DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

causal boundary (Geroch, Kronheimer and
Penrose, 1972). A boundary construction using
only the causal properties of a space-time.
causal curve In relativity, a time-like or null
curve, i.e., one that can be the history of a particle
moving no faster than the speed of light. See
null vector, time-like vector.
causal future/past The causal future/past
J ± (S) of a set S is defined as the union of
all points that can be reached from S by a
future/past-directed causal curve. See causal
curve.
causality relations A trip from point x to
point y of a causal space is an oriented curve
with past endpointx and future endpointy consistingoffuture-orientedtime-likegeodesicsegments.
A causal trip is similarly defined, with
the time-like geodesic segments replaced by
causal geodesic segments.
1. Pointx chronologically precedes pointy (i.e.,
x≪y) if and only if there exists a trip from x
to y.
2. Pointx causally precedes pointy (i.e.,x≺y)
if and only if there exists a causal trip fromx to
y. For an arbitrary point x of a space-time, the
relation x ≺x holds, since a causal trip may
consist of a single point.
cavity, magnetic A region of weak magnetic
field created by a relatively dense plasma expelling
its field lines. Produced in some distant
barium releases.
CCD Acronym of charge-coupled device,
presently the most widely used detector in optical
astronomy. The CCD is a two-dimensional
detector, like a photographic film or plate. Each
picture element (pixel) of a CCD is a photodiode
where electrons, freed by the incoming
radiation via photoelectric effect, are held in a
positive potential for an arbitrary time (i.e., the
exposure time). At readout time, an oscillating
potential transfers the stored charges from pixel
to pixel across each row of pixels to an output
electrode where the charges are measured. Unlike
photographic plates, CCD possess linearity
of response (i.e., the number of electrons freed is
proportional to the number of photons detected)
© 2001 by CRC Press LLC
celestial sphere
and detection quantum efficiency (i.e., the ratio
between detected and incident photons) which
is very high, close to 100% for red light. The
pixel size can be as small as 15µm × 15µm; arrays
of 2048 ×2048 pixels are among the largest
available CCDs.
cD galaxies Luminous, large-size elliptical
galaxies that are located at the center of dense
clusters of galaxies. The notation “cD” indicates
a cluster D galaxy in the Yerkes classification
scheme. The photometric profile of a
cD galaxy is different from that of other elliptical
galaxies, since there is an excess of light
at large radii over the prediction of the de Vaucouleurs
law. cD galaxies possess a stellar halo
that may extend up to 1 Mpc, exceptional mass
and luminosity, and are thought to result from
multiple merging of galaxies and from cannibalism
of smaller galaxies belonging to the cluster.
An example of a cD galaxy is Messier 87, located
at the center of the Virgo cluster. See de
Vaucouleurs’ law, Yerkes classification scheme
of galaxies.
Celaeno Magnitude 5.4 type B7 star at RA
03 h 44 m , dec +24.17’; one of the “seven sisters”
of the Pleiades.
celerity Phase speed; the speed of a wave
deduced from tracking individual wave crests.
celerity The translational speed of a wave
crest. Given by C=L/T , where C is celerity
in units of length per time,L is wavelength, and
T is period.
celestial equator Extension to the celestial
sphere of the plane of the Earth’s equator; the
set of locations on the celestial sphere that can
appear directly overhead at the Earth’s equator.
See nutation, precession.
celestial poles The points on the celestial
sphere that are directly overhead the Earth’s
poles. See nutation, precession.
celestial sphere The imaginary sphere representing
the appearance of the sky, in which
all celestial objects are visualized at the same
distance from the Earth and at different direc-