Fr.: lacune DB
The observed absence of helium white dwarfs with temperatures between 45,000 and 27,000 K. A possible explanation lies in the chemical evolution of white dwarfs. Four processes can change the structure of a white dwarf: gravitational settling, interstellar medium accretion, mass loss, and subsurface convective mixing.
DB white dwarf
sefid kutule-ye DB
Fr.: naine blanche DB
A → white dwarf whose spectrum shows strong He I in the absence of hydrogen or metal lines.
DC white dwarf
sefid kutule-ye DC
Fr.: naine blanche DC
A → white dwarf showing a continuous spectrum with no readily apparent lines.
de Broglie equation
hamugeš-e de Broglie
Fr.: équation de de Broglie
According to the → de Broglie hypothesis,
which has been verified by experiments, every → particle
of matter, whatever its nature, has a characteristic → wavelength
associated with its wavelike quantum aspect. The de Broglie equation gives the
equivalent wavelength of a moving particle: λ = h/mv, where h is
→ Planck's constant, m the mass of the particle,
and v its velocity.
Named after Louis Victor de Broglie (1892-1987), French physicist, creator of a new field in physics, wave mechanics, who won the Nobel prize in physics in 1929. → equation
de Broglie hypothesis
engâre-ye de Broglie
Fr.: hypothèse de de Broglie
The suggestion by Louis de Broglie in 1924 whereby if → electromagnetic waves possess particle properties (→ particle nature), then it might be reasonable to suppose that material particles, such as → electrons, should possess wave properties (→ wave nature). The de Broglie hypothesis was based on the intuitive feeling that nature seems to have strong attachment to symmetry. In other words, if radiation has particle-like properties, then material particles should possess wave-like properties. At the time no direct experimental evidence was present for the validity of this suggestion. The first confirmation of de Broglie's hypothesis was provided by the → Davisson-Germer experiment. See also → wave-particle duality;
de Broglie wavelength
mowjtul-e de Broglie
Fr.: longueur d'onde de Broglie
de Sitter Universe
giti-ye de Sitter
Fr.: Univers de de Sitter
A solution to → Einstein's field equations of → general relativity which contains no ordinary matter (ΩM = 0) or radiation (ΩR = 0), is → Euclidean (k = 0), but has a → cosmological constant (ΩΛ > 0). The Universe expands exponentially forever. This solution was the first model expanding of → expanding Universe. See also → empty Universe, → Milne Universe.
After the Dutch mathematician and physicist Willem de Sitter (1872-1934) who worked out the model in 1917; → universe.
de Vaucouleurs law
qânun-e de Vaucouleurs
Fr.: loi de Vaucouleurs
de Vaucouleurs radius
šo'â'-e de Vaucouleurs
Fr.: rayon de Vaucouleurs
After the French-born American astronomer Gérard de Vaucouleurs (1918-1995); → radius.
vâ- (#), foru- (#)
A prefix occurring in loanwords from L. to indicate:
M.E., from O.Fr. de-, des-, partly from L. de- "from, down, away," and partly from L. → dis-.
Prefix vâ- denoting "reversal, opposition; separation; repetition; open;
variant of bâz-, from Mid.Pers. abâz-, apâc-;
O.Pers. apa- [pref.] "away, from;" Av. apa- [pref.] "away, from,"
apaš [adv.] "toward the back;" cf. Skt. ápāñc
Transition from an excited energy state to a lower energy level, as in spectral line formation or particle emission from an atomic nucleus.
To cause a spacecraft to leave its operational orbit to enter a descent phase or to change course.
Of a spacecraft, the act or process of departing from an operational orbit. → de-orbit.
Noun form of → de-orbit.
A method for estimating the real orientation of a field and/or related velocities/separations from two-dimensional images.
Fr.: magnitude dérougie
A magnitude which has been corrected for the interstellar reddening.
The process that de-reddens. The state of being de-reddened.
Verbal noun of → de-redden.
1) Deprived of life.
M.E. deed, O.E. dead "dead;" cf. O.S. dod, Dan. død, Swed. död, Du. dood, O.H.G. tot, Ger. tot; PIE *dhou-toz-, from base *dheu- "to die."
Mordé "dead," p.p. of mordan, mir- "to die," → death.
Fr.: pixel mort
Of a → CCD detector, a pixel that is not sufficiently active.