An Etymological Dictionary of Astronomy and Astrophysics

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.

See also: → boson.

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.

See also: → Canis Major.

A member of a class of → low-mass X-ray binary systems
containing low-magnetic field → neutron stars. See also → atoll source.

See also: 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

Same as → atomic number.

See also: Z, from the German word Zahl “number, numeral, figure,” which was used to specify an element’s numerical place in the → periodic table; → number.

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.

See also: Named after the Dutch astrophysicist Herman Zanstra (1894-1972), who first introduced the method in 1927.

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.

See also: 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.

Etymology (EN): M.E. cenith, from O.Fr. cenith, from M.L. cenit, senit, incorrect transliteration of Ar. samt (سمت) “path, direction,” abbreviation of samt ar-ras (سمت‌الرأس), literally “road above one’s head.”

Etymology (PE): Sarsu, literally “the way over the head,” from sar→ head + su, → direction.

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.

See also: → zenith; → distance

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.

See also: → 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.

See also: → zenith; → telescope

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

See also: → 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.

See also: → zenithal; → well.

A prefix of the → SI units, denoting 10^-21. Symbol z.

See also: 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.

See also: 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.

See also: → 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).

See also: → zero; → age; → main sequence.

Same as → null matrix.

See also: → zero; → matrix.

same as → null method.

See also: → 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.

See also: → 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.

See also: → zero; → point; → energy.

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

See also: → zero; → polynomial.

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

See also: → 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.

See also: → zero; → velocity; → surface.

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

See also: → 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.

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

Same as → Alnitak.

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

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

See also: 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.

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

The generic name applied to books in Arabic and Persian that tabulate parameters used for astronomical calculations of positions of the Sun, the Moon, and the five planets of antiquity. Some examples: Zij al-Sindhind, by Khwarizmi (c. 780-850), Az-Zij as-Sabi by al-Battâni (Albatenius) (853-929), Zij-i Ilkhâni by Nassireddin Tusi (1201-1274), Zij-i Sultâni by Ulugh Brg (1437), → Toledan Tables, → Alfonsine Tables.

See also: The word is derived from Mid.Pers. zig, variant zih “cord, string” (Mod.Pers. zeh “cord, string”); Av. jiiā- “bow-string,” cognate with Skt. jiyā- “bow-string,” PIE base *g^whi- “thread, tendon” (from which derive also Gk. bios “bow”, L. filum “thread”, Russ. žca “thread”).

The term zig originally referred to the threads in weaving, but because of the similarity between the rows and columns of astronomical tables and the parallel threads, it came to be used for an astronomical table, and subsequently a set of tables.

A metallic, lustrous, bluish-white → chemical element; symbol Zn. → Atomic number 30; → atomic weight 65.38; → melting point 419.58 °C; → boiling point 907 °C; → specific gravity 7.133 at 25 °C. It was recognized as a → metal as early as 1374.

Etymology (EN): From the Ger. Zink of unknown origin. Zinc compounds were known in prehistoric times, where they were used for healing wounds and for making brass.

Etymology (PE): Ruy, from Mid.Pers. rôy, rôd “copper, brass;” Av. raoiδita- “red, reddish;” cf. Skt. rohita- “red, reddish,” lohá- “red, reddish, reddish metal.”

A common mineral, zirconium silicate, ZrSiO₄, occurring in small tetragonal crystals. The color is variable, usually brown to reddish brown, but also colorless, pale yellowish, green, or blue. A red variety, used as a gem, is called hyacinth. Zircon contains trace amounts of uranium and thorium and therefore can be used for radiometric datings. Also called jargon.

See also: From Ger. Zirkon, from Ar. zarqun “cinnabar, bright red,” from Pers. zargun “gold-colored,” from zar→ gold

• -gun “resembling; manner, fashion; color”
  (Mid.Pers. gônak “kind, species;” Av. gaona- “color”).

A metallic chemical element; symbol Zr. Atomic number 40; atomic weight 91.22; melting point about 1,852°C; boiling point 4,377°C; specific gravity 6.5 at 20°C. Zirconium was discovered in the mineral zirconia by the German chemist Martin-Heinrich Klaproth in 1789. It was first isolated by the Swedish chemist Jons Jacob Berzelius in 1824 in an impure state and finally by the chemists D. Lely Jr. and L. Hamburger in a pure state in 1914.

See also: From → zircon.

A → diatomic molecule made up of → zirconium and → oxygen atoms. See → ZrO band.

See also: → zirconium; → band.

An imaginary belt around the heavens extending about 9° on either side of the → ecliptic. The orbits of the Moon and of the principal planets also lie entirely within the zodiac.
The zodiac was created during the first half of the first millennium B.C. by Babylonian astronomers, who divided it into 12 constellations (→ sign of the zodiac) each considered to occupy 1/12, or 30°, of its great circle. They were named after living creatures, with the exception of Libra:
→ Aries, → Taurus, → Gemini, → Cancer, → Leo, → Virgo, → Libra, → Scorpius, → Sagittarius, → Capricornus, → Aquarius, → Pisces. However, as a result of → precession, these signs no longer correspond to the astronomical constellations in which the Sun actually appears. The constellations are irregular in size and shape, and the Sun regularly passes through 13 constellations as it moves along the ecliptic. The additional 13th constellation is → Ophiuchus, situated between → Scorpius and → Sagittarius. See also → dodekatemorion.

Etymology (EN): From M.E. zodiaque, from O.Fr. zodiaque, from L. zodiacus, from Gk. zodiakos (kyklos) “zodiac (circle),” literally “circle of little animals,” from zodiaion, diminutive of zoion “animal,” literally “a living being,” from PIE base *gwei- “to live, life;” cognate with Pers. zist, → bio-.

Etymology (PE): Borjgân, from borj, → sign of the zodiac,

• -gân suffix denoting multiplicity, order, organization.

Of or relating to the → zodiac.

See also: → zodiac; → -al.

→ zodiac.

See also: → zodiacal; → belt.

A cone-shaped faint glow along the → ecliptic, visible to the naked eye in the west after sunset or in the east before sunrise. Zodiacal light results from sunlight reflected by interplanetary dust concentrated in the plane of the ecliptic.

See also: → zodiacal; → light.

→ sign of zodiac.

See also: → zodiacal; → sign.

The portion of the surface of a sphere included between two parallel planes.

Etymology (EN): From L. zona “geographical belt, celestial zone,” from Gk. zone “a belt,” related to zonnynai “to gird,” from PIE base *yes- “to gird, girdle;” cognate with Pers. parhun “circle,” as below.

Etymology (PE): Zonâr most probably loan from from Gk., as above; it is related to Pers. parhun “circle,” ultimately from Proto-Iranian *pari-iâhana- “girdle, belt,” from pari-, variant pirâ-, → circum-, + iâhana- “to girdle,” cf. Av. yâh- “to girdle.”
The Pers. pirâhan “shirt” is a variant of parhun.

The region on the sky covered by the plane of the → Milky Way. It is characterized by an apparent absence of galaxies, due to the obscuring effect of → interstellar dust in the → Galactic plane.

See also: → zone; → avoidance.

Any of the three → absorption bands due to the molecule → zirconium oxide present in the blue and visual spectral regions → S-type stars. The bands are centered on the wavelengths 4614, 5551, and 6468 Å.

See also: → zirconium oxide; → band.

A member of a class of non-radially pulsating stas that change their brightnesses with periods from 30 seconds to 25 minutes and amplitudes from 0.001 to 0.2 mag in V. Their location on the
→ Hertzsprung-Russell diagram identifies them as → white dwarf stars that have entered the → instability strip as they evolve along the white dwarf track.

See also: → Cetus; → star.