Fr.: boson Z
An electrically neutral subatomic particle that along with → W boson mediates the → weak nuclear force. Like the photon, the Z boson is its own antiparticle.
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
A member of a class of → low-mass X-ray binary systems containing low-magnetic field → neutron stars. See also → atoll source.
The name derives from the fact that on X-ray → color-color diagrams Z sources usually form a Z shape that is traced on timescales of hours to days; → source
adad-e atomi (#)
Fr.: nombre atomique
Same as → atomic number.
Z, from the German word Zahl "number, numeral, figure," which was used to specify an element's numerical place in the → periodic table; → 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
Sarsu, literally "the way over the head," from sar→ head + su, → direction.
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.
zenith hour rate (ZHR)
nerx- dar sâ'at dar sarsu
Fr.: taux par heure au zénith
The number of → meteors expected to be seen under ideal conditions if the → radiant of the → meteoroid stream is at the → zenith of the → observer.
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.: puits 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
A star that has arrived on the → horizontal branch after leaving the → red giant branch. It begins → helium burning in its core.
→ zero; → age; → horizontal; → branch; → star.
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).
→ zero; → age; → main sequence.
Fr.: matrice nulle
Same as → null matrix.
Fr.: méthode de zéro
same as → null method.
Fr.: point zéro
1) General: A starting point for making a measurement.
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.