| E component | see L Component. [H76] | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| e-Folding Time | The time within which the amplitude of an oscillation increases or decreases by a factor e (e = 2.718...). [H76] | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| E galaxy | In both Hubble's and Morgan's classifications, an elliptical galaxy. [H76] | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| E layer | The part of Earth's ionosphere (about 150 km) where the temperature gradient reverses and starts to rise. It reflects "short-wave" radio waves. (also called Kennelly-Heaviside Layer) [H76] | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| E+A Galaxy | One of the most enigmatic types of galaxy,
spectrally, are the "E+A"
galaxies. They appear to be a combination of an old elliptical galaxy
spectrum (E) with that of a significantly younger A-star population
(A), thought to be formed in a recent (t < 1 Gyr) episode of star
formation. "E+A" galaxies show strong Balmer absorption lines
(EW A Fraunhofer line at 5270 Å. It is a
blend of Fe I and Ca I. [H76]
A hypothetical group of nuclear reactions by which the
iron group is assumed to be synthesized. At temperatures > 5 ×
109 K and densities > 3 × 106 g cm-3
there are great numbers of collisions between high-energy photons and
nuclei. These collisions break up the nuclei, the fragments of which
promptly combine with other particles. Thus, there is in effect an
equilibrium between formation and breakup. Since the iron group has
the largest binding energies, the particles over the long run will
tend to be trapped in these nuclei. The e-process (the e
stands for equilibrium) is presumed to occur in a supernova
explosion. [H76]
see Be Stars. [H76]
In Hubble's classification, a spiral with a large
nuclear bulge and closely coiled arms. [H76]
Hot stars of spectral types O, B, A, and early
F. [H76]
Third planet from the Sun. Mass 5.977 ×
1027 g; polar
radius 6356.9 km; equatorial radius 6378.17 km; mean density 5.517 g
cm-3; effective temperature 287 K. Rotation period
23h56m4s.1. Mean distance from Sun
149,598,500 km (8.3 lt-min): perihelion distance (early January)
147,100,000 km; aphelion distance 152,100,000 km; vorb
29.78 km s-1; orbital period 365d.2564; e =
0.0167, i = 0; obliquity (1973) 23°26'34". Albedo 0.39
(water and land about 0.2; snow and clouds about 0.8). Surface gravity
980 cm s-2; Vesc 11.19 km
s-1. Precession 50'.256 per year; relativistic advance of
perihelion 4'.6 per century. Atmosphere (by volume) 78% N2;
20.9% O2; 0.9% Ar, 0.03% C02. Particle density
of Earth's atmosphere at sea level 1019 per cm3
= 1.3 × 10-3 g cm-3. Atmospheric pressure at sea
level 1.013 × 106 dyn cm-2.
Magnetic field at surface, about 0.5 gauss; in core, about 100
gauss. Core temperature about 6400 K; core density about 10 g
cm-3. Age 4.6 ± 0.1 × 109 years. First
forms of life appeared about 3.2 to 3.5 × 109 years ago
(Homo sapiens appeared as a species about 105 years
ago). [H76]
Light reflected from the Earth's atmosphere onto
the dark part of the Moon. [H76]
The point on the celestial horizon 90°
clockwise from the
north point. At the equinoxes the Sun rises in the east point. [H76]
Electron-Bombarded CCD An imaging device
containing a thin
target material which emits electrons by the photoelectric effect when
illuminated and then magnetically focuses these electrons to impact
onto a silicon CCD where they generate a large charge. [McL97]
Electron Bombarded Silicon [LLM96]
An orbit that has a high eccentricity that is, highly
elliptical. [C95]
In undisturbed elliptic motion, the angle measured
at the center of the ellipse from pericenter to the point on the
circumscribing auxiliary circle from which a perpendicular to the
major axis would intersect the orbiting body. (see Mean Anomaly; True
Anomaly.) [S92]
(a) In astronomy, the extent to which an
elliptical orbit
departs from a circular one. It is usually expressed as a decimal
fraction, regarding a circle as having an eccentricity of 0. [A84]
In Ptolemaic cosmology, displacement of the center
of a rotating celestial sphere from the center of the Universe. [F88]
A type of diffraction grating with groove angles
of 90°. With the grating at an angle of 45° the grooves
resemble a staircase. [McL97]
A type of diffraction grating consisting of a
number of
equal thin glass sheets stacked on a slant. In use the light is
reflected from the stepped side of the stack; d in the grating
equation is very large, so that very high spectral orders are
possible. [DC99]
(a) Occultation of one celestial body by another
which passes
between it and the observer. The Solar eclipse is caused by the
passing of the Moon between the Sun and the Earth in this way; such an
eclipse may be complete (total) or incomplete (partial). Eclipsing
binary stars also accord with this pattern. Alternatively - and
exceptionally - a lunar eclipse is caused by the passage of the Earth
between the Sun and the Moon, so that the Earth's shadow falls across
the Moon, again either totally or partially, depending upon the
position of the observer. [A84]
A Solar eclipse (see Eclipse, Solar) in which the
Solar disk is never completely covered but is seen as an annulus or
ring at maximum eclipse. An annular eclipse occurs when the apparent
disk of the Moon is smaller than that of the Sun. [S92]
An eclipse in which the Moon passes through the shadow
cast by the Earth. The eclipse may be total (the Moon passing
completely through the Earth's umbra), partial (the Moon passing
partially through the Earth's umbra at maximum eclipse), or penumbral
(the Moon passing only through the Earth's penumbra). [S92]
An eclipse in which the Earth passes through the
shadow cast by the Moon. It may be total (observer in the Moon's
umbra), partial (observer in the Moon's penumbra), or annular. (see
Eclipse, Annular.) [S92]
The interval of time (346.62 days) between two
successive passages
of the Sun through the same node of the Moon's orbit. [H76]
(a) A binary star of which, from the viewpoint of
Earth,
one of the two bodies regularly passes in front of the other. The
resulting variation is perceived luminosity of some eclipsing binaries
has led to their classification as variable stars. [A84]
(a) Apparent linear path through the 12
constellations of the
zodiac that the Sun seems to take during one Earth year, also
representing therefore the "edge" of the plane of Earth's
orbit. Because the equator of the Earth is at an angle of more than
22° to the plane of its orbit, the ecliptic is at the identical
angle to the celestial equator, intersecting it at two points: the
vernal and autumnal equinoxes. [A84]
An approximation used in the study of radiative
transfer. It is
the assumption that the ratio of the second moment of the radiation
field to the mean intensity is everywhere equal to 1/3, the value of
this ratio for an isotropic field. [H76]
A cosmological model in which the
cosmological constant plays a crucial role by allowing an initial
phase that is identical to the Einstein static Universe. After an
arbitrarily long time, the Universe begins to expand. The difficulty
with this model is that the initiation of galaxy formation may
actually cause a collapse rather than initiate an expansion of the
Universe. [Silk90]
In essence, radiation pressure must not exceed
gravity. It is the
limit beyond which the radiation force on matter in
the emitting region is greater than the gravitational forces that
hold the star together. LE = 4 A stellar model in which energy is transported by
radiation
throughout the whole star and the ratio of the radiation pressure to
the gas pressure is assumed to be constant. [H76]
Induced currents set up in a conductor by a
changing magnetic field. They occur in transformers and other
electrical devices. The currents produce a heating effect
corresponding to a loss of useful energy (eddy-current
loss). Metal cores in electrical machines are usually laminated
(built of thin sheets) to reduce such losses; the surface layers
between the laminations have high electrical resistance.
Absorption in the spectra of galaxies at the edges of
some passbands by lines broadened by velocity dispersion. [H76]
The distance from the center of a galaxy within
which half of the total luminosity is included (cf. Holmberg radius). [H76]
The temperature that a blackbody would
have which emitted the same amount of energy per unit area as
the star does: it is a temperature characteristic of the surface
region. Teff of the Sun is 5800 K. [H76]
Each part of the physical world can be described by a sub-theory that
applies over a certain distance scale or energy scale. Such sub-theor
ies are called effective theories. Explanations in a given effective
theory can ignore much of the rest of the world, which has effects on
the part of inter est through a few inputs or parameters. Every part
of our description of the physical world is an effective theory,
except the ultimate theory that is called the Primary Theory. [K2000]
The wave functions corresponding to the
eigenvalues. Eigenfunctions represent the stationary states ("standing
waves") of a system. [H76]
(a) The eigenvalue of a matrix M is a
number classification scheme for elementary particles
established c. 1960. Forerunner of quark model. [D89]
An approximation in which the oscillation of
a wave front is replaced by the direction of the ray which is
normal to the oscillation. [H76]
An emission (or absorption)
coefficient. Aji is the
coefficient of spontaneous emission; Bji, is the
coefficient of stimulated emission, where i is the lower level
and j is the upper level. [H76]
(a) A Friedmann model of the Universe in which the
spacetime continuum is not curved. [C97]
Displacement of spectral lines due to the
gravitational redshift. [H76]
The equations of Einstein's theory of gravity,
called general relativity. The Einstein equations quantitatively
specify the gravity produced by matter and energy. Since gravity is
believed to be the principal force acting over very large distances,
the Einstein equations are used in modern theories of
cosmology. [LB90]
Foundation for curvcd space-time, it
states that bodies fall with the same acceleration and that physics in
freely falling reference frames is independent of the velocity and
location of the frames. [D89]
The theory of gravity in
which the gravitational force is described mathematically by a
curvature in space or space-time. [D89]
A cosmological model in which a static
(neither expanding nor collapsing) Universe is maintained by
introducing a cosmological repulsion force (in the form of the
cosmological constant) to counterbalance the gravitational
force. [Silk90]
A world model of a static Universe with a positive
cosmological constant, whose radius of curvature is constant
and independent of time. [H76]
A radioactive transuranic element of the actinoid
series, not found naturally on Earth. It can be produced in milligram
quantities by bombarding 239Pu with neutrons to give
253Es (half-life 20.47 days). Several other short-lived
isotopes have been synthesized.
Upper boundary layer within which the amplitude
changes exponentially. [H76]
A collision between two particles which conserves
the total kinetic energy and momentum of the system. For atomic
collisions it is one involving energy less than the excitation
potential of the atom. [H76]
Particle reactions in which the same
particles emerge from the
reaction as entered it (e.g. Also called gates or phases. Small electrically
conducting
plates connected to a voltage source (battery or power supply) and
arranged in strip patterns to define the picture elements or pixels of a
CCD. The plates create an electric field within the semiconductor which
therefore forms a storage site for photo-generated charges. Also used as
a generic term for any conductor with an applied voltage. [McL97]
Study of the behavior of electromagnetic force in
motion. [F88]
Force field of the electromagnetic force,
consisting of electric and magnetic lines of force at each point in
space. [G99]
(a) Fundamental force of nature that acts on all
electrically charged
particles. Classical electromagnetics is based on Maxwell's and
Faraday's equations, quantum electromagnetics on the theory of quantum
electrodynamics (QED). [F88]
Gauge symmetry underlying quantum
electrodynamics. [G99]
EMU A system of electrical units based on the
electromagnetic properties of an electric current. [H76]
(a) "Waves" of electrical and magnetic
"disturbance", radiated as visible light, radio waves, or any other
manifestation of the electromagnetic spectrum. The distance between
successive crests of each wave is known as the wavelength, and varies
considerably between electromagnetic forms. The velocity of such
radiation in a vacuum is the speed of light. The units of
electromagnetic radiation are quanta or photons ("packets" of
energy). [A84]
(a) Complete range of electromagnetic radiation,
from very short-wavelength (high-frequency) gamma-rays, through X-rays
and ultraviolet light to the small range of visible light, and further
to infrared radiation, microwave, and the comparatively
long-wavelength low-frequency radio waves. [A84]
A pattern of electric and magnetic fields that
moves through space. Depending on the wavelength, an electromagnetic
wave can be a radio wave, a microwave, an infrared wave, a wave of
visible light, an ultraviolet wave, a beam of X rays, or a beam of gamma
rays. (see Photon) [G97]
(a) One of the four fundamental forces of nature,
governing the electric and
magnetic interaction between particles. [C97]
(a) Negatively charged fundamental particle (also
called a beta
particle) found in the atoms of all elements, where it "orbits" (at
different energy levels and with different directions of spin) round
the central nucleus. The combined charge of the orbiting electrons is
balanced (in a neutral atom) by the charge of an equal number of
positively charged protons in the atomic nucleus. An electron is also
the fundamental unit of electricity. [A84]
(a) The work needed to remove an electron from
a negative ion and move it to infinity.
Short for
electron-positron collider. One import ant way to study particle
interactions and search for new particles is to accelerate an electron
and a positron to high energies and then collide them, using a
detector to study what emerges. The energy to which they are
accelerated is chosen to f it the question of interest. For example,
to study CP violation in b-qua rk decays, the energy is chosen to
maximize the production of b's in an appropriate way, whereas to
produce new heavy particles, the energy i s made as large as
possible. All uses of electron colliders require ver y large
luminosity (intensity). [K2000]
A process in astrophysics occurring in highly
ionized stellar interiors where the
density is high, whereby the bulk of the energy is transported by
"hot" electrons moving in one direction and cooler electrons in the
other. In degenerate matter electron conduction, not radiation, is the
main mechanism of energy transport. [H76]
a system of electrons whose mutual interactions are
sufficiently weak that they can be regarded as moving independently,
subject only to the effects of the exclusion principle. [D89]
When a photon is absorbed in silicon its energy
causes an electron in the valence band to be ejected into the conduction
band leaving a (positively charged) vacancy or hole in the valence
band. [McL97]
Electron scattering by ions oscillating
about equilibrium positions which form a perfect lattice. [H76]
Zones in which the electrons in atoms reside. Their
radius is determined by the quantum principle, their population by the
exclusion principle. [F88]
The temperature that appears in the Maxwell
distribution of velocities for electrons. [H76]
Single fundamental force thought to have
functioned in the very early Universe and to have combined the
attributes thereafter parceled out to the electromagnetic and the
strong and weak nuclear forces. (see Grand Unified Theory) [F88]
(a) Unit of energy. Typically 1-10 eV is the amount
of energy per atom involved in chemical reactions. 1 eV is the energy
gained when an electron is accelerated by a potential of one volt. [D89]
A unit of charge defined as the charge which
exerts a force of 1 dyne on a charge of equal magnitude at a
distance of 1 cm. [H76]
(a) The combination of the electromagnetic force and
the weak nuclear force which takes place at high energy. [C97]
The unified description of the weak interactions and
electromagnetism, developed between 1967 and 1970 by Sheldon Glashow,
Steven Weinberg, and Abdus Salam. [G97]
(a) Theory
demonstrating links between the electromagnetic and the weak nuclear
forces. Indicates that in the high energies that characterized the
very early Universe, electromagnetism and the weak force functioned
as a single, electroweak force. Also known as the Weinberg-Salam
theory. [F88] Different elements are distinguished by the number of
protons in their nuclei. All hydrogen atoms have one proton; all
helium atoms have two protons; all oxygen atoms have eight
protons. [C95]
Quantities tabulated for the calculation of accurate
predictions of an eclipse or occultation for any point on or above
the surface of the Earth. [S92]
(a)
Parameters that specify the position and motion of a body in
orbit. (see Osculating Elements; Mean Elements) [S92]
The angle in
degrees above the horizon toward the zenith or overhead
point. Sometimes loosely called the "altitude" of a star, but not to
be confused with height above sea level. Elevation angle is 90°
minus the zenith distance (or zenith angle). [McL97]
Promising higher-dimensional supergravity theory
developed in the 1970s, subsequently ignored, and more recently shown
to be an important part of string theory. [G99]
A plane curve
in which the sum of the distances of each point along its periphery
from two points - its "foci" - are equal. [F88]
(a) A galaxy that looks round or
elliptical. One example is M87, in the constellation Virgo. [C95]
(b)
A galaxy without spiral arms and with an ellipsoidal
shape. Ellipticals have little interstellar matter and no blue giants
- the only giants are red, and they give ellipticals a slightly
redder color than spirals. The most massive galaxies known (about
1013 M A
quantitative measure of the shape of a galaxy. A completely spherical
galaxy has zero ellipticity. A galaxy shaped like a cigar has a very
high ellipticity. [LB90]
The instants when the geocentric angular
distances of Mercury and Venus are at a maximum from the Sun. [S92]
(a) The geocentric angle between a planet and the
Sun, measured in the plane of the planet, Earth and Sun. Planetary
elongations are measured from 0° to 180°, east or west of
the Sun. [S92] The geocentric angle between a satellite and
its primary, measured in the plane of the satellite, planet and
Earth. Satellite elongations are measured from 0° east or west
of the planet. [S92]
Eggen,
Lynden-Bell and Sandage An influential paper published in 1962 by
Olin Eggen, Donald Lynden-Bell, and Allan Sandage, who argued that
the Galaxy formed from a single huge cloud of gas that rapidly
collapsed. [C95]
The
reappearance of a celestial body after eclipse or occultation. [H76]
The process
of transition of an electron from an outer orbit to an inner orbit
around the nucleus results in a characteristic amount of energy being
radiated (as line emission) that corresponds to the lost energy of
the electron. [Silk90]
Radiant flux emitted per unit volume per unit
solid angle. [H76]
Bright
lines produced in a spectrum by a luminous source, such as a star or
a bright nebula. Compare absorption lines. [F88]
The product of the square of the
electron density times the linear size of the emitting region (in
parsecs). [H76]
An HII region whose spectrum consists of emission
lines. [H76]
A spectrum consisting of emission lines produced
in the laboratory by a glowing gas under low pressure. [H76]
A measure of the efficiency of a source to radiate
like a perfect black body; 0% is perfectly black and 0% is perfectly
reflecting. [McL97]
An emphasis on sense data as a source of knowledge, in
opposition to the rationalist belief that reasoning is superior to
experience. [F88]
Third satellite of Saturn, about 500 km in
diameter. Orbital
period 1.37 days. Discovered by Herschel in 1789. [H76]
The comet with the shortest known period (3.30 years)
(a = 2.21 AU, e = 0.847, i =
12°.4). It has been observed at
every apparition since its discovery in 1819. Its period is gradually
decreasing. Named after J. F. Encke, who computed its
orbit. (It was discovered by Pons.) [H76]
(a) Gap within Saturn's Ring A. [A84]
see Gravitational Encounter. [H76]
A process in which some of the energy of the
incoming particle is
transferred to the nucleus. [H76]
An adjective applied to a reaction in which a net
input of energy is required for the reaction to occur. [H76]
(1) The capacity to do work. (2) Manifestation of
a particular
variety of force. [F88]
a continuous range of energies in a solid in which
there are possible states for the electrons. Energy bands are separated
from one another by energy gaps. [D89]
A plot of the intensity of the continuous
spectrum versus the wavelength. [H76]
The amount of energy in the form of radiation per unit
volume, expressed in ergs cm-3. The energy density of blackbody
radiation at temperature T is aT4, where the
radiation constant a =
7.56 × 10-15 erg cm-3
(K)-4. [Silk90]
The amount of energy radiated at each range of
wavelengths. [H76]
A range of energies in a solid for which there are no
quantum states of the electrons. [D89]
(a) Any of the several discrete states of energy
in which an
atom or ion can exist. For example, an orbital electron can exist
only in those energy levels that correspond to an integral number of
deBroglie wavelengths in a Bohr atom. [H76]
(In cosmic-ray studies, a plot of number of particles versus energy. [H76]
An average over an ensemble of all possible
systems. [H76]
A hypothetical group of many universes of
varying properties. Some physicists attempt to estimate how "probable"
are the properties of our Universe by imagining it as a sample from
an ensemble of universes. [LB90]
the impossibility of expressing certain quantum
mechanical states of a system with two or more parts as the
conjunction of definite quantum states of the separate parts. [D89]
The heat content of a body. H = U +
pV, where
U is the internal energy, p is the pressure, and
V is the volume. [H76]
The real object or image which defines the limit of
valid light paths through an optical system. [McL97]
(a) A thermodynamic property of a macroscopic body
which corresponds intuitively to the degree of disorder. [D89]
(a) An experiment performed in 1909 by the Hungarian
physicist
Eötvös to establish that the gravitational acceleration of a
body does not depend on its composition - i.e., that
inertial mass and gravitational mass are exactly equal. [H76]
In Morgan's classification, an elliptical galaxy
with dust
absorption. [H76]
The age of the Moon; the number of days since New
Moon, diminished by one day, on January 1 in the Gregorian ecclesiastical
lunar cycle. (see Gregorian Calendar; Lunar Phases) [S92]
(a) A list or tabulation of astronomical phenomena
that change with time. [McL97]
An hour angle referred to the ephemeris
meridian. [S92]
Longitude (see Longitude, Terrestrial) measured
eastward from the ephemeris meridian. [S92]
A fictitious meridian that rotates independently of
the Earth at the uniform rate implicitly defined by Terrestrial
Dynamical Time (TDT). The ephemeris meridian is 1.002738 The length of a tropical second
(1/31,556,925.97474 of the tropical year) on 1900 January 0.5
ephemeris time. [H76]
(a) Time based on the ephemeris second. Ephemeris
time is determined primarily from observations of the Moon
against the background of stars, whereas Universal Time is determined
from observations of the stars and depends on the
Earth's current rate of rotation. [H76]
The passage of a celestial body or point across the
ephemeris meridian. [S92]
(a) Circular orbit of a body round a point that is
itself in a
circular orbit round a parent body. Such a system was formulated to
explain some planetary orbits in the Solar System before they were
known to be elliptical. [A84]
A means of accounting for the apparent motions of the
planets in terms of circular motions in a geocentric cosmology. Each
planet moves in a circle, the center of which moves in a circle of
larger radius, and so on, the largest circles being centered on the
earth. [Silk90]
A thin layer of differently doped semiconductor
used in the
construction of solid-state devices such as the CCD. [McL97]
(a) A point of time selected as a fixed
reference. [H76]
Abbreviation of A. Einstein,
B. Podolsky and N. Rosen, who
presented an argument in 1935 that the quantum mechanical description
of certain composite physical systems cannot be complete. [D89]
Erasable Programmable Read Only Memory A small
silicon chip
containing thousands of individual locations which can be set to either
a low or a high voltage level; a 0 or a 1. The settings can be erased by
exposure to ultraviolet light. [McL97]
A young orange dwarf star in the constellation
Eridanus that is visible to the naked eye and lies just 10.7
light-years away from the Sun. [C95]
An old orange dwarf star in the southern constellation
Indus that lies 11.2 light-years away from the Sun. [C95]
In elliptic motion the true anomaly minus the mean
anomaly. It is the difference between the actual angular position in
the elliptic orbit and the position the body would have if its angular
motion were uniform. [S92]
The Right Ascension of the mean equinox (see
Mean Equator; Equinox) referred to the true equator and equinox;
apparent sidereal time minus mean sidereal time. (see Apparent Place;
Mean Place) [S92]
(a) A relation between the pressure, temperature, and
density of a fluid. [H76]
(a) The difference between Apparent and Mean Solar
Time. At Greenwich, apparent Solar noon varies between
11h44m05s and
12h14m19s. Maximum contribution from
Earth's orbital eccentricity, ~ 8 min; from Earth's obliquity, ~ 10 min.
Apparent and Mean Solar Time agree 4 times a year. [H76]
The great circle on the surface of a body formed
by the
intersection of the surface with the plane passing through the center
of the body perpendicular to the axis of rotation. (see Celestial
Equator.) [S92]
The classic type of telescope mount with one axis
parallel to the Earth's polar axis (i.e. pointing at the celestial pole)
and the other at right angles. Once the object is located. only the
polar axis need be driven by a motor to counteract the Earth's
rotation. [McL97]
A condition of balance between the forces operating on
or within a physical system, so that no accelerated motions exist
among the parts of a system. For stable equilibrium, a small
disturbance will eventually damp out. If a small disturbance
continues to grow, the system is said to be in unstable equilibrium. [H76]
The position of an oscillating body at which no
net force acts on it. [H76]
(a) One of two points in the sky that represent
where the Sun appears
to cross the plane of the Earth's equator. From the Earth's viewpoint
therefore, the Sun reaches one point at a quarter, the other at three
quarters of the way through the sidereal year: the vernal (spring)
equinox is thus on or around 21 March, the autumnal on or around 22
September. The actual points in the sky change slightly every year
through a process called precession. [A84]
(a) If all stars have the same kinetic energy,
equipartition of energy prevails. Because kinetic energy depends on
both a star's mass and its velocity, high-mass stars must move more
slowly than low-mass stars, if equipartition of energy prevails. [C95]
(a) The principle that it is impossible to
distinguish between
gravitational and inertial forces; gravitational
mass is precisely equal to inertial mass. [H76]
A measure of the total amount of energy subtracted
from the continuous spectrum by an absorption line on a graph
of relative intensity versus wavelength. Since the shapes of line
profiles vary - e.g., one may be broad and shallow whereas another is
narrow and deep - measurement is facilitated by transforming each
profile into a rectangle whose base corresponds to
zero intensity and whose area is the same as that of the true
absorption line. [H76]
A system of chronological notation reckoned from a
given date. [S92]
Extremely Red Galaxy
(a) The cgs unit of energy; the work
done by a force of 1 dyne acting over a distance of 1 cm. 1 erg
= 10-7joules = 1 g cm2 s-2. (sometimes
called dyne cm) [H76]
Motion by one or more particles which fills phase
space with uniform density after a sufficiently long time. [H76]
That part of space in which no physical object
can remain
at rest with respect to an observer at infinity; the dragging of
inertial frames is so extreme that all timelike world lines rotate
with the star. Technically, it is the region in which the
asymptotically timelike Killing vector becomes spacelike. [H76]
The region surrounding the event horizon (but
inside the
stationary limit) of a rotating Kerr black hole (see Ergoregion). [H76]
see Achernar. [H76]
An eclipsing binary whose secondary is close to
its Roche limit. [H76]
A fourth-magnitude K2 V star 3.30 pc distant. In
1973 van de
Kamp announced that it has a planet-like object in orbit around
it at a distance of about 8 AU and with a period of about 25
years. [H76]
Extremely Red Object
A nearby triple system, 5 pc distant. Component A
is K0 V; component B is a DA white dwarf; component C is M5e V. [H76]
A small asteroid, No. 433 (axes 35 × 16
× 17 km) whose closest
approach to Earth is less than 0.15 AU. Rotation period
5h16m12s.913, orbital period 642
days, a = 1.48 AU, e = 0.223, i =
10°.8; perihelion distance 1.084 AU. Discovered by
G. Witt in 1898. [H76]
see Violent Galaxy. [H76]
see Cataclysmic Variable. [H76]
European Space Agency [LLM96]
(a) Speed an object must attain in order to free
itself
from returning to the parent body under the effects of gravity. [A84]
European Southern Observatory [LLM96]
European Synchrotron Research Facility [LLM96]
Ephemeris Time [LLM96]
A pulsating star in the constellation Aquila. It was the
first Cepheid variable star discovered, in 1784. [C95]
Essentially an optical filter that operates by
multiple-beam
interference of light reflected and transmitted by a pair of parallel
flat reflecting plates. [McL97]
A system for linking computers with a single
serial cable. [McL97]
The geometry developed by the Greek Euclid about
300 BC. Euclidean geometry, like all geometries, deduces certain
results from a set of starting assumptions. One of the critical
assumptions of Euclidean geometry is that given any straight line and
a point not on that line, there is exactly one line that can be drawn
through that point parallel to the first line. One of the results of
Euclidean geometry is that the interior angles of any triangle sum to
180 degrees. Euclidean geometry is the geometry we learn in high
school. [LB90]
A number used in fluid dynamics defined by
p /
One of the Galilean satellites of Jupiter 3600 km in
diameter. Period 3.55 days, e = 0.00, i =
0°.01, mean density 3.07 g cm-3. [H76]
Extreme UltraViolet [LLM96]
The small irregularity in the Moon's orbital
motion due to Solar and planetary perturbations. [H76]
see 4N Nuclei. [H76]
Nuclei that contain even numbers of protons but
odd numbers of neutrons. [H76]
(a) A happenstance in the spacetime continuum
referenced by three spatial
coordinates and a complementary temporal ordinate. [C97]
(a) The "edge" of a black hole; the interface between
four-dimensional space and a singularity. [A84]
a contingency concerning a system which is either true
or false if it is definite, but which (in view of a fundamental
conceptual innovation of quantum mechanics) may he indefinite. A
near-synonym for this term is `proposition'. [D89]
The radial motion outward (from the central umbra)
of the gases in the penumbral regions of sunspots. [H76]
(a) In Biology the theory that coniplex and
multifarious
living things developed from generally simpler and less various
organisms.
The correlation of particles and spins which is
embodied in a Slater-determinant wave function. [H76]
the spin-dependent part of the interaction
between particles with spin. [D89]
Amount of energy required to bring an electron
from its ground state to a given excited state (measured in electron
volts). [H76]
(a) Pauli's exclusion principle says that there
could not be more than one electron in each quantum state. [D89]
these are the theorems that assert the existence
of mathematical objects satisfying a specific set of axioms. In the
case of differential equations describing the time evolution of a
physical system, they guarantee the existence of solutions to the
equations that become unique if enough initial conditions are
specified. [D89]
A process in which energy is liberated. [H76]
A spiral arm of neutral hydrogen lying between 2.5
and 4 kpc beyond the Galactic center and receding from it at
about 135 km s-1. [H76]
Constant increase, with time, in the distance
separating distant galaxies from one another. Expansion does not take
place within individual galaxies or clusters of galaxies, which are
bound together gravitationally, but evidences itself on the
supercluster level. [F88]
see Violent Galaxy. [H76]
A US series of satellites, many
of which remain in orbit round the Earth fulfilling scientific
functions. Explorer 1 was in fact the first US orbital satellite
(launched on 31 January 1958) and was instrumental in discovering the
inner Van Allen belt. [A84]
A theory of galaxy formation wherein the
explosion of a large number of stars creates a giant shock wave that
travels outward and compresses the surrounding gas. Galaxies form in
the regions of high-density gas. [LB90]
(a) The nucleosynthetic processes which are
thought to occur in supernovae. These explosive processes are
thought to produce the nuclei from neon up to and including the
e-process nuclei and possibly the r-process
nuclei. Explosive carbon burning occurs for a temperature of about 2
× 109 K and a density of 104-107 g
cm-3 and produces nuclei
from neon to silicon. Explosive oxygen burning occurs for a
temperature of about 4 × 109 K and produces nuclei from
silicon to calcium, and the e-process occurs at a temperature
greater than 5 × 109 K and produces the iron peak
nuclei. [H76]
see Cataclysmic Variables. [H76]
(a) Extremely rapid expansion. "Exponential" is a
mathematical term that precisely defines the rate of expansion. For
example, a balloon that doubles its size every second is expanding
exponentially. By contrast, a balloon whose radius is one inch after
one second, two inches after two seconds, three inches after three
seconds, and so on, is expanding linearly with time, rather than
exponentially. According to the inflationary universe model, the early
Universe went through a brief period of exponential expansion, during
which its size increased enormously. [LB90]
A space (and spacetime) dimension that is large and
directly apparent; a dimension with which we are ordinarily familiar, as
opposed to a curled-up dimension. [G99]
A version of the inflationary
Universe theory proposed in 1989 by Paul Steinhardt and Daile La.
Its key new feature was the suggestion that a new field interacts
directly with the gravitational field, causing the strength of gravity
to change with time. This causes the expansion of the Universe to
slow down, allowing the bubbles forming at the end of inflation to
catch up with the expansion and smoothly fill the Universe. [G97]
In radio astronomy, formerly a source whose
angular extent could
be measured, as distinguished from a point source. Now, one which has
a large angular extent and is strongest at longer wavelengths
(distinguished from a compact source). Most extended sources tend to
be polarized. [H76]
(a) Attenuation of starlight due to absorption and
scattering
by Earth's atmosphere, or by interstellar dust. The longer the
path through the dust, and the denser the dust, the more the
starlight is reddened. The normal relation is AV =
0.8 mag per kpc. The total visual extinction toward the galactic
center is on the order of 25 magnitudes. [H76]
The field that deals with objects beyond the
Milky Way, especially galaxies and quasars. [C95]
The set of distances to astronomical
objects outside our galaxy. It is difficult to obtain distances to
objects further than about 10 million light years with accuracies
better than about 25%. [LB90]
Black holes endowed with the maximal amount of
force charge possible for a given total mass. [G99]
A semiconductor, such as silicon, which has been
doped with
impurity atoms to provide smaller energy band gaps for detection of
lower-energy photons. [McL97]
|
cGM/Ks, where
Ks = Thomson and/or Compton scattering
opacity. Eddington limit for a 1 M
star, 1038
ergs s-1. 
which satisfies the
equation
= 
0.
e,
and the electron antineutrino 
of the ascending node, the
argument 
of the perihelion,
and the epoch T (usually the time of perihelion passage). To
determine the orbit of a binary star system in which the mass is not
known, a seventh element, the period, must be established. 
T east of the
Greenwich meridian, where 
Eri
Eri
v2,
where p is pressure,