An Etymological Dictionary of Astronomy and Astrophysics
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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".

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 stellar designation system in which a specific star is identified by a Greek letter, followed by the genitive form of its hosting → constellation's Latin name. For example, Alpha Eridani, Delta Cephei, Lambda Bootis. The Greek alphabet has only 24 letters. In case a single constellation contained a larger number of stars, Bayer amended with Latin letters: upper case A, followed by lower case b through z (omitting j and v), for a total of another 24 letters. Bayer did not go beyond z, but later astronomers added more designations using both upper and lower case Latin letters, the upper case letters following the lower case ones in general. Examples include, for Vela: a Vel (Velorum), z Vel, A Vel, Q Vel; for Scorpius: d Sco (Scorpii), A Sco; for Leo: b Leo (Leonis), o Leo, A Leo, → c Orionis. Compare with the → Flamsteed designation.

First introduced by Johann Bayer (1572-1625) in his atlas Uranometria, published in 1603 at Augsburg, Germany; → designation.

The episodic occurrence of abrupt → mass loss in → Be stars resulting in → Balmer lines in emission and → infrared excess. The Be phenomenon results from a combination of a long-term secular effect and short-term instabilities, such as pulsation. The secular evolution brings the star close enough to the critical → break-up velocity, so that the additional velocity field due to the instability may allow some mass ejection (Maeder 2011).

→ Be star; → phenomenon.

A bright, blue → giant star ( → spectral type B2 III), one of the main stars of the constellation → Orion. With a visual magnitude of 1.64, it is about 1000 times more luminous than the Sun, and lies at a distance of 243 → light-years.

From L. bellatrix "a female warrior," fem. of bellator, from bellum "war."

Merzam, Ar. name of the star; its other name is Nâjed.

1) (with preposition to) To be the property of.
2) (with preposition to) To be a part or adjunct of.

M.E. belongen, from be- intensive prefix, + longen "to go," from O.E. langian "pertain to, to go along with;" akin to Du. belangen, Ger. belangen; of unknown origin.

Pargetidan, literally "to surround, to relate with" (on the model of L. pertinere "pertain," Skt. parigraha- "surrounding; relation to"), from parget "to hold, seize, take around," from par- "around," → peri-, + get "to take, sieze," as in Tâleši gate "to take," Târi gata, Sorxeyi, Lâsgardi, Semnâni, Šâhmerzâdi -git- "take, seize," variants of gereftan "take, hold," → concept.

If x is an → element of a → set S, then x belongs to S and this is written x ∈ S.

Third person present verb of → belong.

→ Orion's Belt.

→ belt; → Orion.

The equation expressing → Bernoulli's theorem: P + (1/2)ρV² + ρgz = constant, where P is the fluid → pressure, V is → velocity, ρ is → density, g is the acceleration due to → gravity, and z is the vertical reference → level. The theree terms are called → static pressure, → dynamic pressure, and → hydrostatic pressure, respectively. The Bernoulli equation states that the total pressure along a → streamline is → constant.

→ Bernoulli's theorem; → equation.

A linear second-order differential equation, the solutions to which are called Bessel functions.

From → Bessel; → equation

Hamugeš, → equation.

Any of the eight quantities denoted by a, b, c, d (for → right ascension) and a', b', c', d' (for → declination) used in conjunction with → Besselian day numbers for the reduction of star's → mean catalog place.

→ Besselian; → star; → constant.

The → red supergiant that is the second brightest star in the constellation → Orion. Betelgeuse is one of the biggest stars known with a size of almost 1,000 times larger than the Sun, corresponding to an angular diameter of 43.76 ± 0.12 milli-arcseconds (Perrin et al. 2004, A&A 418, 675). It is a → semiregular variable whose → apparent visual magnitude varies between 0.2 and 1.2 shining very rarely more brightly than its neighbor → Rigel. The energy released by Betelgeuse is estimated to be only 13% in the form of visible light, with most of its radiation being at → infrared wavelengths. The distance of Betelgeuse is 643±146 → light-years (Harper et al. 2008, AJ 135, 1430), while its luminosity is about 140,000 times that of the Sun (→ solar luminosity). Its → spectral type is M2 Iab, its → surface temperature about 3,600 K, and its → initial mass 10 to 20 → solar masses (M_sun). Neilson & Lester (2011, arXiv:1109.4562) recently proposed a mass of 11.6 (+5.0, -3.9) M_sun for Betelgeuse, while Dolan et al. (2008, BAPS 53, APR.S8.6) obtained about 21 M_sun. Its → rotation period is estimated to be about 17 years (Uittenbroek et al. 1998, AJ 116, 2501). Recent observations with the → Very Large Telescope resolve not only the apparent surface of Betelgeuse, but also reveal a large and previously unknown plume of gas extending into space from the surface of the star (Kervella et al. 2009, A&A 504, 115). The plume extends to at least six times the diameter of the star, corresponding to the distance between the Sun and Neptune. This detection suggests that the whole outer shell of Betelgeuse is not shedding matter evenly in all directions. More recently, an image of the surface of the star was obtained using long → baseline → interferometry at infrared wavelengths (Haubois et al. 2009, A&A 508, 923). It shows the presence of an irregular flux distribution possibly caused by enormous → convective cells. A very large dusty envelope has also been observed at larger distances from the star (Kervella et al. 2011, A&A 531, A117).

Betelgeuse, from Ar. Ibt al-Jauza' (ابط‌الجوزاء) "the armpit of Jauza'," from ibt "armpit" + Jauza' "Orion."

Ebtoljowzâ, from Ar. Ibt al-Jauza'.

The theory that bright galaxies form preferentially from anomalously overdense perturbations in the → early Universe.

→ biased; → galaxy; → formation.

Describing a lens with two concave faces.

→ bi-; → concave; → lens.