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

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 (annus) "year containing an intercalary day," from bisextus, from bis "twice, two, doubled" + 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.

Note. Some authors have used the Pers. term behizaki for "bissextile," however this is problematic as far as the calendar history is concerned. The term comes from Mid.Pers. vihezagig or vihezakik "movable," which was the name of the additional whole month of 30 days which Iranians used in their calendar system in the Sasanid era. In fact they did not use an intercalary day every four years, but instead added a whole month after a period of 120 years. The operation was called vihezag. The term originates from Mid.Pers. vihez- "to move, progress," vihezag(k) "movement, progression." Therefore, this dictionary cautions against equating behizaki and bissextile. → embolismic month.

The condition in which a physical system is capable of assuming either of two stable states.

→ bi-; → stable.

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).

→ bistability; → jump.

The mechanism that accounts for the → bistability jump.

→ bistability; → mechanism.