An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

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.

→ zenith; → hour; → rate.

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.

→ zenith; → telescope

Of or relating to the → zenith; located at or near the zenith.

→ zenith; → -al.

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.
2) A vertical tunnel in → Paris Observatory (built in 1667), from the top roof to the underground vaults, 55m deep. This well had the purpose of observing the stars near the → zenith and measure their → parallaxes resulting from the Earth motion around Sun. A long plumb line was attached to the top of the well. Astronomers thought they could measure the stellar shifts with respect to the plumb line. The problem was, however, the lack of stability of the images, because the well acted in fact as a chimney generating turbulence. So that the zenithal well was hardly used. See also: → zenith telescope.

→ zenithal; → well.

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.
2) The only integer that is neither negative nor positive.
3) The → identity element for addition.

From Fr. zéro, from It. zero, from M.L. zephirum, from Ar. sifr (صفر) "cipher," translation of Skt. śūnya "empty place, void, naught." The rules governing the use of zero appeared for the first time in the Indian mathematician Brahmagupta's book Brahmasputha Siddhanta "The Opening of the Universe," written in 628.

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.

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.

Same as → null matrix.

→ zero; → matrix.

same as → null method.

→ zero; → method.

1) General: A starting point for making a measurement.
2) A basic parameter on which a → photometric system relies and must be determined each night. The zero points of → magnitudes and color indices (→ color index) depend on atmospheric transparency that varies from night to night.

→ zero; → point.

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.

→ zero; → point; → energy.

A → polynomial whose → coefficients are all zero. See also → non-zero polynomial.

→ zero; → polynomial.

The removal of non-significant zeroes from a number. For example, replacing 531.2300 by 531.23.

→ zero; → suppression.

In the → restricted three-body problem, a surface which limits the region of space in which a small body can move. In the expression for the → Jacobi integral, the left side value is always positive or nul; hence the particle motion is confined to the region where U ≤ C_J. The surface that limits this region, defined by U = C_J, is called the zero-velocity surface.

→ zero; → velocity; → surface.

Two objects that are in → thermal equilibrium with a third object will be in thermal equilibrium with each other.

→ zero; → law; → thermodynamics.

A blue star, also called HD 149757 and HR 6175, which is the nearest, and probably the most widely studied, → massive star. It is variable in several wavelength bands and has a mean visual magnitude of V = 2.58, B - V = 0.01. It lies ~ 222 pc away and has a formal spectral type of O9.5 Ve and a luminosity of 10⁵ Lsun. ζ Ophiuchi is a very rapid rotator with a v sin i ~ 400 km s^-1, i.e. ~ 85% of the → break-up velocity. It is one of the earliest prototypes of the → Be phenomenon. Moreover, it shows episodes of Hα emission variability, a common feature of Be/Oe stars. It also shows periodic non-radial pulsations and UV → P Cygni profile variability, as is evident in the periodic behavior of → discrete absorption components (DACs). It has a → mass loss rate of 10^-7 Msun yr^-1 and a → terminal velocity of wind v_∞ = 1550 km s^-1. ζ Ophiuchi is a well-known → runaway star with a velocity of 30 km s^-1. The interstellar → CH molecule and → CN molecule were first detected toward ζ Ophiuchi. It has been recognized for some time that this star lies close to the blue edge of the → beta Cephei instability strip.

Zeta (ζ), according to the → variable star designation system; Ophiuchi, → Ophiuchus.

Same as → Alnitak.

Zeta (ζ), Gk. letter in the → Bayer designation scheme.

A prefix of the → SI units, denoting 10²¹. Symbol: Z.

From L. septem, → hepta-, by replacing the initial s with z to distinguish from → zepto-. It is equal to 1000⁷.

One of several regions of the stellar interior where increased → opacity can provide the necessary → valve mechanism to drive → stellar pulsations. Also called → partial ionization zone.

Named after the Russian astronomer Sergei A. Zhevakin (1916-2001), who identified these zone in the 1950s; → mechanism.