Fr.: théorème de Birkhoff
For a four dimensional → space-time, the → Schwarzschild metric is the only solution of → Einstein's field equations which describes the gravitational field created by a spherically symmetrical distribution of mass. The theorem implies that the gravitational field outside a sphere is necessarily static, and that the metric inside a spherical shell of matter is necessarily flat.
The theorem was first demonstrated in 1923 by George David Birkhoff (1884-1944), an American mathematician; → theorem
1) zâymân; 2) zâdmân, zâd; 3) zâyeš
Fr.: 1) acouchement; 2;3 ) naissance
1) The act or process of bearing or bringing forth offspring.
M.E. byrthe; O.E. gebyrd; cf. O.H.G. giburt, Ger. geburt; PIE *bhrto, from *bher- "to bear."
1) Zâymân, from zây present stem of zâyidan, zâdan "to give birth"
Av. zan- "to bear, give birth to a child, be born,"
infinitive zazāite, zāta- "born;"
cf. Skt. janati "begets, bears;"
Gk. gignesthai "to become, happen;" L. gignere
"to beget," gnasci "to be born;" PIE base *gen-
"to give birth, beget") + -mân a suffix forming verbal nouns,
birth binary population (BBP)
porineš-e dorinhâ hengâm-e zâdmân
Fr.: population binaire à la naissance
In star formation models, the population of binary components formed via random pairing of stars distributed according to the → canonical IMF.
Fr.: ligne de naissance
The path in the → H-R diagram that continuously → accreting stars follow. Stars on the birthline are difficult to observe, because they are hidden by the → accretion disk and a dense → cocoon of → interstellar matter. The star becomes visible once a sufficient amount of the surrounding matter has been dispersed. It follows that the young starsare observed generally between the birthline and the → zero-age main sequence (ZAMS). Before hydrogen ignition, gravitational contraction and deuterium burning are the main energy sources for the star. The localization of the birthline depends on the deuterium → accretion rate. First introduced by R. Behrend & A. Maeder, 2001 (A&A 373, 190).
Fr.: méthode de Biruni
A method devised by the Iranian astronomer Biruni (973-1048) to measure the Earth radius, using trigonometric calculations. In contrast to foregoing → Eratosthenes' method and → Mamun's method, which required expeditions to travel long distances, Biruni's method was on-site. He carried out the measurement when he was at Nandana Fort (at the southern end of the pass through the Salt Range, near Baghanwala in the Punjab). He first calculated the height of a hill (321.5 m). To do this he used the usual method of observing the summit from two places in a straight line from the hill top. He measured the distance, d, between the two places and the angles θ1 and θ2 to the hill top from the two points, respectively. He made both measurements using an astrolabe. The formula that relates these angles to the hill height is: h = (d. tan θ1 . tan θ2) / (tan θ2 - tan θ1). He then climbed to the hill top, where he measured the → dip angle (θ), that is the angle of the line of sight to the horizon. He applied the values he obtained for the dip angle and the hill's height to the following trigonometric formula to derive the Earth radius: R = h cosθ / (1 - cos θ). The result for the Earth radius was 12,851,369.845 cubits (or 6335.725 km, using favorable conversion units). Despite the fact that the method is very ingenious, such a precise value is only by chance, because of several drawbacks: The plane was not perfectly flat to serve as the smooth surface of the sea. A measuring instrument more accurate than the alleged 5 arc minutes was needed. And the method suffered from the → atmospheric refraction (See, e.g., Gomez, A. G., 2010, Journal of Scientific and Mathematical Research).
Abu Rayhân Mohammad Biruni (973-1048 A.D.), one of the greatest scholars of the medieval era, was an Iranian of the Khwarezm region; → method.
A white, crystalline, brittle metallic chemical element with a pinkish tinge; symbol Bi. → Atomic number 83; → atomic weight 208.9804; → melting point 271.3°C; → boiling point about 1,560°C; → specific gravity 9.75 at 20°C; → valence +3 or +5. Bismuth is the most → diamagnetic of all metals. Its thermal conductivity is lower than any metal, except → mercury. There is only one naturally occurring → isotope of bismuth, 209Bi. Bismuth is used in a number of very different applications, chiefly in bismuth alloys, and in pharmaceuticals and chemicals.
From Ger. Bismuth, Wismut, Wissmuth, probably from weisse Masse "white mass," indicating how the element appears in nature.
Of or relating to a → leap year or to the extra day falling in a leap year.
L.L. bissextlis "containing an intercalary day," from bisextus, from bis "twice" + sextus "sixth," because in the Julian calendar the sixth day before the Calends of March was doubled every four years. Same as → leap and → intercalary day.
Andarheli, of or relating to andarhel→ intercalation.
The condition in which a physical system is capable of assuming either of two stable states.
Fr.: bistabilité par saut
An abrupt discontinuity in the → stellar wind properties of → hot stars near → effective temperatures about 21,000 K and 10,000 K, corresponding to O9.5-B3 supergiants (Castor et al. 1975, ApJ 195, 157; Lamers et al., 1995, ApJ 455, 269). At these temperatures the → terminal velocity of the wind drops steeply by about a factor two and the → mass loss rate increases steeply by about a factor three to five, when going from high to low temperatures. Bistability jump is related to the degree of ionization in the wind. With a little drop in the temperature, the dominant driving element (Fe) will recombine to lower ionization stages which produces a lower terminal velocity and a relatively high density in the wind. → wind momentum. Additional bistability jumps may occur at higher temperatures where CNO may provide the dominant line driving, especially for low metallicity stars (Vink et al. 2001, A&A 369, 574). However, a recent study using a larger sample finds that there is a gradual decline in the wind terminal velocities of early B supergiants and not a "jump" (Crowther et al. 2006, A&A 446, 279).
Fr.: mécanisme de bistabilité
The mechanism that accounts for the → bistability jump.
bit, raqam-e dorin
A contraction of → binary digit, either 0 or 1.
Bit, from binary + digit
Fr.: logique bivalente
A logical system, such as → classical logic, in which every declarative sentence expressing a → proposition has exactly one → truth value, either → true or → false. Bivalent logic is just a sub-set of a more powerful type of logic known as → fuzzy logic. See also → polyvalent logic.
BL Lac object
barâxt-e BL Calpâsé
Fr.: objet BL Lac
A member of a family of → quasars, or extragalactic → Active Galactic Nuclei, which displays a high radio emission and/or important optical variability over a short period of time. BL Lac objects appear star-like but their spectrum is flat, and partially polarized. Also called → blazars.
BL Lac, from object BL in the constellation → Lacerta (BL Lacertae). The reason for this terminology is that it was originally thought to be an irregular variable star in our Galaxy; hence its variable star designation. In the 1970s the "star" was identified with a bright, variable → radio source and a very faint galaxy; → object.
Fr.: mécanisme de Blaauw
A mechanism aimed at explaining the → disruption of a → binary system. As one component loses mass dramatically, the resulting loss of → gravitational attraction changes the orbit of, or ejects completely, the → companion star.
Adriaan Blaauw (1914-2010), 1961, Bull. Astron. Inst. Netherlands 15, 265; → mechanism.
siyâh (#), siyah (#)
Very dark in color; absorbing light without reflecting any of its various rays.
Black, from O.E. blæc "black," from P.Gmc. *blak-, from PIE *bhelg- "to shine, flash, burn" (cf. Gk. phlegein "to burn, scorch," L. flagrare "to blaze, glow, burn," fulgur "lightning").
Siyâh or siyah, from Mid.Pers. siyâ, siyâk, siyâvah "black," Av. sâma-, sayâva- "black, dark," cf. Skt. syama-, syava- "black, brown," Gk. skia "shadow."
Fr.: goutte noire
The appearance of a band linking the solar limb to the disk of a transiting planet (Venus or Mercury) near the point of internal tangency. This effect increases the uncertainty in measuring the period from when the planet fully enters the solar disk to when it begins to depart. Historically, the black drop phenomenon limited the accuracy of the determination of the Astronomical Unit and the scale of the Solar System in the 18th and 19th centuries. While there have been many proposed theories over the years, the true cause of the effect was revealed during a transit of Mercury in 1999, which was observed by the NASA's TRACE satellite. Two effects could fully explain the black drop: the inherent blurriness of the image caused by the finite size of the telescope (→ point spread function), and an extreme dimming of the Sun's surface just inside the apparent outer edge (→ limb darkening). See Schneider et al. 2004, Icarus 168, 249.
siyah câl (#), ~ surâx (#)
Fr.: trou noir
A fantastically → compact object, predicted by the theory of → general relativity, whose → gravity is so powerful that not even light can escape from it. A black hole forms when matter → collapses to → infinite → density, producing a → singularity of infinite → curvature in the fabric of → space-time. Each black hole is surrounded by an → event horizon, at which the → escape velocity is the → speed of light. The → Schwarzschild radius for the Sun is about 3 km and for the Earth about 1 cm. There is observational evidence for black holes on a remarkable range of scales in the Universe: → stellar black hole, → intermediate-mass black hole, → primordial black hole, → mini black hole, → supermassive black hole, → Schwarzschild black hole, → Kerr black hole.
Historically, the Newtonian concept of such a celestial body appeared at the end of the 18th century when light was shown to have particle characteristics. In fact the English geologist John Mitchell (1724-1793) and French mathematician and astronomer Pierre Simon Laplace (1749-1827), independently, suggested that regions of space, where gravitational attraction was so strong that not even light could escape, may exist in the Universe. However, the term black hole was coined in 1967 by the Princeton physicist John A. Wheeler (1911-2008); → black; → hole.
black hole binary
siyah câl-e dorin
Fr.: trou noir binaire
black hole candidate
nâmzad-e siyah câl (#)
Fr.: candidat trou noir
An object that seems likely to be a → black hole, but waits for more observational confirmations.