Movatterモバイル変換
[0]ホーム
ALPHA SCL (Alpha Sculptoris). East of brightFomalhaut, the luminary of the SouthernFish (Piscis Austrinus), lies the modernconstellationSculptor, the Sculptor'sStudio, one of the few constellations in which the first fourGreek letters actually fall in order ofbrightness, surely by coincidence because the differences amongthem are so small. Alpha Sculptoris, toward the faint end offourth magnitude (4.31), tops Beta by just 0.07 magnitude. It lies just 2.7 degrees to the southeast of the South GalacticPole (the perpendicular to the center line of the Galactic disk)could thus carry the name "Polaris Galacticus Australis," in linewith 31 Comae Berenices, "Polaris Galacticus Borealis." Alpha Scl's faintness results from its rather large distance of 670light years, and belies a curious character. At first it seemslike just one more warm (14,000 Kelvin) blue-white class B (B7)star, of the kind that abound in the naked-eye sky. Classed as agiant, it radiates 1700 times more brightly than theSun, which with temperature yields a radius of7 times solar, a large mass of 5.5 times solar, and an age of 81million years. The star is thus right on the edge of the hydrogen-fusing main (dwarf) sequence and has probably quenched the nuclearengine in its now-helium core as it prepares to make a run tocooler surface temperatures and to become a much larger red giant. Spinning with an equatorial velocity of only 14 kilometers persecond, very slow for a class B star, it takes a precisely-known21.652 days to make a rotation. Alpha Scl is part of a rare breedcalled "helium weak" stars, in which the abundance of surfacehelium is anomalously low, here only 45 percent of normal (which isusually 10 percent of hydrogen). On the other hand, other elementslike silicon, titanium, and manganese are greatly enhanced. Thisoddness is caused by the slow rotation that keeps the outer layersundisturbed, allowing some kinds of atoms to drift downward, othersto rise to the surface. The effect is enhanced by a magnetic field(which renders Alpha Scl a "Bp" star) that helps concentrate thechemicals into spots that in turn allow the precise measurement ofthe rotation period, as their movement in and out of sight causesvariations in the spectrum (which were at one time interpreted asthe result of a non-existent orbiting companion that could havebeen a black hole!). Alpha Sculptoris is thus the prototype of thefew-known "Si-Ti helium weak" stars"(rendering them "Alpha Sclstars"). The magnetic field occasionally flips its direction, andcontrols the behavior of a close-in cloud of circumstellar gas.
[8]ページ先頭