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X SGR (X Sagittarii). This wonderful fifth magnitude (typically4.5, near fourth magnitude) star has a several things going for it,in part as a result of its name. "X" denotes the "unknown," even"mystery." Well, not in this case, as the name simply tells usthat it is variable, the seventh variable discovered withinSagittarius (thesequence starting with "R"). "X" also "marks the spot," X Sagittarii doing that quite well, asit is the nearest naked-eye star to the direction to theGalaxy's Center (Galactic longitude andlatitude both zero degrees), and acts rather like aPolaris, which marks theNorth Celestial Pole. Perhaps then itdeserves a Latin proper name: "Stella Centri Galaxiae" perhaps, or"Stella Mediogalactica" (which erroneously implies that the star isAT the center), or for a real mouthful "Stella iter ad CentrumGalaxiae Indicans." Best, rather obviously, to stick with "X,"which in "mystery mode" guides our eye to toward the three-million-solar-mass black hole that lies at the Galaxy's center 25,000 lightyears away (and thus so obscured by interstellar dust that it isvisible only in the infrared and radio parts of the spectrum). Ata distance of 1075 light years (as determined byparallax), X Sgr falls farshort of the actual Center. A nominal class F (F7) brightgiant, X Sgr is one of the sky'sfew naked-eyeCepheidvariables, like better-knownDeltaCephei,Mekbuda,Eta Aquilae, and by odd coincidence, thatmost famous of other indicators,Polaris(which is a distinctly odd case). Changing like clockwork betweenmagnitudes 4.2 and 4.9 and back every 7.01283 days, its classranges between about F5 and G9 with a G2 average. Lying deep inSagittarius (oddly only five degreesnorthwest of another bright Cepheid,WSgr), X Sgr is afflicted by a fair degree of interstellar dustabsorption, best estimated at about 0.75 magnitudes. Without theobscuration, the star would at its brightest reach close to thirdmagnitude. Using the famed Cepheid period-luminosity relation tocalculate the absolute visual brightness, and comparing that to theapparent magnitude, we find a distance of 1075 light years, exactlythat determined through direct parallax. With a typicaltemperature near 5300 Kelvin, there is not much infrared orultraviolet to deal with, giving us a total luminosity of 3100times that of theSun, which in turn givesus a radius of 66 times solar. A slow projected equatorialrotation speed of 24 kilometers per second then leads to aponderous rotation period of less than 138 days. Direct measure ofangular diameter coupled with distance yields a smaller radius 53times that of the Sun, but that is in infrared light, implying thatthe size of the star depends on the color with which you examine it(as is the case for many large stars). A mass of 7 or 6 solarmasses respectively depends on whether the star is cooling with adead helium core or heating as a core helium-burner. X Sgr isunsual in that pairs of shock waves (rather like sonic booms) havebeen observed moving through its outer layers during the pulsationcycle, the only Cepheid for which they have been seen. Havingstarted life as a hot class B dwarf, X Sgr's age falls between 43and 65 million years. Its fate is to make a massivewhite dwarf of between 0.9and 1.0 solar masses, all the rest of the mass ejected back intointerstellar space from which it came. (Thanks to Latin scholarDavid Bright for the wonderful names.) Written byJim Kaler 10/31/08. Return toSTARS.
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