VEGA (Alpha Lyrae). One of the most famed stars of the sky, Vega is the luminary of the exquisite constellation Lyra, the Lyre, which represents the harp of the great mythical musician Orpheus. Its name derives from an Arabic phrase that means "the swooping eagle." Vega is one of three brilliant stars that divide the northern heavens into rough thirds, the others Arcturus and Capella, and with Altair and Deneb forms the great Summer Triangle, lying at its northwestern apex. At magnitude zero (0.03), it is the sky's fifth brightest star, falling just behind Arcturus and just ahead of Capella. It is also one of the closer stars to the Earth, lying just 25.0 light years away.
A closeup view of Vega (the bright star at top) and its surroundings reveals the duplicity of the famed "double-double star" Epsilon Lyrae at 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 of the right-hand edge, while the unrelated pair Delta-1 and Delta-2 Lyrae are at bottom right, Delta-2 the brighter.
Vega is a classic class A (A0) white main sequence dwarf star, like the Sun quietly running off the nuclear fusion of hydrogen deep in its core, with a sort of average effective surface temperature of about 9500 degrees Kelvin. Its white color and apparent brightness made it a basic standard against which the apparent magnitudes of other stars are compared. Studies of Vega have a serious problem, however. While it appears to be a slow rotator, it is really a rapid rotator viewed pole-on, its axis nearly pointing at the Earth. Rotation will make a star flatten at its poles, turning it from a sphere into an oblate spheroid (as it does the Earth). The poles therefore become hotter, the equator cooler, a well-known phenomenon called "gravity darkening." Detailed interferometer measures that can image the star's surface, plus subsequent analysis, reveal a severe temperature gradient that runs from 10,150 at the poles to 7950 at the equator, a polar diameter of 2.26 times that of the Sun, and an equatorial diameter 2.75 solar, the result of a rotation period of only half a day (and an equatorial spin speed of around 270 kilometers per second). Calculation of luminosity is therefore much more difficult than for a slowly rotating star. In Vega's case, it comes out to about 36 times that of the Sun, which gives a mass of 2.3 solar and an age of about 400 million years. Like the Sun, Vega is halfway through its stable hydrogen-fusing life. Vega was one of the first stars to be discovered with a large luminous infrared-radiating halo that reveals a circumstellar cloud of warm dust. Since Vega is rotating with its axis directed toward the Earth, the dust cloud represents a face-on disk that may not be unlike 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 that they may indicate the existence of planetary systems, though no planets have ever been detected. Even if they exist, it seems unlikely that life would have developed to any degree because of the short lifetimes of these hot stars. (Vega is included in Jim Kaler's "The Hundred Greatest Stars.")
Written by Jim Kaler. Return to STARS.