DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

comet, artificial
in 2003, rendezvous with Comet Wirtanen in
2112-13, and RoLand (the lander) will make further
measurements on the surface of the comet.
comet, artificial See barium release.
comminution The breaking up and fragmentation
of a rock or other solid. The Earth’s crust
is comminuted on a wide range of scales by tectonic
process. In California the crust has been
fragmented into blocks on scales of microns to
tens or hundreds of kilometers. Fault gouge is an
example of comminution on the smallest scale.
common envelope A binary star system enters
a common envelope phase when one, or
both, of the stars in the binary overfills its Rochelobe
and the cores orbit within one combined
stellar envelope. Common envelope phases occur
in close binaries where the Roche-lobe overflowing
star expands too rapidly for the accreting
star to incorporate the accreting material.
Most common envelope phases occur when a
star moves off the main sequence and expands
toward its giant phase (either in Case B or Case
C mass transfer phases). The standard formation
scenarios of many short-period binary systems
(e.g., low-mass X-ray binaries, double neutron
stars) require a common envelope phase
which tightens the orbital separation and ejects
the common envelope.
common envelope binary Binary stars that
are so close to one another that both fill their respective
Roche surfaces, resulting in a common
envelope that surrounds both stars.
comoving frame In general relativity coordinates
are just labels for space-time points and
have no a priori physical meaning. It is, however,
possible to associate those labels to part
of the matter present in the universe, in which
case one has a material realization of a reference
frame. Since such coordinates follow the
matter in its motion, the corresponding frame is
said to be comoving. A fundamental requisite
is that the trajectories of the objects considered
do not cross at any point; otherwise the map
of coordinates would become singular in such
points. A useful, but not necessary, property
is that those objects interact only gravitation-
© 2001 by CRC Press LLC
ally, so as to track geodesics in the background
spacetime.
compact group of galaxies Isolated group
of galaxies for which the separation between the
galaxies is comparable to the size of the galaxies
themselves; groups of galaxies isolated by
Hickson from the Palomar Observatory Sky Survey
according to three criteria: (1) there are at
least four members whose magnitudes differ by
less than three magnitudes from the magnitude
of the brightest member; (2) if RG is the radius
of the circle on the sky containing all group
members, then the distance to the nearest galaxy
outside the group must be larger than 3 RG (in
other words, the group must be reasonably isolated
and not an obvious part of a larger structure);
(3) the mean surface brightness within RG
should be brighter than 26 mag per square second
of arc, i.e., the group must not contain vast
empty sky areas and hence should be “compact”.
compaction As rocks are buried to a greater
depth in a sedimentary basin, the “lithostatic”
pressure increases. This causes the rock to compact.
The void space, or “porosity” of the rock,
decreases with increasing depth, and the density
increases.
compact steep spectrum radio sources
A class of radio sources which includes radio
galaxies and quasars unresolved at resolution
≈ 2 arcsecs. They are differentiated from
other core-dominated radio sources by showing
a steep radio spectrum. Observations at higher
resolution show that compact steep spectrum radio
sources are either classical lobe dominated
sources whose lobe size is less than the size of
the galaxy or quasars with a core single-side jet
morphology. In both cases, the radio morphology
appears often to be disrupted and irregular.
Compact steep spectrum radio sources are
thought to be young radiogalaxies which are expanding
or, alternatively, radio sources in which
the expansion of the radio plasma is hampered
by interstellar or intergalactic medium.
compensation In geophysics, the positive
mass of major mountain belts is compensated by
the negative mass of crustal mountain roots. The
crustal rocks are lighter than the mantle rocks be-