Spectral type for white stars, such as Sirius, Vega, Altair, Deneb, and Fomalhaut.
One of about a dozen of the strongest Fraunhofer lines seen in the Solar spectrum, the A band at 7600 angstoms is due to telluric lines of molecular oxygen in the Earth's atmosphere.
Einstein coefficient, where Aji is the coefficient of spontaneous emission from upper level j to lower level i.
Atomic Mass Number: The total number of protons and neutrons in an atom's nucleus. For example, Oxygen-16 has a mass number of sixteen, because it has eight protons and eight neutrons.
|A Shell Stars|
A-type stars in which two different types of line profiles co-exist.
A star of spectral type A with a surface temperature of about 10,000 K, in whose spectrum the Balmer lines of hydrogen attain their greatest strength. Helium lines can no longer be seen. Some metallic lines are present; in late A stars the H and K lines of ionized calcium appear. A0 stars have a color index of zero. Examples of A stars are Vega and Sirius.
A sub-class of Bailey type RR Lyrae variables, having asymmetric lioght curves of large amplitude.
|AB Magnitude System|
The AB magnitude system is defined such that for any bandpass or filter being considered, the magnitude zero-point corresponds to a flux density of 3631 Jy (1 Jy = 1 Jansky = 10-26 W Hz-1 m-2 = 10-23 erg s-1 Hz-1 cm-2) .
Abbreviation employed in this book to mean After the Beginning of Time, which is here defined as the beginning of the expansion of the Universe.
|Ae or A Emission Stars|
A-type stars with emission in one or several Balmer lines.
(a) Peculiar stars whose metallic lines are as strong
as those of the F stars but whose hydrogen lines are so strong as to
require that they be classed with the A stars. They are generally short-period
(<300d) spectroscopic binaries with high atmospheric turbulence
and variable spectra, and are slower rotators than normal A stars.
Peculiar A-type stars ("magnetic" A stars) that show abnormally strong lines, sometimes of varying intensity, of certain ionized metals. Recent evidence indicates that all Ap stars are slow rotators compared with normal A stars.
A mathematical group of transformations with the property that the end result of a series of transformations does not depend on the order in which they are performed.
(a) Defect in the image formed by a lens, mirror
or optical system. Spherical aberration results when different rays
of light are brought to more than one focus, producing a blurred image
or coma; chromatic aberration when different wavelengths within
a ray of light are brought to more than one focus, producing an image
distorted by colored fringes. Aberration in lenses can be overcome by
the use of an achromatic lens or a combination of lenses made
of glasses of different refractive indices.
A defect in an optical system such that the image is
not a true picture of the object. For instance, colored fringes may
appear, the image may not be focused, or the shape may show
distortion. Techniques of aberration correction exist; these can,
however, be complex and costly.
The component of stellar aberration (see Aberration, Stellar) resulting from the motion of the Earth about the Sun.
The component of stellar aberration resulting from the observer's diurnal motion about the center of the Earth.
|Aberration, E-terms of|
Terms of annual aberration depending on the eccentricity and longitude of perihelion of the Earth (abbreviation for Ecliptic Aberration).
The apparent angular displacement of the observed position of a celestial body produced by the motion of the observer and the actual motion of the observed object
The component of stellar aberration resulting from the essentially uniform and rectilinear motion of the entire Solar System in space. Secular aberration is usually disregarded.
Always occurs with rays that are distant from the axis and incident on a spherical mirror or lens. It is the cause of the caustic curve. Spherical aberration is corrected by using parabolic reflecting and refracting surfaces.
(a) Difference in a star's apparent position in the
sky from the apparent position it would have if the Earth were stationary.
Such displacement caused by the Earth's sidereal motion results
in an optical positioning difference of up to about 20.5 seconds of
arc, much greater than any displacement observed by parallax.
Effects associated with the performance of optical components which give rise to imperfect optical images.
Erosion of an object (generally a meteorite) by the friction generated when it passes through the Earth's atmosphere.
The total luminosity radiated by an object.
(a) A measure of the intrinsic brightness of a star
or galaxy. Absolute magnitude is defined as the apparent magnitude the
star or galaxy would have if it were 32.6 light-years (10 parsecs) from
Earth. The lower an object's absolute magnitude, the greater
its intrinsic brightness. For example, the Sun has an absolute magnitude
of +4.83, while Sirius, whose intrinsic brightness is greater, has an
absolute magnitude of +1.43. A star that is one absolute magnitude brighter
than another (e.g., +4 versus +5) is 2.5 times intrinsically brighter;
a star that is 5 absolute magnitudes brighter is 100 times intrinsically
brighter; and a star that is 10 absolute magnitudes brighter is 10,000
times intrinsically brighter.
Newtonian space, hypothesized to define a cosmic reference frame independent of its content of matter or energy. The existence of absolute space, enshrined in aether theory, was denied in relativity.
(a) Temperature measured on the Kelvin scale: 0 Kelvin
= -273.15° Celsius. Absolute temperature is directly related to (kinetic)
energy via the equation E = kBT, where
kB is Boltzmann's constant. So, a temperature of 0
K corresponds to zero energy, and room temperature, 300 K = 27°C, corresponds
to an energy of 0.025 eV.
T = + 273.15
A unit defined in terms of fundamental quantities (such as length, mass, time, and electric charge).
(a) The zero value of thermodynamic temperature;
0 kelvin or -273.15°C.
Symbol: The ratio of the radiant or luminous flux absorbed by a body or material to the incident flux. It was formerly called the absorptivity.
(a) A process in which a gas is taken up by a liquid
or solid, or in which a liquid is taken up by a solid. In absorption,
the substance absorbed goes into the bulk of the material. Solids that
absorb gases or liquids often have a porous structure. The absorption
of gases in solids is sometimes called sorption. Compare adsorption.
see Band Spectrum
Fraction of the incident radiation absorbed at a certain wavelength per unit thickness of the absorber. The absorption coefficient is in general a function of temperature, density, and chemical composition. ( or k in cm-1) see Lambert's Law
Sudden rises superposed on the smooth decrease of the curve of the attenuation coefficient, which cause the curve to have a typical sawtooth aspect. They generally occur at the limit of spectral lines.
Dark lines in a spectrum, produced when light or other electromagnetic radiation coming from a distant source passes through a gas cloud or similar object closer to the observer. Like emission lines, absorption lines betray the chemical composition and velocity of the material that produces them.
|Absorption of Radiation|
No medium transmits radiation without some energy loss. This loss of energy is called absorption. The energy is converted to some other form within the medium. see also Lambert's Law
Dark lines superposed on a continuous spectrum, caused by the absorption of light passing through a gas of lower temperature than the continuum light source.
Range of wavelengths (around 21 cm) at which atomic hydrogen absorbs (or emits) radiation; this is a concept used in the attempt to detect intergalactic matter.
Symbol: The ratio of the radiant or luminous flux absorbed by a body or material to the incident flux. It is now called the Absorptance.
(a) The relative amount of a given element among
others; for example, the abundance of oxygen in the Earth's crust is
approximately 50% by weight.
The ratio of the number of atoms of an isotope to the number of atoms of another isotope of the same element in a sample.
|Acausal Initial Conditions|
Initial conditions that could not have been caused by any prior physical process.
(a) The SI unit is the meter per
second per second (m s-2). 1. When considering
motion in one dimension, and in unscientific usage, acceleration means
rate of increase of speed. This is a scalar quantity, which can be
positive or negative. Negative values mean that the speed is
decreasing and may be called deceleration or retardation. 2.
In scientific study of motion in two or three dimensions acceleration
means rate of change of velocity; a = dv / dt. This is a vector
quantity having magnitude (which is always positive) and
direction. Whenever speed changes (increasing or decreasing), or
direction changes, or both speed and direction change, this is an
(a) A machine for speeding subatomic particles to
high velocity, then colliding them with a stationary target or with
another beam of particles moving in the opposite direction. (In the
latter instance, the machine may be called a collider.) At velocities
approaching that of light the mass of the particles increases dramatically,
adding greatly to the energy released on impact. The resulting explosion
promotes the production of exotic particles, which are analyzed according
to their behavior as they fly away through a particle detector.
(a) Collection of material together, generally to
form a single body.
A disk of gas that accumulates around a center of gravitational attraction, such as a white dwarf, neutron star, or black hole. As the gas spirals in, it becomes hot and emits light or even X-radiation.
The theory by which planetesimals are assumed to collide with one another and coalesce, eventually sweeping up enough material to form the planets.
A subgiant of spectral type B5, about 35 pc distant. ( Eridani)
Asteroid No. 588, a Trojan 60° ahead of Jupiter (P = 11.98 yr, a = 5.2 AU, e = 0.15, i = 10°.3). It was the first Trojan to be discovered (in 1906).
An achromatic lens.
A color that has no hue; i.e. black, white, or gray.
(a) Lens (or combination of lenses) that brings different
wavelenghts within a ray of light to a single focus, thus overcoming
chromatic aberration. It was first successfully made by Joseph von Fraunhofer.
1 P1 + 2 P2 = 0
where 1 and 2 are the dispersive powers of the glasses of the lenses, and P1 and P2 are the powers of the lenses. Achromatic lenses are corrected for chromatic aberration at two different wavelengths. see also Apochromatic Lens
A lens of two or more components with different refraction indices (e.g., crown glass and flint glass), used to correct for chromatic aberration.
Radiation that can cause a chemical reaction; for example, ultraviolet radiation is actinic.
A soft silvery-white radioactive metallic element that
is the first member of the actinoid series. It occurs in minute
quantities in uranium ores. It can be produced by neutron bombardment
of radium and is used as a source of alpha particles. The metal glows
in the dark.
A quantity related to the momentum and position of a body or system of particles. The Principle of Least Action asserts that the integral, or sum of this action, taken over a particular path must be a minimum. This Principle of Least Action can be used instead of Newton's Laws to determine the motion of a system.
A description of a force, such as Newton's law of gravity, in which two separated bodies are said to directly exert forces on each other. In the modern description, the bodies produce a gravitational field, which in turn exerts forces on the two bodies. see Gravitational Field
|Active Galactic Nucleus|
AGN -- An unusually bright galactic nucleus whose light is not due to starlight.
Any galaxy which is emitting large quantities of
Controlling the shape of a telescope mirror at a relatively slow rate.
The Sun during its 11-year cycle of activity when spots, flares, prominences, and variations in radiofrequency radiation are at a maximum. [H76]
Symbol: A For a radioactive substance, the average number of atoms disintegrating per unit time.
The ability of the eye to see separately two points close to each other. It is a measure of the resolving power of the eye's optical system and depends on the density of cells in the retina. The maximum acuity of the normal human eye is around 0.5 minutes of arc - points separated by this angle at the eye should be seen as separate. see Resolution
Compensating for atmospheric distortions in a wavefront by high-speed changes in the shape of a small, thin mirror.
Analog-to-Digital Converter -- An electronic circuit which takes an input voltage in a given range (typically 0-10 volts) and provides a corresponding digital output by setting output lines (bits) high or low. A 16-bit ADC has 16 output lines.
Astrophysical Data Facility, located at the NASA Goddard Space Flight Center (GSFC), is responsible for designing, developing, and operating data systems that support the processing, management, archiving and distribution of NASA mission data. The ADF serves three broadly-defined astrophysics disciplines: high-energy astrophysics, UV/optical astrophysics, and infrared/submillimeter/radio astrophysics. The ADF collaborates with the GSFC Laboratory for High Energy Astrophysics (LHEA) and the Laboratory for Astronomy and Solar Physics (LASP) in managing data for specific missions. The ADF staff also support the astrophysics community's access to multi-mission and multi-spectral data archives in the National Space Science Data Center (NSSDC).
A force of attraction between atoms or molecules of different substances. For example, adhesion between water molecules and glass creates a meniscus.
A change taking place in a system that has
perfect thermal insulation, so that heat cannot enter or leave the
system and energy can only be transferred by work. In practice, a
close approximation to an adiabatic change can be achieved by the
process being too rapid for significant heat transfer, or by the large
scale of the system (e.g. a large volume of air in the atmosphere).
where K1, K2, and K3 are constants and is the ratio of the principal specific heat capacities. Compare isothermal change.
A method of producing temperatures close to absolute zero. A sample of a paramagnetic salt is cooled in liquid helium in a strong magnetizing field. The sample is then thermally isolated by pumping away the helium, and the magnetic field is removed. The sample demagnetizes itself at the expense of its internal energy so that the temperature falls. Temperatures of the order of a millikelvin can be obtained.
|Adiabatic Index ()|
The ratio of the fractional change in pressure to the fractional change in density as an element of fluid expands (or contracts) without exchange of heat with its surroundings.
Fluctuations in both the matter and radiation density, as though a volume of the Universe were slightly squeezed but allowing no radiation to escape. Prior to the Decoupling Era, adiabatic fluctuations behaved like waves, on scales smaller than the horizon size. After decoupling, gravitational instability sets in on scales above about 1013 M, smaller adiabatic fluctuations having been damped at earlier eras.
A process in which a layer of atoms or molecules of one substance forms on the surface of a solid or liquid. All solid surfaces take up layers of gas from the surrounding atmosphere. The adsorbed layer may be held by chemical bonds (chemisorption) or by weaker van der Waals' forces (physisorption). Compare absorption.
Analog-to-Digital Units see DN
|Advance of the Perihelion|
The slow rotation of the major axis of a planet's orbit in the same direction as the revolution of the planet itself, due to gravitational interactions with other planets and/or other effects (such as those due to general relativity).
The transfer of matter such as water vapor or heat through the atmosphere as a result of horizontal movement of an air mass.
|Aeon (or Eon)|
In astronomical terms, 1,000 million years.
A stony meteorite, composed primarily of silicates. About 93 percent of all known falls are aerolites. They include the carbonaceous chondrites, other chondrites, and achondrites. (lit. "air stone.")
An image seen after the eye's retina has been exposed for a time to an intense or stationary light source. It may be negative or positive, or appear in complementary colors.
(1) In Aristotelian physics, the fifth element, of which the stars and planets are made. (2) In Classical physics, an invisible medium that was thought to suffuse all space.
|Age of the Universe|
The time elapsed since the singularity predicted by the Big Bang theory, estimated to be around 13 billion years.
Astronomical Image Processing System -- National Radio Astronomy Observatory
Light in the nighttime sky caused by the collision of atoms and molecules (primarily oxygen, OH, and Ne) in Earth's geocorona with charged particles and X-rays from the Sun or outer space. The airglow varies with time of night, latitude, and season. It is a minimum at zenith and maximum about 10° above the horizon. (also called nightglow)
|Airy Diffraction Disk|
The central spot in the diffraction pattern of the image of a star at the focus of a telescope. Named for Sir George Airy (1801-1892), seventh Astronomer Royal.
(a) The ratio of the amount of light reflected from
a surface to the amount of incident light.
Art of bringing parts of the Universe to the perfect state toward which they were thought to aspire - e.g., gold for metals, immortality for human beings.
( Tau) -- The brightest star in the Pleiades (spectral type B5).
Tau) -- (a) The brightest star in the constellation Taurus, Aldebaran
is an orange K-type giant that lies 60 light-years away.
Explanation of the big-bang theory in terms of nuclear physics, proposed by Ralph Alpher, Hans Bethe and George Gamow in 1948; it was later slightly corrected by Chushiro Hayashi.
A dimensionless number characterizing steady fluid flow past an obstacle in a uniform magnetic field parallel to the direction of flow. It has a partial analogy to the Mach number. The Alfven number is given by vl( µ)1/2 B-1/2 where v is the velocity of flow, l is length of obstacle, is density, µ is permeability and B is magnetic flux density. It is named after H. 0. G. Alfven (1908- ), the Swedish astrophysicist and Nobel Prize winner, who introduced the term magnetohydrodynamics.
A cosmological model in which the early Universe is depicted as a giant collapsing spherical cloud of matter and antimatter. When a critical density is reached, the matter and antimatter begin to annihilate, the resulting release of radiation and energy causing the Universe to expand. There are many difficulties with this model of the expanding Universe, which is largely discredited on observational grounds.
The speed at which hydromagnetic waves are propagated along a magnetic field: (VA) = B / (4 )1/2. [H76]
Waves moving perpendicularly through a magnetic field. They are caused by the oscillation of magnetic lines of force by the motions of the fluid element around its equilibrium position, which in turn is caused by the interactions between density fluctuations and magnetic variations.
Per) (a) The most famous eclipsing binary, Algol was probably the first
variable star discovered. It lies in the constellation Perseus and consists
of two stars that orbit each other every 2.87 days. When one star passes
in front of the other, the light of the system dims.
In a discrete Fourier transform, the overlapping of replicas of the basic transform, usually due to undersampling.
The existence of a solid substance in different physical forms. Tin, for example, has metallic and non-metallic crystalline forms. Carbon has two crystalline allotropes: diamond and graphite.
Arabic title for Ptolemy of Alexandria's Syntaxis, the writings in which he combined his own astronomical researches with those of others. Although much of the work is inaccurate even in premise, until Nicolaus Copernicus published his results fourteen centuries later the Almagest remained the standard reference source in Europe.
The nucleus of a 4He atom, consisting of two protons and two neutrons. Mass of -Particle 4.00260 amu.
Nuclei formed by the -process. see Even-Even Nuclei
A hypothetical process of nucleosynthesis (now considered obsolete terminology), which consisted of redistributing -particles in the region from 20Ne to 56Fe (and perhaps slightly higher). The -process has been replaced by explosive and nonexplosive C, O, and Si burning occurring in rapidly evolving or even explosive stages of stellar evolution which at higher temperatures and densities becomes the e-process.
(a) Bright binary star in which both components contribute
to a magnitude of -0.27: it is also the nearest of the bright stars
(at a distance of 4.3 light years).
(a) A type of radioactive decay in which the unstable nucleus emits a helium nucleus. The resulting nuclide has a mass number decreased by 4 and a proton number decreased by 2. An example is:
88226 Ra 86222 Rn + 24 He
The particles emitted in alpha decay are alpha particles. Streams
of alpha particles are alpha rays or alpha radiation.
They penetrate a few centimeters of air at STP or a metal foil of mass/area
a few milligram/cm2. see also Beta Decay
Particles first discovered in radioactive decay, and later identified as helium nuclei (two protons and two neutrons bound together).
Aql) (a) A bright (mv = 0.78) A7 V star about 4.8
(a) Comprising a means of measuring or precisely
locating in coordinates the position of objects at any altitude or azimuth.
The term is now used mainly to describe a type of mounting for a telescope.
(a) Angular distance above the horizon.
A soft moderately reactive metal. Aluminum has the
electronic structure of neon plus three additional outer
electrons. There are numerous minerals of aluminum; it is the most
common metallic element in the Earth's crust (8.1% by weight) and the
third in order of abundance.
|am||Abbreviation for the Latin Ante Meridiem (before noon).|
A unit of molar volume at 0° C and a pressure of 1.0 atmosphere. This unit varies slightly from one gas to another, but in general it corresponds to 2.24 × 104 cm3. Also, a unit of density equal to 0.0446 gram mole per liter at 1 atm pressure.
Jupiter V, the innermost satellite of Jupiter. Diameter about 140 km; i = 0°.4, e = 0.0028, period 0.498 days. Discovered by Barnard in 1892. (also called Barnard's satellite)
A highly toxic radioactive silvery element of the
actinoid series of metals. A transuranic element, it is not found
naturally on Earth but is synthesized from plutonium. 241Am
has been used in gamma-ray radiography.
Denoting a solid that has no crystalline structure; i.e. there is no long-range ordering of atoms. Many substances that appear to be amorphous are in fact composed of many tiny crystals. Soot and glass are examples of truly amorphous materials.
The SI unit of electric current. "The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 × 10-7 newton per meter of length" (CIPM , Resolution 2, approved by the 9th CGPM 1948). A current of 1 A is equivalent to the passage along the filament of a light bulb of about 6 × 1018 electronic charges per second.
A device that increases an electrical signal applied
to it as an input. If the input is an alternating voltage, the output
voltage has a similar waveform with an increased amplitude.
(a) The maximum value of a varying quantity from
its mean or base value. In the case of a simple harmonic motion - a
wave or vibration - it is half the maximum peak-to-peak value.
(AM) A type of modulation in which the
amplitude of a carrier wave is modulated by an imposed signal, usually
at audio frequency.
A device for determining the plane of polarization of plane-polarized radiation. Maximum intensity is transmitted if the plane is parallel with the analyzer's direction of polarization; the intensity is a minimum (theoretically zero) if the two are perpendicular. For visible radiation, analyzers are usually Polaroid sheets or Nicol prisms.
The difference in magnification along the spectrum and perpendicular to the spectrum in a spectrograph.
A lens designed so as to minimize its astigmatic aberration. Anastigmatic lenses have different curvatures in different directions; the surface of an anastigmatic lens is part of a toroid.
A constellation near Perseus and Pegasus.
|Andromeda Galaxy (M31)|
(a) Major spiral galaxy, 2.2 million light-years
from Earth. Gravitationally bound to the Milky Way galaxy with which
it shares membership in the Local Group, it is currently approaching
us, rather than receding as is the case for most galaxies.
|Andromeda I, II, III|
Three dwarf spheroidal galaxies, in the Andromeda subgroup of the Local Group, discovered by van den Bergh in 1972. They are among the intrinsically faintest members of the Local Group.
A supernova seen in 1885 in the Andromeda Galaxy.
(a) A unit that measures the wavelength of light
and equals 0.00000001 of a centimeter. Blue light has a wavelength of
about 4400 angstroms, yellow light 5500 angstroms, and red light 6500
The unit was introduced by the International Union for Solar Research in 1907. It was named after A. J. Ångström (1814-1874), the Scandinavian scientist who used units of 10-10 m to describe wavelengths in his classical map of the Solar spectrum made in 1868. The ångström was not confirmed as a unit of length by the International Congress of Weights and Measures until 1927. For over half a century the ångström was equal to 1.0000002 × 10-10 m but when the metre was defined in terms of the wavelength of krypton in 1960 the ångström became equal to 10-10 m exactly. The ångström is sometimes called a tenth metre.
X-ray wavelengths are often given ångström stars (Å*). This unit was devised by J. A. Bearden in 1965 and is based on the wavelength of the K1 line of tungsten which he took to be 0.2090100 Å*, where Å* = 1.00001481 Å = 1.00001481 × 10-10 m.
Symbol: The rotational acceleration of an object about an axis:
= d / d t or = d2 / d t2
Here is angular velocity; is angular displacement. Angular acceleration is directly analogous to linear acceleration, a.
Symbol: The rotational displacement of an object about an axis. If the object (or a point on it) moves from point P1 to point P2 in a plane perpendicular to the axis, is the angle P1OP2, where O is the point at which the perpendicular plane meets the axis.
The rate of change of angle (due to refraction or diffraction) with wavelength of the emergent beam in a spectrograph.
(Pulsatance) Symbol: The number of complete rotations per unit time. A simple harmonic motion of frequency f can be represented by a point moving in a circular path at constant speed. The foot of a perpendicular from the point to a diameter of the circle moves backward and forward along the diameter with simple harmonic motion. The angular frequency of this motion is 2f, where f is the frequency. The unit is the hertz.
(a) The angular momentum of a system about a specified
origin is the sum over all the particles in the system (or an integral
over the different elements of the system if it is continuous) of the
vector products of the radius vector joining each particle to the origin
and the momentum of the particle. For a closed system it is conserved
by virtue of the isotropy of space.
The angle subtended by an object on the sky. For example, the angular size of the moon is 30 arcminutes.
A system whose vibration, while still periodic, cannot be described in terms of simple harmonic motions (i.e. sinusoidal motions). In such cases, the period of oscillation is not independent of the amplitude.
see Mixmaster Model
A system of fermions in which Cooper pairs form in a state of finite relative orbital motion and possibly finite total spin.
(a) A medium is anisotropic if a certain physical
quantity differs in value in. different directions. Most crystals are
anisotropic electrically; important polarization properties result from
differences in transmission of electromagnetic radiation in different
A reaction between a particle and its
antiparticle; for example, between an electron and a positron. The
energy released is equal to the sum of the rest energies of the
particles and their kinetic energies. In order that momentum be
conserved two photons are formed, moving away in opposite
directions. This radiation (annihilation radiation) is in the
gamma-ray region of the electromagnetic spectrum. The quantum energy
is about 0.51 MeV.
The direction and strength of the Earth's magnetic field at any point changes with time. This must be allowed for by navigators. One such change is a variation with a period of a year, but there are others. The amplitude of the annual variation is greatest during maximum sun-spot activity.
An eclipse of the Sun in which the Moon is too far from Earth to block out the Sun completely, so that a ring of sunlight appears around the Moon.
The interval (27.555 days) between two successive perigee passages of the Moon.
The interval (365.2596 ephemeris days) between two successive perihelion passages of Earth.
The refractive index of a transparent medium normally increases as the wavelength is reduced. There is then a range of wavelengths (usually in the ultraviolet) in which the radiation is absorbed fairly strongly. Such little radiation as is transmitted in this region shows anomalous dispersion, that is the refractive index decreases as the wavelength is reduced. see Dispersion
An increase in volume resulting from a decreased temperature. Most liquids increase in volume as their temperature rises. The density of the liquid falls with increased temperature. Water, however, shows anomalous behavior. Between 0 and 4°C the density increases with increasing temperature.
|Anomalous Zeeman Effect|
Splitting of spectral lines into several components, in contrast to the normal Zeeman effect which results in only two distinct components. The anomalous Zeeman effect is due to the fact that the electrons in the magnetic field have opposite directions of spin.
An angular value used to describe the position of one member of a binary system with respect to the other. The true anomaly of a star is the angular distance (as measured from the central body and in the direction of the star's motion) between periastron and the observed position of the star. The mean anomaly is the angular distance (measured in the same manner) between periastron and a fictitious body in the direction of the star, which is moving in a circular orbit with a period equal to that of the star.
(a) The "handles", or extremities, of Saturn's
rings as viewed from
An old name for an RR Lyrae star.
The direction in the sky (in Columba) away from which the Sun seems to be moving (at a speed of 19.4 km s-1) relative to general field stars in the Galaxy.
Sco) (a) A red supergiant star in the constellation Scorpius. Antares
is the brightest star in Scorpius and lies about 500 light-years from
Earth, on the inner edge of the Orion spiral arm.
The part of a radio telescope responsible for detecting an electromagnetic wave. (or Aerial)
A measure of the directivity of a radio telescope. It is the ratio of the amount of power received in the direction the dish is pointing to the smaller amount of power from other directions in the sidelobes.
A term used to describe the strength of a signal received from a radio source. It is the convolution of the true brightness distribution and the effective area of the antenna.
A famous pair of interacting galaxies in the constellation Corvus. Each galaxy's tidal force has drawn out a long tail of stars from the other. The Antennae are also known as NGC 4038 and NGC 4039.
(a) The doctrine that the value of certain fundamental
constants of nature can be explained by demonstrating that, were they
otherwise, the Universe could not support life and therefore would contain
nobody capable of worrying about why they are as they are. Were the
strong nuclear force slightly different in strength, for instance, the
stars could not shine and life as we know it would be impossible.
The belief that humans are central to the Universe.
The projection of human attributes onto nonhuman entities such as animals, the planets, or the Universe as a whole.
The antiparticle of a baryon.
The direction of the sky (in Auriga) opposite to that toward the center of the Milky Way.
A particle counter in which the circuit has been designed so as not to register the passage of an ionizing particle through more than one counting tube.
a solid in which the spins of neighboring atoms are oppositely aligned. The lattice is composed of two equivalent sublattices, and on each sublattice the spins are magnetized, as in a ferromagnet, but the directions of the magnetisations are opposed so that there is no net magnetisation.
A kind of magnetism found in many solids at low temperatures. The molecular magnets form two arrays, aligned antiparallel. At the lowest temperatures there are equal numbers with equal magnetic moments in opposite directions, giving zero resultant magnetization. As the temperature is raised, the susceptibility increases up to the Néel temperature above which the substance is paramagnetic.
(a) For every variety of particle there exists an
antiparticle with opposite properties such as sign of electrical charge.
When a particle and its antiparticle meet they can mutually annihilate
and produce energy. Thus, antiquark, antiproton, etc.
A metalloid element existing in three allotropic
forms; the most stable is a brittle silvery metal. It is used in
alloys - small amounts of antimony can harden other metals. It is also
used in semiconductor devices.
The antiparticle of a neutrino.
The antiparticle of a neutron. A neutron and antineutron both have the same mass and zero electric charge, but can be differentiated by their interactions: a neutron and an antineutron can annihilate into gamma rays, while two neutrons cannot.
(a) An elementary particle of opposite charge but
otherwise identical to its partner. Most of the observable Universe
consists of particles and matter, as opposed to antiparticles and antimatter.
The antiparticle of a proton, identical in mass and spin but of opposite (negative) charge.
The antiparticle of the quark.
Also AR coating. A layer of material of lower refractive index of just the right thickness (1/4 wave) is deposited on the optical surface to be coated. More complex coatings are possible which cover a large wavelength range.
The point in the orbit of one component of a binary system where it is farthest from the other.
(a) The effective diameter of the primary mirror
or lens of a telescope.
aperture = d/f
Thus a 50-mm camera lens may be used with an aperture diameter of 12.5
mm. Then, aperture = 12.5/50. This is usually described with the
f-number. In this case the aperture diameter is f/4,
often written as f4.
The ratio of the effective aperture of the antenna, A, to its geometric aperture, Ag = d2 / 4. The beam and aperture efficiencies are related by A = B 2 / AgM, where M is the solid angle of the main beam.
In radio astronomy, a distribution of direction assignments applying to a uniform background.
Usually refers to magnitude measurements made from digital images by deriving the flux that would have been recorded within a circular aperture large enough to enclose the star's seeing disk.
The ratio of the aperture of a telescope to the focal length.
The method of combining the signals received by several smaller telescopes distributed over a very large area or baseline to provide the angular resolution of a much large telescope. Used extensively in radio astronomy, e.g. the VLA.
see Solar Apex
The point in a planetary orbit that is at the greatest distance from the Sun.
A lens designed so as to minimize both its astigmatic and coma aberration.
A system of three lenses which, taken together, correct for spherical aberration, chromatic aberration, and coma.
The point in the orbit of one component of a binary system which is farthest from the center of mass of the system.
A lens designed to correct for chromatic aberration at three different wavelengths. Apochromatic lenses are constructed of three or more kinds of glass. They thus have better correction than achromatic lenses, which correct at two different wavelengths (usually in the red and blue regions of the spectrum). see Achromatic Lens
A mathematical process performed on the data received from an interferometer before carrying out the calculations of the Fourier transformation to obtain the spectrum, in order to modify the instrumental response function.
The point at which a body in orbit around the Earth reaches its farthest distance from the Earth.
One of a small group of asteroids whose orbits intersect that of Earth. They are named for the prototype, Apollo (P = 622d, a = 1.486 AU, e = 0.57, i = 6°.4).
|Apollo Space Program|
Successful US lunar exploration program in which the Apollo spacecraft 1 to 6 were unmanned; 7 to 10 were manned but did not land; and 11, 12 and 14 to 17 landed and returned safely. (Apollo 13 was an aborted mission.) The first men to land on the Moon were Neil Armstrong and Edwin Aldrin, from Apollo 11, on 20 July 1969. The final Apollo flight (17) lasted from 7 to 19 December 1972, and left a considerable quantity of exploratory devices on the lunar surface.
The point in a star's orbit farthest from the Galactic center.
Because radiation travels at different speeds in
different media, the apparent depth or thickness of a transparent
sample is not the same as its real depth or thickness. The effect is
very obvious when one looks down into a glass of water or a clear
pool. It is associated with the fact that a long object partly
submerged in water seems bent at the water surface.
refractive index = real depth / apparent depth
The relation is used in a number of methods for finding the refractive constant of a transparent medium. It applies to all wave radiations, not just to visible radiation.
(a) A measure of how bright a star looks in the
sky. The brighter the
star, the smaller the apparent magnitude. A star that is one magnitude
brighter than another (e.g., +1 versus +2) looks 2.5
times brighter. The brightest star of all, of course, is the Sun,
whose apparent magnitude is -26.74, followed by Sirius, whose apparent
magnitude is -1.46, Canopus (-0.72), Alpha Centauri (-0.27), Arcturus
(-0.04), and Vega (+0.03). Stars of the Big Dipper are fainter, most
of them around magnitude +2. On a clear, dark night, the unaided eye
can see stars as faint as apparent magnitude +6, and the largest
telescopes penetrate to apparent magnitude +30. [C95]
The position on a celestial sphere, centered at the Earth, determined by removing from the directly observed position of a celestial body the effects that depend on the topocentric location of the observer; i.e., refraction, diurnal aberration. see Aberration, Diurnal), and geocentric (diurnal) parallax. Thus the position at which the object would actually be seen from the center of the Earth, displaced by planetary aberration (except the diurnal part - see Aberration, Planetary; Aberration, Diurnal) and referred to the true equator and equinox.
|Apparent Solar Day|
Interval between two successive culminations of the Sun - i.e., the period from apparent noon to apparent noon. The apparent Solar day is longest in late December.
|Apparent Solar Time|
The measure of time based on the diurnal motion of the true Sun. The rate of diurnal motion undergoes seasonal variation because of the obliquity of the ecliptic and because of the eccentricity of the Earth's orbit. Additional small variations result from irregularities in the rotation of the Earth on its axis.
The period during which a celestial body is visible.
(F-layer) The upper of the two main layers in the ionosphere, at a height above about 150 km. It reflects radio waves. see Ionosphere
A penumbral eclipse of the Moon.
Rotation of the line of apsides in the plane of the orbit; (in a binary) precession of the line of apsides due to mutual tidal distortion.
|Apsides, Line of|
The major axis of an elliptical orbit.
One sixtieth of a degree of angular measure. The Moon is 31 arcminutes across.
Making a permanent record which can be accessed later at any time.
One sixtieth of an arcminute, or 1/3600 of a degree. Jupiter is 40 arcseconds across.
The spectra of neutral atoms produced in a laboratory arc (cf. spark spectra).
Boo) (a) A beautiful orange star that is the brightest in the constellation
Bootes and the fourth brightest in the night sky. It lies 34 light-years
away and is a member of the thick-disk population. Historically, Arcturus
is famous because it was one of the first stars to have its proper motion
|Areas, Law of|
see Kepler's Second Law
a diagram in which the length and phase-angle of a complex quantity is displayed.
A method of classifying stars according to image size. If the sequence stars are labeled a, b, etc., in order of image size and if the image size of a variable appears to be, say, 0.7 of the way from sequence star a to sequence star b, its brightness is listed as a7b. (also called the step method )
An inert colorless odorless monatomic element of the
rare-gas group. It forms 0.93% by volume of air. Argon is used to
provide an inert atmosphere in electric and fluorescent lights, in
welding, and in extracting titanium and silicon. The element forms no
|Argument of the Perihelion ()|
Angular distance (measured in the plane of the object's orbit and in the direction of its motion) from the ascending node to the perihelion point.
Second satellite of Uranus about 1600 km in diameter, discovered by Lassell in 1851. Period 2.52 days.
Physics as promulgated by Aristotle; includes the hypothesis that our world is comprised of four elements, and that the Universe beyond the moon is made of a fifth element and so is fundamentally different from the mundane realm.
Young stars typical of those found in spiral arms (Population I stars).
Ancient Greek, Arabic and medieval alt-azimuth device, comprising a calibrated ring fixed in the meridian plane, within which a second concentric ring, also calibrated, was mobile around a vertical axis.
In organic chemistry, carbon which has bonds that are between single and double bonds (e.g., the molecule has a bond with a delocalized electron.)
In radio astronomy, an arrangement of antenna elements designed to produce a particular antenna pattern.
|Arrow of Time|
The direction, apparently inviolable, of the "flow" of time that distinguishes the past from the future.The direction, apparently inviolable, of the "flow" of time that distinguishes the past from the future.
A toxic metalloid element existing in several
allotropic forms; the most stable is a brittle gray metal. It is used
in semiconductor devices, alloys, and gun shot.
Advanced Satellite for Cosmology and Astrophysics
In the orbit of a Solar-System body, the point where the body crosses the ecliptic from south to north: for a star, out of the plane of the sky toward the observer.
A faint glow from the unlit side of Venus when it is in the crescent phase. Its cause is unknown; it may be the Venusian analog to terrestrial airglow.
Magnitudes expressed as the inverse hyperbolic sine (or ``asinh''), sometimes referred to informally as luptitudes. The transformation from linear flux measurements to asinh magnitudes is designed to be virtually identical to the standard astronomical magnitude at high signal-to-noise ratio, but switches over to linear behavior at low S/N thereby accommodating even negative values of flux, where the logarithm, as used in the Pogson magnitude, fails. [sinh-1(x) = asinh(x)*ln(x+SQRT(x2+1))]
Application Specific Integrated Circuit
The apparent position of any of the planets or the Moon relative to the Sun, as seen from Earth.
Ratio of the major axis (e.g., of a rocket) to the minor axis. (Of a fusion device) ratio of the plasma diameter to the major diameter of the torus.
An optical surface with departures in shape from a perfect sphere in order to cancel optical imperfections or aberrations.
A sparsely populated grouping (mass range 102-103 M) of very young, massive stars lying along a spiral arm of the Milky Way, whose spectral types or motions in the sky indicate a common origin. The star density is insufficient for gravitation to hold the group together against shear by differential galactic rotation, but the stars have not yet had time to disperse completely. OB associations are composed of stars of spectral types O-B2; T associations have many young T Tauri stars. The internationally approved designation for associations is the name of the constellation followed by an arabic numeral - e.g., Perseus OB2.
A radioactive element belonging to the halogen
group. It occurs in minute quantities in uranium ores. Many
short-lived radioisotopes are known, all alpha-particle emitters.
(a) A small rocky body that orbits a star. In the
Solar System, most asteroids lie between the orbits of Mars and Jupiter.
The largest asteroid is Ceres, about 900 kilometers in diameter.
A region of space lying between Mars (1.5 AU) and Jupiter (5.2 AU), where the great majority of the asteroids are found. None of the belt asteroids have retrograde motion.
(a) A common eye defect in which the observer cannot
focus clearly on objects at any distance. The cause is usually a non-spherical
cornea. Visual astigmatism may be corrected with a lens with a suitable
degree of cylindrical curvature.
The processing of matter through stars.
(a) Ancient Arabic and medieval alt-azimuth device
comprising two or more flat, metal, calibrated discs, attached so both
or all could rotate independently. For early navigators and astronomers
it acted as star-chart, compass, clock and calendar.
(a) The belief that human affairs and people's personalities
and characters are influenced by (or encoded in) the positions of the
see Binary System
An ephemeris of a Solar-System body in which the tabulated positions are essentially comparable to catalog mean places of stars at a standard epoch. An astrometric position is obtained by adding to the geometric position, computed from gravitational theory, the correction for light-time. Prior to 1984, the E-terms of annual aberration (see Aberration, Annual; Aberration, E-terms of) were also added to the geometric position.
The branch of astronomy that deals with measuring the positions of celestial objects, especially stars. Astrometrists measure parallaxes and proper motions, which allow astronomers to determine the distances and velocities of the stars.
|Astronomical Color Index|
Difference in a star's brightness when measured on two selected wavelengths, in order to determine the star's temperature. Cooler stars emit more light at longer wavelengths (and so appear redder than hot stars). Modern methods involve photoelectric filtering and the UBV system.
The longitude and latitude of a point on the Earth relative to the geoid. These coordinates are influenced by local gravity anomalies. see Zenith; Longitude, Terrestrial; Latitude, Terrestrial)
The period from sunset to the time that the Sun is 18° below the horizon; or the corresponding period before sunrise.
(a) Mean distance between the Earth and the Sun:
The science that studies the natural world beyond the earth.
(a) The science that studies the physics and chemistry
of extraterrestrial objects. The alliance of physics and astronomy,
which began with the advent of spectroscopy, made it possible to investigate
what celestial objects are and not just where they are.
The negative of the mean V velocity of a stellar population. In general, the older the stellar population, the more negative the V velocity and therefore the greater the asymmetric drift. The young thin disk has an asymmetric drift of 0 kilometers per second, whereas the halo has an asymmetric drift of 200 kilometers per second.
A violation of symmetry.
|Asymptotic Branch (AGB) Stars|
Globular cluster stars, which are found in that part of the HR diagram that connects the top pf the giant tip with the horizontal branch.
(a) A term used to describe the observed decrease
in the intrinsic strength of the color force between quarks as they
are brought closer together. At asymptotically small separations, the
quarks are virtually free. This is in contrast to the electromagnetic
force whose intrinsic strength increases as two charged particles approach
Acronym for After The Bang; usually used in reference to time elapsed since the big bang.
(a) Mantle of gases round a star planet or moon,
sometimes even forming the apparent surface of the body. For a body
to retain an atmosphere depends on the body's gravity, and the temperature
and composition of the gases. The atmosphere of the Earth is, by volume,
78% nitrogen and 21% oxygen (with 1% of other gases); mean atmospheric
pressure at the surface is 10,330 kg/m2, and is also referred
to as atmosphere.
The gaseous outer layers of the Sun, including, from the deeper layers outward, the photosphere, the chromosphere, and the corona. The atmosphere constitutes those layers of the Sun that can be observed directly.
|Atmospheric Dispersion Corrector|
An optical device usually comprising two thin prisms which can rotate to compensate for the elongation of a star image caused by the wavelength dependence of the refractive index of air.
Decrease in the intensity of light from a celestial body due to absorption and scattering by the Earth's atmosphere. The extinction increases from the zenith to the horizon and affects short wavelengths more than long wavelengths, so that objects near the horizon appear redder than they are at the zenith.
(a) The smallest component of matter that retains
its chemical properties. An atom consists of a nucleus and at least
Individual hydrogen atoms that do not belong to molecules. In its neutral form (HI), atomic hydrogen consists of a proton and an electron and generates radio waves that are 21 centimeters long. In its ionized form (HII), atomic hydrogen is simply a proton. HII regions look red because a few of the protons capture electrons, which can radiate red light as they settle into position around the protons.
|Atomic Mass Number|
The total number of protons and neutrons in an atom's nucleus. For example, oxygen-16 has a mass number of sixteen, because it has eight protons and eight neutrons.
|Atomic Mass Unit|
(a) A unit of mass used for atoms and molecules,
equal to 1/12 of the mass of an atom of carbon-12. It is equal to 1.660
33 × 10-27 kg.
In 1960 the International Union of Pure and Applied Physics followed the proposal made a year earlier by the International Union of Pure and Applied Chemistry that all atomic weights should be based on the 126C scale. This enabled more isotopes to have integral mass numbers than would have been possible on the oxygen scale. This reduced the values given on the previous chemical scale by a factor of 1.000043 and changed the values of the Faraday and Avogadro's constant by a similar amount. Values on the physical scale can be converted to the new scale by multiplying by 0.999685.
(a) The number of protons in an atom's nucleus. This
determines the type of element. For example, hydrogen has an atomic
number of one, so all hydrogen atoms have one proton; helium has an
atomic number of two, so all helium atoms have two protons; and oxygen
has an atomic number of eight, so all oxygen atoms have eight protons.
Representation of the electron cloud surrounding an atom. Named by primary quantum number and shape (i.e., 1s, 2s, 2p).
see Second; Systeme International
The theory that matter is made up of atoms that combine to form molecules. Each chemical element has a particular type of atom, which may join with like atoms to form molecules of the element, or with atoms of other elements to form molecules of a compound. The atom consists of a dense positively charged nucleus containing protons and neutrons, surrounded by electrons. The number of protons in the nucleus determines the number and distribution of the electrons, which are held by the positive charge of the nucleus. Because the outer electrons form the chemical bonds between atoms, the chemical properties of an element depend on the electronic structure of the atom, and therefore also on the number of protons. The number of neutrons in the nucleus may vary, forming different isotopes of an element. These cannot usually be separated by chemical means.
Time based on the atomic second (see Second). Atomic time was officially adopted 1972 January 1. From 1972 January to 1974 January 1, 3 leap seconds had to be introduced to keep atomic time within 0.7 seconds of Universal Time.
The mean atomic mass of a particular element in atomic mass units.
The fundamental units of a chemical element. An atom consists of a nucleus, which may contain protons and neutrons, and electrons, which occupy shells that surround the nucleus and are centered on it.
(a) The reduction of intensity of a
radiation as it
passes through a medium. It includes reductions due to both absorption
In a rocket-borne or satellite-borne telescope, the ratio of the expected counting rate to the observed counting rate.
Position of a rocket with respect to the horizon or some other fixed reference plane.
(a) A mechanical system may be such that its dynamical
evolution causes it to approach a stable end-state. In the phase space
representing the system, the representative point tends to a fixed set
of points called an attractor. The attractor may be a point, a line,
or a fractal.
A prefix meaning 10-18.
Having a stellar spectral type of A, that is, hot and white, like Sirius and Vega.
(a) The ejection of an electron from an atom or ion
without the emission of radiation (x-rays or gamma rays). It results
from the de-excitation of an excited electron within the atom. It can
be regarded as the internal conversion of the photon that would otherwise
have been emitted.
The amount by which the apparent semidiameter of a celestial body, as observed from the surface of the Earth, is greater than the semidiameter that would be observed from the center of the Earth.
An O9.5 V runaway star.
An eclipsing binary with an invisible supergiant companion. The primary is an extremely luminous A8 Ia supergiant of 30 M in a post-main-sequence stage of evolution; the secondary may be a collapsed star or black hole. The period of the system is about 27 years. Probably on the order of 1 kpc distant. It has at least six components.
A dG5e T Tauri star with a strong ultraviolet excess.
In general, binaries with a K supergiant primary and a main-sequence secondary.
(a) Spectacular array of light in the night sky,
caused by charged particles from the Sun hitting the Earth's upper atmosphere.
The aurora borealis is seen in the north of the Northern hemisphere;
the aurora australis in the south of the Southern.
the ability of certain chemicals to enhance by their presence the rate of their own production in a sequence of chemical reactions. Part of the more general class of feedback processes.
(a) The spontaneous ionization of excited atoms,
ions, or molecules, as in the Auger effect. see Auger Effect; Ionization
A process such as that in which a single ionization leads to a large number of ions. The electrons and ions produced ionize more atoms, so that the number of ions multiplies quickly.
see Mean Life
(a) Symbol: NA number of particles
in one mole of a substance. Its value is 6.002 52 × 1023.
Equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules. It is often called Avogadro's hypothesis. It is strictly true only for ideal gases.
(6.02 × 1023): The number of atoms in 12 grams of 12C; by extension, the number of atoms in a gram-atom (or the number of molecules in a mole) of any substance.
The fact that galaxies appear to "avoid" the Milky Way, and are most numerous in other parts of the sky. When galaxies were known as spiral nebulae and their nature was not yet understood, avoidance was thought by some researchers to indicate a connection between them and the Milky Way. Now the effect is understood to be due to dark clouds of dust and gas in our galaxy, which obscure our view of the Universe beyond in those quarters of the sky.
Advanced X-ray Astronomical Facility
A hypothetical spin-0 particle with a very small mass of 10-5-10-3 eV. It was postulated in order to provide a natural solution to the "strong CP problem".
Theoretical straight line through a celestial body, around which it rotates.
Collapse of mass in such a way that the mass maintains the symmetry of a cylinder.
Aseotrope: A mixture of two liquids that boils without any change in composition. The proportions of components in the vapor are the same as in the liquid. Azeotropic mixtures cannot be separated by distillation.
(a) Directional bearing around the horizon, measured
in degrees from north (0°).
|Azimuthal Quantum Number (k)|
A measure of the minor axis of an elliptic orbital of an electron according to the Bohr-Sommerfeld theory.