Fr.: boson Z
Z Canis Majoris (Z CMa)
Fr.: Z CMa
A luminous → FU Orionis object consisting of two young stars in a → binary system, a → Herbig Be star embedded in a dust cocoon and a less massive component located 0.1 arcsecond south-east. It is associated with a very large-scale and high-velocity → bipolar outflow extending in total 3.6 pc (for a distance of 1150 pc) with radial velocities up to -620 km/sec. The outflow is traced by a → jet and at least 15 → Herbig-Haro objects. In 2008 Z CMa showed the largest "outburst" ever reported in the past 90 years.
→ Canis Major.
Fr.: source Z
adad-e atomi (#)
Fr.: nombre atomique
Same as → atomic number.
Fr.: méthode de Zanstra
The method of using the nebular observations to estimate the stellar ultraviolet radiation and the temperature of the central star in a planetary nebula. The basic assumptions are that the flux from a star could be approximately represented by the Planck function and that the nebula absorbs all the ultraviolet photons from the star which can cause ionization. For each ultraviolet photon absorbed an Hα photon is emitted when the ionized hydrogen subsequently recombines with an electron. Thus the strength of the Hα line is related to the ultraviolet flux of the star. However, modern theoretical work on stellar atmospheres shows that there are important deviations between the emergent fluxes from stars and Planck functions. Moreover, some of the stellar ultraviolet photons may be missed.
Named after the Dutch astrophysicist Herman Zanstra (1894-1972), who first introduced the method in 1927.
Fr.: effet Zeeman
The splitting of spectral lines into closely-spaced components when the radiating substance is placed in a strong → magnetic field. Distinction is made between the normal and → anomalous Zeeman effects, and also the longitudinal and transverse Zeeman effects. In the normal → longitudinal Zeeman effect each spectral line is split into two components with frequencies ν ± Δν. In the normal → transverse Zeeman effect un un-displaced line is observed along with a doublet, i.e. three lines in all, with the frequencies ν and ν ±Δν. In the classical theory of the normal Zeeman effect, the motion of an electron in an atom is regarded as the harmonic oscillation of a linear harmonic oscillator. Arbitrary linearly polarized oscillation of the electron can be resolved into two oscillations: one along the magnetic field and the other in a plane perpendicular to this field. The latter can be further resolved into two oscillations, circularly polarized with opposite directions of rotation that occur in the Larmor precession frequency. Classical theory cannot explain the anomalous Zeeman effect. Both effects are accounted for in quantum mechanics as the result of changes in the energy levels of atomic electrons due to the interaction of their → orbital angular momentum and → spin angular momentum with each other and with the external magnetic field. See also → inverse Zeeman effect.
Named after Pieter Zeeman (1865-1943), Dutch physicist who discovered the phenomenon; → effect.
The point on the celestial sphere directly above the observer's head, opposite to the direction in which gravity acts. Opposite of → nadir.
M.E. cenith, from O.Fr. cenith, from M.L. cenit, senit,
incorrect transliteration of Ar. samt
durâ-ye sarsu, duri-ye ~
Fr.: distance zénithale
The angular distance of a celestial body from the zenith. The zenith distance is 90° minus the body's altitude above the horizon (i.e. the complement of the altitude) and hence is also known as coaltitude.
teleskop-e sarsu, dvrbin-e ~
Fr.: télescope zénithal
A → telescope that is mounted on a → vertical axis or moves only a small amount from the vertical. It is primarily used to determine positional measurement of stars moving near the → zenith. The advantage is that there is no → atmospheric refraction occurring at the zenith. If a star on one night passes through the center of eyepiece, one must observe it six month later, and see if the star has been offset by the center. A shift would mean a measure of parallax. See also: → zenithal well.
Of or relating to the → zenith; located at or near the zenith.
Fr.: puit zénithal
1) A well used in Antiquity from bottom of which the sky could be observed
during the day with a better contrast. The aperture of the well reduced the
light diffused by the sky.
A prefix of the → SI units, denoting 10-21. Symbol z.
From L. septem, → hepta-, since it is equal to 1/10007.
1) The cardinal number of a class that has no members.
From Fr. zéro, from It. zero, from M.L. zephirum, from Ar.
zero age horizontal branch star (ZAHB)
setâre-ye šâxe-ye ofoqi bâ senn-e sefr
Fr.: étoile de la branche horizontale d'âge zéro
zero age main sequence (ZAMS)
rešte-ye farist bâ senn-e sefr
Fr.: séquence principale d'âge zéro
The position on the → Hertzsprung-Russell diagram for newborn stars which have just started → hydrogen burning in their cores. The ZAMS forms the lower luminosity boundary of of the → main sequence strip. See also → terminal age main sequence (TAMS).
Fr.: matrice nulle
Same as → null matrix.
Fr.: méthode de zéro
same as → null method.
Fr.: point zéro
A basic parameter on which a → photometric system relies and must be determined each night. The zero points of magnitudes and color indices depend on atmospheric transparency that varies from night to night.
zero point energy
kâruž-e noqte-ye sefr
Fr.: énergie du point zéro, point zéro
The lowest possible energy that a → quantum mechanical system may possess. It is the energy of the → ground state of the system. The term "zero point" refers to the observed fact that → vacuum fluctuations persist at → absolute zero temperature. Same as → vacuum energy.
Fr.: polynôme nul