DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

detritus
detritus The particulate decomposition or
disintegration products of plankton, including
dead cells, cell fragments, fecal pellets, shells,
and skeletons, and sometimes mineral particles
in coastal waters.
deuterium 2H; the isotope of hydrogen
whose nucleus consists of 1 proton and 1 neutron.
The nucleus is called a deuteron.
deuterium burning The capture of a proton
by a deuteron to produce 3 He (which then
easily burns through to 4 He; see proton-proton
chain). Deuterium burning occurs at the lowest
temperature of any important nuclear reaction,
about 10 6 K. Thus, its onset marks the end of the
protostellar collapse stage of star formation (see
proto-star). It is the only reaction that occurs in
thelowestmassstars, near0.08solarmasses(see
brown dwarf), and it has long ago destroyed all
the deuterium in the sun and other normal stars.
Thus, the presence of deuterium in the Earth and
other planets means they are made of material
that was never inside the sun. Deuterium is produced
only in the early universe (see Big Bang
nucleosynthesis).
de Vaucouleurs’ classification scheme A
classification scheme that refines and extends
the Hubble scheme of classification for galaxies,
introduced by G. de Vaucouleurs in 1959.
de Vaucouleurs’ scheme attempts to account for
the variety of morphologies observed for each
Hubble type. His scheme employs three main
parameters: (1) a refined Hubble type, where
severalintermediatestagesareaddedtotheHubble
sequence, notably E + ,S0 − ,S0 + , which
account for some lenticular features in ellipticals
or weak arms in S0 galaxies, and Sd, Sm,
and Im, which more closely detail the transition
from Sc to Magellanic irregulars; (2) a parameterdescribingthespiraldesign,
asringshaped(r)
or s-shaped (s), or intermediate (rs); and (3) a parameter
designating barred galaxies (SB), nonbarred
(SA), and intermediate (SAB), for galaxies
where the bar is less developed than in classical
Hubble’s barred galaxies. In addition, the
presence of an outer ring or of a ring-like feature
formed by joining spiral arms is indicated with
an uppercase R preceding all other labels. The
de Vaucouleurs classification scheme has been
© 2001 by CRC Press LLC
extensively used in the three editions of the Reference
Catalogue of Galaxies, where de Vaucouleurs’
types are given for several thousand
galaxies. For example, Messier 31, the spiral
galaxy nearest to the Galaxy, and the nearby
spiral Messier 101 are classified as Sb and as
Sc according to Hubble, and as SA(s)b and as
SAB(rs)cd according to de Vaucouleurs.
de Vaucouleurs’ law Empirical law describing
the brightness profile of an elliptical galaxy.
The surface brightness of typical giant elliptical
galaxies, apart from constants, decreases
with radius as
log(r)∝(r/re) −1/4 ,
where re denotes a scaling parameter, the effective
radius, within which half the light of
the galaxy is emitted. de Vaucouleurs’ law applies
more frequently to giant elliptical galaxies;
dwarf elliptical galaxies are often better fitted by
other laws. See elliptical galaxies.
deviative absorption Deviative absorption
ofaradiowaveoccursnearthepointofreflection
in the ionosphere. See critical frequency.
deviatoric strain A state of strain defined
by subtracting one-third of the volumetric strain
from the total strain. If εij is the total strain
tensor, the deviatoric strain tensor is ε ′ ij = εij −
θδij /3, where θ = εii = ε11 + ε22 + ε33, and
δij is the Kronecker delta.
deviatoric stress A state of stress with pressure
subtracted. If σij is the total stress tensor,
the deviatoric stress tensor is σ ′ ij = σij − pδij ,
where p = σii/3 = (σ11 + σ22 + σ33)/3, and
δij is the Kronecker delta.
dew During night or early morning, due to
radiational cooling, water condensed onto objects
near the ground whose temperature has
fallen below the dew point of adjacent air but
still above freezing. Dew often appears in warm
seasons with clear weather and light winds. Although
the amount of water is small, in dry season
and regions it amounts to a significant contribution
to plant growth requirements.