Astronomy

This blog entry is about one of the most famed stars of the sky, Vega. As the luminary of the exquisite constellation Lyra, the Lyre, which represents the harp of the great mythical musician Orpheus, Vega also known as “Alpha Lyrae”. Its name derives from an Arabic phrase that means “the swooping eagle.”

Vega has been extensively studied by astronomers, leading it to be termed, “arguably the next most important star in the sky after the Sun”. Vega was the first star, other than the Sun, to have its photograph taken and the first to have its spectrum photographed. It was also one of the first stars to have its distance estimated through parallax measurements. Vega has served as the baseline for calibrating the photometric brightness scale, and was one of the stars used to define the mean values for the UBV photometric system.

Physically, 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 an 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 2.78 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 37 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.

Observed from Earth, 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) at all wavelengths, for many years, Vega was used as a baseline for the calibration of absolute photometric brightness scales. As 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 25.3 light years away. Historically, Vega served as the pole star at about 12,000 BCE and will do so again at around 14,000 CE (current polar star, however, is Polaris).

Popularity: 38%