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

A state of a fluid in which the surfaces of constant density coincide with surfaces of constant pressure (isobaric).

→ barotropic gas.

Having a bar like structure.

→ bar.

A transitional class of object between the classic spiral galaxies and true irregular systems. The → Large Magellanic Cloud, the nearest and best studied example of the class, is, contrary to popular opinion, not an irregular galaxy. The LMC and other members of the SBm class have definite structural signatures. They are generally dominated by a pronounced asymmetric bar -- one that is offset from the optical center of the galaxy -- with a nascent spiral arm emanating from one end. As is the case with irregular galaxies, the optical centers of SBm type systems are not particularly special places. Disk systems later than Sc characteristically lack a central stellar concentration in addition to having weak spiral structure; this is true of SBm-type galaxies. SBm galaxies are typically very active in their star formation activity, often containing a large star-forming complex situated at one end of the bar. Beyond these general trends there is a tremendous amount of dispersion in physical properties within the SBm class, particularly in the strength of the spiral structure. At one extreme are the "one-armed" spirals such as NGC 3664 and NGC 4027 which are dominated by single, looping spiral arm. On the other hand NGC 4861 shows little evidence of spiral structure and it is dominated by a large star-forming complex at one end of its bar. The class smoothly leads to the Barred Magellanic irregulars (IBm) which show no indication of spiral structure (Wilcots et al. 1996, AJ 111, 1575).

→ Magellanic; → spiral; → galaxy.

A → spiral galaxy that exhibits a bar-shaped structure in its nucleus. → galactic bar.

→ barred; → spiral; → galaxy.

A defect in an optical system in which magnification decreases with distance from the optical axis, whereby the image of a square appears barrel-shaped. Opposite of → pincushion distortion.

Barrel, M.E. barel, from O.Fr. baril; → distortion.

Cowlegi, → distortion; celiki, relating to celik "barrel".

General: Anything that prevents passage or blocks.
Physics: 1) A region where the potential energy is greater than the total energy of a particle, whereby the particle cannot go through.
2) The depletion layer of a P-N junction in a diode.

O.F. barrière "obstacle," from V.L. *barraria, from *barra "bar, barrier."

Varqé, from varq "a mound, a dam" + -é nuance suffix. Varq is probably related to Av. vâra- "barrage," vara- "enclosure," var- "castle," Mid.Pers. var "enclosure," from Av. root var- "to cover, to conceal;" variants: barq (Torbat Heydariyei), valgâ (štiyâni), var (Qomi); cf. Skt. vatra- "a dike, a dam,"varana- "rampart, wall," from vr- "to obstruct, close, cover, hide; to choose."

Same as → Meteor Crater.

Names after Daniel Barringer (1860-1929), American geologist, who bought the Crater in 1903, convinced that it was made by a huge → meteorite; → crater.

The center of mass of a system of bodies.

From Gk. barus "heavy," → bar, + → center.

Gerânigâh, from gerâni "weight;" cognate with Gk. barus, → bar, + gâh "place."

A → coordinate time having its spatial origin at the solar system barycenter. It is intended to be used as the independent variable of time for all calculations pertaining to orbits of planets, asteroids, comets, and interplanetary spacecraft in the solar system. → Barycentric Dynamical Time (IDB).

→ barycenter; → coordinate; → time.

A time scale previously used in calculations of the orbits of solar system objects (planets, asteroids, comets, and interplanetary spacecrafts). It was based on the Terrestrial Dynamical Time, but took the relativistic effect of time dilation into account to move the origin to the solar system barycenter. It is now superseded by → Barycentric Coordinate Time (TCB).

→ barycenter; → dynamical; → time.

The → Julian Date referenced to the → barycenter of the → solar system. The BJD is more precise than the → Heliocentric Julian Day because the Sun is not stationary. It moves due to the → gravitational attraction of Jupiter and the other planets.

→ barycentric; → Julian Date.

The hypothetical mechanism of creating the → baryon asymmetry in the → Universe. Universe. Explaining the observed matter asymmetry is an important open question in physical cosmology. → Sakharov conditions.

From baryo-, from → baryon + → -genesis.

Any of the class of the heaviest → subatomic particles that includes → protons, → neutrons, as well as a number of short-lived particles whose decay products include protons. Baryons obey the → Fermi-Dirac statistics. They form a subclass of the → hadrons and are further subdivided into → nucleons and → hyperons.

Gk. barys "heavy" + → -on, from "fermion."

In cosmology, one of a series of peaks and troughs that are present in the power spectrum of matter fluctuations after the → recombination era, and on large scales. At the time of the Big Bang, and for about 380,000 years afterwards, Universe was ionized and photons and baryons were tightly coupled. Acoustic oscillations arose from perturbations in the primordial plasma due to the competition between gravitational attraction and gas+photons pressure. After the epoch of recombination, these oscillations froze and imprinted their signatures in both the → CMB and matter distribution. In the case of the photons, the acoustic mode history is manifested as the high-contrast Doppler peaks in the temperature anisotropies. As for baryons, they were in a similar state, and when mixed with the non-oscillating → cold dark matter perturbations, they left a small residual imprint in the clustering of matter on very large scales, ~100 h^-1Mpc (h being the → Hubble constant in units of 100 km s^-1 Mpc^-1). The phenomenon of BAOs, recently discovered using the Sloan Digital Sky Survey data, is a confirmation of the current model of cosmology. Like → Type Ia supernovae, BAOs provide a → standard candle for determining cosmic distances. The measurement of BAOs is therefore a powerful new technique for probing how → dark energy has affected the expansion of the Universe (see, e.g., Eisenstein 2005, New Astronomy Reviews 49, 360; Percival et al. 2010, MNRAS 401, 2148).

→ baryon; → acoustic; → oscillation.

The observation that in the present → Universe there is → matter but not much → antimatter. Observations do not show the presence of galaxies made of antimatter, nor gamma rays are observed that would be produced if large entities of antimatter would undergo → annihilation with matter. However, the → early Universe could have been baryon symmetric, and for some reason the matter excess has been generated, through some process called → baryogenesis. → Sakharov conditions.

→ baryon; → asymmetry.

1) The difference between the total number of → baryons and the total number of → antibaryons in a system of → subatomic particles. It is a measure of → baryon asymmetry and is defined by the quantity η = (n_b - n_b^-)/n_γ, called the → baryon-photon ratio, where n_b is the → comoving number density of baryons, n_b^- is the number of antibaryons, and n_γ is that of photons. The value of η for the → cosmic microwave background radiation (CMBR) has been very well determined by the → WMAP satellite to be η = (6.14 ± 0.25) x 10^-10. The baryon number is assumed to be constant. The photons created in stars amount to only a small fraction, less than 1%, of those in the CMBR.
2) A property of an → elementary particle represented by a → quantum number. It is equal to +1 for a baryon and -1 for an antibaryon. → Bosons, → leptons, and → mesons have a baryon number B = 0. → Quarks and → antiquarks have baryon numbers of B = +1/3 and -1/3, respectively. The baryon number is → conserved in all observed types of particle-particle interaction.

→ baryon; → number.

The → baryon number compared with the number of photons in the → Universe. The baryon-photon ratio can be estimated in a simple way. The → energy density associated with → blackbody radiation of → temperature  T is aT⁴, and the mean energy per photon is ~kT. Therefore, the number density of blackbody photons for T = 2.7 K is: n_γ = aT⁴/kT = 3.7 x 10² photons cm^-3, where a = 7.6 x 10^-15 erg cm^-3 K^-4 (→ radiation density constant) and k = 1.38 x 10^-16 erg K^-1 (→ Boltzmann's constant). The number density of baryons can be expressed by ρ_m/m_p, where ρ_m is the mass density of the Universe and m_p is the mass of the → proton (1.66 x 10^-24 g). → CMB measurements show that the baryonic mean density is ρ_m = 4.2 x 10^-31 g cm^-3 (roughly 5% of the → critical density). This leads to the value of ~ 2 x 10^-7 for the number density of baryons. Thus, the baryon/photon ratio is approximately equal to η = n_b/n_γ = 2 x 10^-7/3.7 x 10² ~ 5 x 10^-10. In other words, for each baryon in the Universe there is 10¹⁰ photons. This estimate is in agreement with the precise value of the baryon-photon ratio 6.14 x 10^-10 derived with the → WMAP. Since the photon number and the baryon number are conserved, the baryon-photon ratio stays constant as the Universe expands.

→ baryon; → photon; → ratio.

→ Dark matter made up of → baryons that are not luminous enough to produce any detectable radiation. It is generally believed that most dark matter is → non-baryonic. The baryonic dark matter could reside in a number of forms, including cold gas and compact objects.

→ baryonic; → dark; → matter.

Ordinary matter composed of → baryons, i.e. → protons and → neutrons, as distinct from → non-baryonic, exotic forms.

Adj. of → baryon; → matter.

A mineral, mercuric sulphide, HgS, which is the primary → ore for the production of → mercury. It is a → crystalline solid with a bright → red color. Cinnabar is highly toxic.

From O.Fr. cinabre, from L. cinnabaris, from Gk. kinnabari, maybe ultimately from Pers. šangarf "red lead, cinnabar," of unknown origin.