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Chemical symbol for hydrogen. The most abundant chemical in the universe. H2 is the symbol for the molecular hydrogen molecule which is abundant in giant clouds in our galaxy and can be detected by its infrared spectrum. The symbols HI and HII are used to indicate neutral and ionized hydrogen respectively. HII regions are usually associated with star formation, e.g. the Orion Nebula, and are detected by their emission lines such as H-alpha at 656.3 nm. Radio emission at 21 cm wavelength can be detected from neutral hydrogen.
The quantity h (Planck's constant) divided by 2: = 1.054 x 10-27 erg seconds.
Hubble's constant in units of 100 km s-1 Mpc-1.
An Mg II resonance line at 2803 Å.
|H and K Emission Line Stars||
Late objects (F4 to M), which exhibit emission features in their H and K lines of Ca II.
|H and K lines||
The two closely spaced lines of singly ionized calcium at 3968 and 3934 Å, respectively.
An H ion with an extra electron in its shell. It is an important source of stellar opacity in stars whose spectral types are later than about A5.
The magnitude derived from infrared observations at 1.6 microns.
Neutral hydrogen gas. It emits radio waves that are 21 centimeters long.
|H I Region||
Region of neutral (atomic) hydrogen in interstellar space. The temperature is about 125 K (the spin temperature of neutral hydrogen - far too low for electrons to emit radiation in the optical part of the spectrum (see 21-cm radiation). At least 95 percent of interstellar H is H I. (Density is about 10 atoms per cm3, about the same as in H I regions.)
Ionized hydrogen - that is, hydrogen with its electron missing.
|H II Condensation||
A high-density H II region.
|H II Region||
(a) An area of ionized hydrogen. Most H II regions are
red and arise from hot blue O and B stars, whose ultraviolet light can
ionize all the hydrogen for dozens or even hundreds of light-years in
every direction. The most famous H II region is the Orion Nebula.
Hertzsprung-Russell diagram. A diagram of stars arranged according to their luminosity (measured on the y axis) and temperature (on the x axis). In the early part of the twentieth century, the Danish astronomer E. Hertzsprung and the American astronomer H. Russell plotted known stars in such a diagram and found a definite correlation between luminosity and temperature. (See main sequence.)
Hydrogenated Amorphous Carbon
An unofficial name for Jupiter IX, the outermost satellite of Jupiter (P = 758 days retrograde, i = 156°, e = 0.28). Discovered by Nicholson in 1914.
(a) The generic name for any particle which experiences
the strong nuclear force.
The interval (t 10-43 [10-23] s after the big bang, when = 1093  g cm-3) during which quantum and general-relativistic effects are expected to modify each other in an unknown way. The quantities in brackets are for a different equation of state.
The interval (t 10-5 s after the big bang) when the Universe was matter-dominated and when kT m. It was followed by the lepton era (q.v.).
The interval lasting until some 10-4 seconds after the Big Bang when the universe was matter-dominated, containing many hadrons in equilibrium with the radiation field. The Hadronic Era ended when the characteristic photon energy fell below the rest mass of a pion or -meson (270 electron masses), and very few hadrons remained (about one hadron for every 108 photons).
A transition metal found in zirconium ores. Hafnium is
difficult to work and can burn in air. It is used in control rods for
nuclear reactors and in certain specialized alloys and ceramics.
|Hagedorn Equation of State||
An equation of state for extremely degenerate matter (density greater than about 1015 g cm-3).
The formation of a halo around bright star images by light reflected from the back of the photographic plate or emulsion.
(a) The time it takes for half of a given quantity of
radioactive material to decay.
(HPBW) The angle across the main lobe of an antenna pattern between the two directions where the sensitivity of the antenna is half the value at the center of the lobe. This is the nominal resolving power of the antenna system.
When an electric current is passed through a conductor and a magnetic field is applied at right angles, a potential difference is produced between two opposite surfaces of the conductor. The direction of the potential gradient is perpendicular to both the current direction and the field direction. It is caused by deflection of the moving charge carriers in the magnetic field. The size and direction of the potential difference gives information on the number and type of charge carriers.
Probably the best known of all comets. Its orbit was computed by Edmund Halley in 1704, at which time he predicted that the bright comet of 1682 would return in 1758 (Halley died in 1742, before he could see his prediction verified). Records of Halley's comet (a = 17.8 AU, e = 0.967, i = 162°.3, P = 76.2 yr perihelion distance 0.587 AU) have been traced back to 240 B.C. Last appearance 1910, next appearance 1986.
(a) Nebulous quality round a celestial body (particularly
round a red giant); the galactic halo, however, describes the spherical
collection of stars forming a surrounding "shell" for our otherwise compact,
Old stars typical of those found in the halo of the Galaxy; also called Population II.
Stars that have high spatial velocity and low metallicity. This is not an observational definition.
|Hamiltonian Function (H)||
The quantity in classical mechanics corresponding to the total energy of a system, expressed in terms of momenta and positional coordinates.
|Hamiltonian Operator (H)||
The dynamical operator in quantum mechanics that corresponds to the Hamiltonian function in classical mechanics.
A theory for calculating the trajectory of a particle under an applied force. Hamiltonian theory, developed in the nineteenth century, is equivalent to Newton's laws of mechanics but is reformulated in a mathematically elegant way to allow easier solutions to some problems.
A method of smoothing out the noise in radio data. For each data point, one-half the value of that point is taken, plus one-quarter the value of the point on each side. The result is usually a smoother curve.
The rule that atoms of even atomic number are more abundant than those of odd atomic number.
An evolutionary sequence of hot subdwarfs and nuclei of planetary nebulae.
See Kepler's third law.
|Harmonic Motion||A motion that repeats itself in equal intervals of time. (also called periodic motion)|
Any oscillating particle in harmonic motion.
Any integral multiple of the fundamental frequency (q.v.).
Blue objects whose spectra show sharp emission lines.
See no-boundary proposal.
See Henry Draper system.
A transactinide element formed artificially.
the radiation produced by a black hole when quantum effects are taken into account. It can be viewed as a type of virtual pair production in which one of the particles falls through the event horizon of the black hole and hence cannot escape to rejoin its partner.
(1) A stationary black hole must be either static (i.e., nonrotating) or axisymmetric. (2) In interactions involving black holes, the surface area of the event horizon can never decrease.
A nearly vertical track of stellar evolution toward the main sequence during phases when the star is largely or completely in convective equilibrium. The luminosity, originally very high, decreases rapidly with contraction, but the surface temperature remains almost constant.
|HB or Horizontal Branch Stars||
Globular cluster stars defined by their position in the color-magnitude diagram. They are located on both sides of the RR Lyr gap and one speaks therefore of blue or red HB stars.
High energy peaked BL Lac object
Henry Draper Catalogue, which lists over 200,000 stars. It was published in nine volumes between 1918 and 1924.
|head (of comet)||
A class of relatively weak radio sources associated with clusters of galaxies and characterized by a high-brightness "head" close to the optical galaxy and a long low-brightness "tail".
High-Energy Astronomical Observatory.
|Heat of Formation||
Energy which would be required to form a molecule from dissociated atoms. If positive, the structure will not be formed spontaneously. Lower heats of formation indicate more stable molecules, which are formed preferentially.
a class of recently discovered materials, usually rare-earth or actinide compounds, in which the `effective mass of the electrons appears to be hundreds or thousands of times the real electron mass.
See E layer.
A class of peculiar giants that includes the Ba II stars and the S stars.
Elevation above ground or distance upwards from a given level (especially sea level) to a fixed point. (See altitude.)
A model of magnetic systems in which each magnetic atom has a spin which is free to point in any direction in space. Neighboring atoms are coupled by a force which tends to align the spins in parallel (for a ferromagnet) or opposite (for an antiferromagnet) directions.
|Heisenberg Uncertainty Principle||
(a) States that the position and momentum of a particle
can only be known to a certain level of precision. The more precisely
one quantity is known, the less certain the precision of the other. A
similarly linked pair of quantities is the time and energy content in
a volume of space.
Asteroid 624, the largest (about 100 km long) of the Trojans. Its shape is apparently as elongated as that of Eros. Rotation period 6.9225 hours. Its visual mean opposition magnitude is near +14.5, which makes it the brightest of the Trojans. Assumed albedo 0.28. It has a large obliquity.
A process of joining two metals using an electric arc in an atmosphere of a noble gas.
(a) The projection of a particle's spin along its direction
of motion. The helicity of a particle is described as being either left-handed
or right-handed depending on whether its spin projection is in the direction
of motion or against it.
Having the Sun at the center.
School of models in which the sun was portrayed as standing at the center of the universe.
Device for recording the positions of SUNSPOTS.
Instrument to measure the apparent diameter of the Sun at different seasons, also used to measure angular distances between stars.
(a) Element which, after hydrogen, is the second lightest
and second most abundant in the Universe. Its atom comprises two protons,
two neutrons and two electrons; its nucleus is sometimes called an alpha
particle. Helium is the product of the nuclear fusion of hydrogen in most
stars, but this does not explain the overall helium abundance.
(a) Presence - and dominance - of helium atoms in the
Universe. The fact that about 8% of all atoms are helium can be traced,
through the --
theory, to the primordial big bang.
The stage when a star fuses helium into carbon and oxygen. All stars born with more than half a solar mass eventually burn helium.
The onset of runaway helium burning under degenerate conditions. The helium flash occurs in the hydrogen-exhausted core of a star in the red-giant phase of evolution. When gravitational pressure has brought the degenerate core to a temperature of about 108 K, the helium nuclei can start to undergo thermonuclear reactions. Once the helium burning has started, the temperature builds up rapidly (without a cooling, stabilizing expansion), and the extreme sensitivity of the nuclear reaction rate to temperature causes the helium-burning process to accelerate. This in turn raises the temperature, which further accelerates the helium burning, until a point is reached where the thermal pressure expands the core and thus removes the degeneracy and limits the flash. The helium flash can only occur when the helium core is less than the 1.4 M Chandrasekhar mass limit and thus it is restricted to low-mass stars.
Poses the question: what physical process caused the current abundance of helium in the Universe?
|Helium Shell Flash||
It has been shown that helium shell burning outside a degenerate core is unstable; the helium-burning shell does not generate energy at a constant rate but instead produces energy primarily during short flashes. During a flash, the region just outside the helium-burning shell becomes unstable to convection. The resultant mixing probably leads to the s-process as well as to the upward movement of carbon produced by helium burning.
B-type stars in which the helium lines are stronger than in normal stars. One distinguishes usually the extreme helium stars (also called hydrogen-deficient stars), in which no trace of hydrogen is seen, and the intermediate helium-rich stars, in which the hydrogen lines are still visible, but weaker than in normal stars. Related to these objects are the hydrogen deficient C stars.
B-type stars in which the helium lines are weaker than in normal stars. Also called Bp helium-weak stars.
|Helium Variable Stars||
Bp stars in which the strength of the helium lines varies periodically. At the extreme phases the objects appear as helium-rich, whereas at other phases He can be very weak or absent.
A planetary nebula about 140 pc distant in Aquarius with the largest known angular diameter of any planetary. (NGC 7293)
See Kelvin-Helmholtz contraction.
|Henry Draper system||
A classification of stellar spectra into the sequence O, B, A, F, G, K, M, in order of decreasing temperature.
An almost horizontal track of stellar evolution between the Hayashi track and the main sequence.
Unofficial name for Jupiter VII. P = 259.65 days, e = 0.21, i = 28°. Discovered by Perrine in 1905.
|Herbig Ae, Be Stars||
Be or Ae stars associated with nebulosity.
An object with many of the characteristics of a T Tauri star (e.g., its spectrum shows a weak continuum with strong emission lines), believed to be a star in the very early stages of evolution. All known Herbig-Haro objects have been found within the boundaries of dark clouds. They are strong infrared sources and are characterized by mass loss.
An unsymmetrical cluster of about 75 bright galaxies (z = 0.036) of which about half are spiral or irregular and about half elliptical or 50. It contains a rather large number of disturbed and peculiar galaxies. The "missing mass", if present, must constitute more than 95% of the total. (3U 1551 + 15)
An X-ray pulsar probably about 5 kpc distant, a member
of an occulting binary system with an orbital period of 1.7 days. The
visible component has been identified as the blue variable HZ Herculis,
whose spectrum varies from late A or early F to B. Her X- l has a pulsation
period of 1.2378 seconds, presumably its rotation period, and exhibits
a 35-day quasi-periodicity in the X-ray region (but not in the optical).
It is probably a rotating neutron star in a circular orbit (e <
0.1) with a mass of about 0.7
A slow nova. It is an eclipsing binary with an orbital period of only 4h39m. It also has a regular flickering period of 71 seconds, the shortest period of regular variations known, except for pulsars and compact X-ray objects. It is probably composed of an M dwarf and a white dwarf with an accretion disk. (Nova Herculis 1934)
Of or relating to Hermes Trismegistus, a mythical philosopher beloved of the Neoplatonists and usually identified with ancient Egypt.
A matrix which remains unchanged if each element is replaced by its complex conjugate and the rows and columns are interchanged. In quantum mechanics all matrices corresponding to observables have this property.
(Hz) A unit of frequency equal to one cycle (or wave) per second.
A gap (from about A0 to F5) in the horizontal branch of the H-R diagram (see instability strip). The few stars that populate this gap are RR Lyrae and other variable stars. It is regarded as a region through which a star moves rapidly in its evolutionary track away from the main sequence.
(a) A plot of stellar color, temperature, or spectral
type versus stellar luminosity. The H-R diagram segregates three principal
types of stars: the main sequence, which forms a diagonal band from bright
blue stars to faint red ones; red giants and supergiants, which appear
in the upper right of the diagram; and white dwarfs, which lie below the
(a) A diagram showing the frequencies with which stars
occur at various positions in an H-R diagram.
Unofficial name for Jupiter VI. Discovered by Perrine in 1904. P = 250 days, e = 0.16, i = 29°.
A detection method used extensively in radio astronomy in which the wave nature of light is used. The method usually involves combining the measured wave with a local oscillator or reference wave and looking for the signal at the difference frequency.
|Heterotic E-String Theory||
Heterotic E8 × E8 string theory One of the five superstring theories; involves closed strings whose right-moving vibrations resemble those of the Type II string and whose left-moving vibrations involve those of the bosonic string. Differs in important but subtle ways from the Heterotic-O string theory.
Gross's version of string theory in which space-times of different dimensions are associated with the same closed loop.
|Heterotic O-String Theory||
heterotic O(32) String Theory One of the five superstring theories; involves closed strings whose right-moving vibrations resemble those of the Type II string and whose left-moving vibrations involve those of the bosonic string. Differs in important but subtle ways from the Heterotic-E string theory.
Asteroid 944, perhaps 20 km in diameter, with the largest known orbit (a = 5.8 AU), second highest inclination to the ecliptic (42°.5), and second highest eccentricity (e = 0.66) of any known minor planet. Period 13.7 years. Discovered by Baade in 1920.
Matter whose presence is inferred from dynamical measurements but which has no optical counterpart. The luminous regions of galaxies have mass-luminosity ratios of about 10. However, the mass-uminosity ratio in the outer halos of many spiral galaxies is 100 or more; one sees the brightness fall off with distance from the center of the galaxy but considerable mass is present. A similar situation prevails in galaxy clusters, where nonluminous matter must provide most of the self-gravitational attraction that holds the clusters together. The missing mass is not really missing; it is present but invisible (at least to current detectors). It is generally believed to consist either of the remnants of massive stars or of planetary-sized objects comparable in mass to Jupiter.
|Hidden Variables Theory||
one of a class of physical theories which deny that the quantum state of a physical system is a complete specification. The hidden variables are those components of the hypothetical complete state which are not contained in the quantum state.
The process by which a system of self-gravitating particles will gradually aggregate into larger and larger gravitationally bound groups and clusters. Small clusters merge into larger clusters, which retain little trace of the subunits from which they formed. Elliptical galaxies may have formed in this way from mergers of globular-cluster-sized star clusters; clusters of galaxies may have formed by a similar process.
|Hierarchical Clustering Model||
A model of galaxy clustering in which different patterns appear at different scales of distance and in which the "average" density of matter depends on the size of the volume over which the average is performed. In a homogeneous model, on the other hand, the average density is independent of the size of the volume over which the average is performed. (See pancake model.)
A cosmology characterized by a system of clusters within clusters within clusters.
In the context of grand unified theories, the hierarchy problem is our inability to understand theoretically why the energy scale at which the unification becomes apparent, about 1016 GeV (billion electron volts), is so much higher than other energy scales of relevance to particle physics, such as the mass/energy of a proton, which is only 1 GeV.
(a) A hypothetical, spinless particle that plays an important
role in the Glashow-Weinberg-Salam electroweak theory (and in other theories
involving spontaneous symmetry breaking, e.g. GUTs).
(a) Mechanism operating in symmetry-breaking events;
in electroweak theory, the Higgs field is said to have imparted mass to
the W and Z particles.
(a) A mechanism by which gauge bosons acquire mass through
spontaneous symmetry breaking. In the Glashow-Weinberg-Salam electroweak
model, for example, Higgs fields are introduced into the theory in a gauge-invariant
way. However, the state of minimum energy breaks the local gauge symmetry,
generating masses for the W± and Z0 bosons, and
giving rise to a real, observable Higgs boson, '.
The particle or particles associated with the bundles of energy in the Higgs field. Such particles are analogous to the photons that are associated with the electromagnetic field. The standard model of particle physics predicts one electrically neutral Higgs particle which has not yet been found, but which will be sought in upcoming particle accelerator experiments. The grand unified theories predict many Higgs particles, but they are too massive to be accessible at existing or foreseeable accelerators.
This is the combined physics that explains the origin of the Higgs field, the reason the Higgs mechanism applies, and the properties and study of the Higgs bosons.
The superpartner of the Higgs boson.
Particles of electromagnetic radiation that contain high energies, measured in terms of electron volts. The energy in gamma radiation is of the order of 8 x 107 to 8 x 105 electron volts and in X-rays of 8 x 103 to 8 × 101 electron volts.
See particle physics.
|High-Luminosity Early Type Objects||
A collective designation for some early type stars with very peculiar spectra, like S Dor and P Cyg.
Generally a celestial object in the galactic halo whose orbital velocity around the galactic center is less than that of the Sun, and that thus, relative to the Sun, has a high space motion. A "high-velocity" object usually travels around the galactic center in an eccentric orbit, often of large inclination to the galactic plane.
(a) A star whose U and/or V and/or W velocities are much
greater or much less than zero. Such stars usually have eccentric orbits
around the Galaxy.
Class of supergravity theories in more than four spacetime dimensions.
A mathematical tool used in the formalism of quantum mechanics. The dimensions of a Hilbert space consist of wave functions, instead of length, width, and breadth. (See quantum mechanics; wave function.)
A reflection nebula (q.v.) discovered by Hind in 1852, which is illuminated by the star T Tauri. It is remarkable for its changes in brightness. (NGC 1554-5)
HIgh-Precision PARallax COllecting Satellite.
Groups of minor planets with similar orbital elements. The members of a given family are widely believed to have resulted from collisions between larger parent bodies.
The time taken to use up all the liquid cryogens, like LN2, in a cooled CCD cryostat.
The radius of an external galaxy at which the surface brightness is 26.6 mag arcsec-2. This criterion was developed by Holmberg in 1958 to estimate the actual dimensions of the major and minor axes of a galaxy without regard to its orientation in space.
A soft malleable silvery element of the lanthanoid
series of metals. It occurs in association with other lanthanoids. It
has few applications.
An interferometric method of recording information about the three-dimensional nature of an object which relies on preserving both the amplitudes and phases of the wavefronts which reach the detector, instead of merely the amplitudes. Hologram means "whole record". The basic principle was outlined by D. Gabor in 1948.
Same as complex analytic.
An approximation in which the lines emitted and absorbed by atoms are subject to the fluctuating electrostatic fields to which the atom is subject in an ionized atmosphere.
(a) In cosmology, the property that any large volume
of the universe looks the same as any other large volume. Most cosmological
models assume homogeneity.
Same as horizon problem.
A universe is called homogeneous if it would look the same to all observers, no matter where they were located. The real universe is not precisely homogeneous, but it appears to be homogeneous on large scales. That is, if we averaged the mass density or other property of the matter in the universe over cubes of a few hundred million light-years on each side, all such cubes would be very similar to each other. See also isotropic.
To a good approximation, our universe appears to be undergoing homogeneous expansion, which means that successive snapshots of a given region would each look like a photographic blowup of the first snapshot. Homogeneous expansion is also called Hubble expansion, since it implies Hubble's law. See Figure 2.1 on page 21.
A construction method for a large mirror in which the back is hollowed-out to leave a ribbed structure that resembles a honeycomb.
(a) The maximum distance that an observer can see. In
cosmology, our horizon is the distance from us that light has traveled
since the beginning of the universe. Objects more distant than our horizon
are invisible to us because there hasn't been enough time for light to
have traveled from there to here.
the maximum distance, at any given time, that a light
signal could have travelled since the beginning of the Universe.
(a) A quandary in standard big bang theory, which indicates
that few of the particles of the early universe would have had time to
be in causal contact with one another at the outset of cosmic expansion.
It appears to have been resolved in the inflationary universe theory.
That part of the H-R diagram of a typical globular cluster that extends shortward from the asymptotic branch at an approximately constant absolute bolometric magnitude of about 0.3. A star appears on the horizontal branch after it has undergone the helium flash and begins to burn helium quietly in its core and hydrogen in a surrounding envelope.
|Horizontal Branch Star||
A metal-poor star, similar in mass to the Sun, that fuses helium into carbon and oxygen at its core. Such stars range in color from blue to yellow. RR Lyrae stars are horizontal-branch stars. Stars bluer than RR Lyraes are called blue horizontal-branch stars; stars redder are called red horizontal-branch stars, even though they are actually yellow. All other things being equal, the more metal-poor a globular cluster, the bluer its horizontal branch; the older a globular cluster, the bluer its horizontal branch, too.
The difference between the topocentric and geocentric positions of an object, when the object is on the astronomical horizon.
An absorption nebula in the middle of Orion. See Omega Nebula. (NGC 2024)
The main or master computer in an instrumentation system. The computer responsible for interacting with the user.
|Hot Big Bang||
Later, but fundamental, concept within the big-bang theory, that the primordial explosion occurred in terms of almost unimaginable heat. The concept, formulated by George Gamow, led to considerable study of thermonuclear reactions and the search for background radiation.
|Hot Dark Matter||
Any form of dark matter which was relativistic at its point of decoupling.
Angular distance on the celestial sphere measured westward along the celestial equator from the meridian to the hour circle that passes through a celestial object.
A great circle passing through the celestial poles - i.e., perpendicular to the celestial equator.
A compact H II region in the center of M8.
A reformulation of the general theory of relativity that incorporates and extends Mach's principle (q.v.). In this theory, the inertial mass of a particle is a function of the masses of all other particles, multiplied by a coupling constant which is a function of cosmic epoch. In cosmologies based on this theory, the gravitational constant G decreases strongly with time.
Half Power Beam Width. The angle across the main lobe of an antenna pattern between the two directions where the sensitivity of the antenna is half the value at the center of the lobe. This is the nominal resolving power of the antenna system.
Highly Polarized Quasar
See Hertzsprung-Russell Diagram.
High-Resolution Spectrograph (Hubble).
Hubble Space Telescope. A space-based reflecting telescope with a primary mirror diameter of 2.4 m (94 in) capable of high-resolution imaging from the far ultraviolet to the near infrared. A joint NASA/ESA mission. Launched in 1990 with a planned lifetime of 15 years. Encountered reduced performance when the mirror was found to have spherical aberration. Solved by the installation of corrective optics (COSTAR) in 1994.
Hyper-Text Mark-up language.
Hyper-Text Transfer Protocol.
A morphological classification sequence of galaxies devised by Edwin Hubble. It splits galaxies into ellipticals, lenticulars, spirals, barred spirals and irregulars.
(a) The present expansion rate of the universe, in units
of kilometers per second per megaparsec. The larger the Hubble constant,
the younger the universe.
Plot of galaxy redsifts against their distances. This was the first evidence of the expansion of the universe.
... of the Universe. There are billions of other galaxies and all except the closest ones (the Local Group) are receding from us as deduced from the Doppler redshift effect of their spectra. Thus the Universe appears to be expanding. Moreover, the greater the distance the faster the speed of recession. This is interpreted as the expansion of spacetime itself since an event called the Big Bang.
The movement of the galaxies away from us caused by the expansion of the Universe.
(a) The law that recessional speed is proportional to
distance for a homogeneous and isotropic universe. Galaxies moving away
from us with a speed precisely following this law are said to follow the
Hubble flow. Because the actual universe is not precisely homogeneous,
with lumpiness arising from clustering of galaxies and voids of empty
space, the motions of actual galaxies deviate somewhat from the Hubble
A cometary nebula whose apex star is R Mon. (NGC 2261)
The research program carried out in the 1920s and 1930s by Edwin Hubble to measure the recessional speeds and distances of a large number of galaxies and to attempt to measure the deceleration parameter. This last parameter can in principle be determined by measuring the apparent brightness and redshift of a large number of objects of identical intrinsic luminosity. (See deceleration parameter; Sandage program; standard candle.)
c/H The radius of the observable universe (> 1027 cm).
(a) The Hubble time is one divided by the Hubble constant,
which gives a number from 10 to 20 billion years. For a flat universe
with no cosmological constant, the age of the universe is two-thirds of
the Hubble time.
Relations expressing conservation of baryon number, momentum, and energy across a shock front.
A binary pulsar discovered in 1974, probably consisting of a neutron star and an even more compact object in an eccentric orbit, with an orbital period of 0.3230 days and a pulsation period of 59 milliseconds. (PSR 1913+16)
The larger the value of S (total spin angular momentum), the lower the value of the average perturbation energy <V>SL.
The brightest portion of the Orion Nebula.
A single member of the Hyades.
A young (5 × 108 yr) moving cluster (radial velocity, + 36 km s-1) of more than 200 stars (spectral types A1-K) visible to the naked eye in Taurus, about 40 pc distant. Aldebaran is a foreground star in that region of the sky.
A device in which the roles of radiation (infrared mostly) detector and signal multiplexer are separated. The device is a sandwich of two slabs. Other names include focal plane array (FPA) and sensor chip assembly (SCA).
Molecule which contains only hydrogen and carbon. Type of organic molecule.
The study of how gases and fluids flow under applied forces.
(a) Element that is the lightest and the most abundant
in the Universe. Its atom comprises one proton and one electron. The element
occurs both in stars and as interstellar clouds, in regions where it may
be neutral (H I regions) or ionized (H II regions).
The fusion of hydrogen into helium and the process by which all main-sequence stars generate energy. Every star born with more than 0.08 solar masses burns hydrogen.
|Hydrogen-Deficient C-type Stars||
A subgroup of high-luminosity C stars with weak or absent hydrogen lines, mostly of types F and G. Variable stars having such characteristics are called R CrB stars.
|Hydrogen-Deficient Early-Type Stars||
Early type stars of type O, B or A in which the hydrogen lines are very weak or absent.
A balance between the gravitational force inward and the gas and radiation forces outward in a star.
A three-dimensional space whose geometry resembles that of a saddle-shaped surface and is said to have negative curvature.
Twice the charge of a charge multiplet (q.v.).
Involving more than the customary four dimensions (three of space plus one of time) of relativistic space-time.
Splitting of spectral lines due to the spin and consequent magnetic moment of an atomic nucleus. It can be observed only at very high resolution.
A system consisting of a dominant spiral galaxy surrounded by a cloud of dwarf satellite galaxies, often ellipticals. Our galaxy and the Andromeda galaxy are hypergalaxies.
Eighth satellite of Saturn about 160 km in diameter. P = 21d6h38m. Discovered by Bond in 1848.
(a) A baryon with non-zero strangeness.
A scientific proposition that purports to explain a given set of phenomena; less comprehensive and less well established than a theory.
(a) The ability to follow two different branches of states, as a parameter built in the system varies first in a monotonic fashion and subsequently comes back to its initial value by varying in the opposite direction.
(b) In general, an apparent lag of an effect behind whatever is causing it. Magnetic Hysteresis is the behavior of ferromagnetic materials as they are magnetized and demagnetized. The flux density, B, lags behind the external field strength H. See hysteresis cycle.
A closed loop obtained by plotting the flux
density, B, of a ferromagnetic substance against the
magnetizing field strength, H. The substance is first brought
to magnetic saturation from an unmagnetized state - this produces
curve OA (see illustration). As the field strength is taken through
one cycle of reductions, reversals, and increases, the curve follows
the path ACDEFGA. This is known as a hysteresis loop.
Hertz A unit of frequency equal to one cycle (or wave) per second.
Blue horizontal-branch stars, the first catalog of which was compiled by Humason and Zwicky.