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VEGA (Alpha Lyrae). One of the most famed stars of the sky, Vegais the luminary of the exquisite constellationLyra, the Lyre, which represents the harp of the greatmythical musician Orpheus. Its name derives from an Arabic phrasethat means "the swooping eagle." Vega is one of three brilliantstars that divide the northern heavens into rough thirds, theothersArcturus andCapella, and withAltair andDenebforms the greatSummer Triangle,lying at its northwestern apex. At magnitude zero (0.03), it isthe sky's fifth brightest star, falling just behind Arcturus andjust ahead of Capella. It is also one of the closer stars to theEarth, lying just 25.0 light years away.

A closeup view of Vega (the bright star at top) and itssurroundings reveals the duplicity of the famed "double-doublestar"Epsilon Lyraeat left (west is up in the picture). The left-hand star of the pair is Epsilon-1, the right hand star Epsilon-2. Each of the two are also double.Zeta Lyrae is at the center ofthe right-hand edge, while the unrelated pair Delta-1 andDelta-2Lyrae are at bottom right, Delta-2 the brighter.

Vega is a classic class A (A0) white main sequence dwarf star, liketheSun quietly running off the nuclearfusion of hydrogen deep in its core, with a sort of averageeffective surface temperature of about 9500 degrees Kelvin. Itswhite color and apparent brightness made it a basic standardagainst which the apparent magnitudes of other stars are compared. Studies of Vega have a serious problem, however. While it appearsto be a slow rotator, it is really a rapid rotator viewed pole-on,its axis nearly pointing at the Earth. Rotation will make a starflatten at its poles, turning it from a sphere into an oblatespheroid (as it does the Earth). The poles therefore becomehotter, the equator cooler, a well-known phenomenon called "gravitydarkening." Detailed interferometer measures that can image thestar's surface, plus subsequent analysis, reveal a severetemperature gradient that runs from 10,150 at the poles to 7950 atthe equator, a polar diameter of 2.26 times that of the Sun, and anequatorial diameter 2.75 solar, the result of a rotation period ofonly half a day (and an equatorial spin speed of around 270kilometers per second). Calculation of luminosity is thereforemuch more difficult than for a slowly rotating star. In Vega'scase, it comes out to about 36 times that of the Sun, which givesa mass of 2.3 solar and an age of about 400 million years. Likethe Sun, Vega is halfway through its stable hydrogen-fusing life. Vega was one of the first stars to be discovered with a largeluminous infrared-radiating halo that reveals a circumstellar cloudof warm dust. Since Vega is rotating with its axis directed towardthe Earth, the dust cloud represents a face-on disk that may not beunlike the disk surrounding the Sun and that contains the planets. Several other stars similar to Vega (Fomalhaut, Denebola, Merak,for example) possess similar disks, and astronomers speculate thatthey may indicate the existence ofplanetarysystems, though no planets have ever been detected. Even ifthey exist, it seems unlikely that life would have developed to anydegree because of the short lifetimes of these hot stars. (Vega is included in Jim Kaler's "The Hundred Greatest Stars.")

Written byJim Kaler. Last revised 6/26/09. Return toSTARS.

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