Vega - Physical Properties

Physical Properties

Vega's spectral class is A0V, making it a blue-tinged white main sequence star that is fusing hydrogen to helium in its core. Since more massive stars use their fusion fuel more quickly than smaller ones, Vega's main-sequence lifetime is roughly one billion years, a tenth of our Sun's. The current age of this star is about 455 million years, or up to about half its expected total main-sequence lifespan. After leaving the main sequence, Vega will become a class-M red giant and shed much of its mass, finally becoming a white dwarf. At present, Vega has more than twice the mass of the Sun and its full luminosity is about 40 times the Sun's value. However, because of its high rate of rotation, the pole is considerably brighter than the equator. Since we see it nearly pole-on, its apparent luminosity from Earth is notably higher, about 57 times the Sun's value. If Vega is variable, then it may be a Delta Scuti type with a period of about 0.107 days.

Most of the energy produced at Vega's core is generated by the carbon–nitrogen–oxygen cycle (CNO cycle), a nuclear fusion process that combines protons to form helium nuclei through intermediary nuclei of carbon, nitrogen, and oxygen. This process requires a temperature of about 15 million K, which is higher than the core temperature of the Sun, but is more efficient than the Sun's proton-proton chain reaction fusion reaction. The CNO cycle is highly temperature sensitive, which results in a convection zone about the core that evenly distributes the 'ash' from the fusion reaction within the core region. The overlying atmosphere is in radiative equilibrium. This is in contrast to the Sun, which has a radiation zone centered on the core with an overlying convection zone.

The energy flux from Vega has been precisely measured against standard light sources. At 5480 Å, the flux is 3,650 Jy with an error margin of 2%. The visual spectrum of Vega is dominated by absorption lines of hydrogen; specifically by the hydrogen Balmer series with the electron at the n=2 principal quantum number. The lines of other elements are relatively weak, with the strongest being ionized magnesium, iron, and chromium. The X-ray emission from Vega is very low, demonstrating that the corona for this star must be very weak or non-existent. However, as the pole of Vega is facing us and a polar coronal hole may be present, confirmation of a corona as the likely source of the X-rays detected from Vega (or the region very close to Vega) may be difficult as most of any coronal X-rays would not be emitted along the line of sight.

Using spectropolarimetry, a magnetic field has been detected on the surface of Vega by a team of astronomers at the Observatoire du Pic du Midi. This is the first such detection of a magnetic field on a spectral class A star that is not an Ap chemically peculiar star. The average line of sight component of this field has a strength of −0.6 ± 0.3 G. This is comparable to the mean magnetic field on the Sun. Magnetic fields of roughly 30 gauss have been reported for Vega, compared to about 1 gauss for the Sun.