derivative along the vector ξ. Homothety is a special one-parameter conformal group when σ is a constant.

homotopy group Mathematical structure that allows classiﬁcation of topological defects formed during phase transitions. Denoting M as the vacuum manifold, one can deﬁne the zeroth homotopy group π0 as a counter of its disjoint parts if it is disconnected. For a connected manifold, it reduces to the identity element.

Inequivalent paths on a plane with a hole. The closed curves in full line, not encircling the hole, are all equivalent, and so are those in dashed line that wind once around the hole. However, because of the hole, it is not possible to smoothly transform full line curves into dashed lines. The ﬁrst homotopy group of the plane with a hole is Z, the group of integers, because the curves can wind an integer a number of times around the hole, in one way or another (hence there is a positive or negative sign associated to each curve).

The ﬁrst, or fundamental, homotopy group π1 counts the inequivalent closed paths on M: for a plane with a hole for instance, all paths encircling the hole are said to be equivalent depending on the number of times they pass around it (see ﬁgure). As they can also wind clockwise or counter-clockwise inequivalently one has, for a plane with one hole, π1 Z, the group of integers.

Similarly, one can deﬁne the nth homotopy group πn of M by means of the inequivalent n- dimensional closed hypersurfaces that exist on

The topological defect classiﬁcation stems from these homotopy groups: when π0(M) is non-trivial, then domain walls must form, while strings, monopoles, and textures will exist, respectively, if π1(M), π2(M), and π3(M) are non-trivial. See cosmic phase transition, cosmic topological defect, spontaneous symmetry breaking.

hopper dredge A dredge that includes a hopper or bin for storage of sediment that will later be pumped out. Used to remove sand from channel entrances and harbors.

horizon On the Earth, or other planet, the distance at which a line from the viewpoint above the surface tangentially touches the surface of the planet. Hence, the maximum distance that can be seen from that viewpoint. The distance to the horizon depends on the height of the viewpoint; for 2 m above sea level the distance on Earth is approximately 5 km. In general relativity one deﬁnes an event horizon, which bounds parts of a spacetime which can never be observed from outside the horizon; and particle horizon, which delimits the boundary visible up to a particular time at a particular location in space. See apparent horizon, black hole horizon, event horizon, particle horizon.

horizontal branch star A star of low mass ( < 1.5 solar masses) and low heavy-element content (0.2%) in the evolutionary phase where the two main sources of energy are helium burning in the core and CNO-cycle hydrogen burning in a thin shell. The star is about 30 times as bright as it was on the main sequence, and the phase lasts about 3% as long. The name comes