Vega

Eighteen years and a month ago I spent a few weeks on a mountain in Armenia near the points where Iran, Armenia and Azerbaijan meet.   I was there with a group of astronomers from the Pulkovo observatory in St. Petersburg to observe spectrophotometric standard stars.  I would swing the telescope from α Aquilae to α Lyrae who straddle either side of Cygnus, the swan.  We looked at other stars that I can no longer remember.   The common name for α Lyrae is, of course, Vega, and Vega is the ultimate standard star that forms the basis for astronomical measurements at least in the optical and nearby.   The apparent brightness of other stars is compared to Vega, so Vega is an especially well-studied star.


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Vega is one of the brighest stars in the northern sky and from Vancouver where I'm sitting, it is visible most nights -- a blue-white beacon in sky.   Vega is about twice the mass of the Sun, so it should be about 20 times brighter when in fact it appears to be nearly sixty times brighter than the Sun, much brighter than other stars of its mass; Sirius for example is 25 times brighter than the Sun.   This presented quite a puzzle until it was discovered that Vega is rotating once every 12 hours or so.  If you looked at the star from the side, it is squished like a melon --- if it rotated just a bit faster, material would fly off of its equator.   We are looking down on the pole of Vega so it is difficult to measure the rotation velocity, but because the pole is closer to the centre of the star, the gravity at the pole is stronger.  It turns out for a particular star, regions where the gravity is stronger put out more light, so from our point of view Vega is about 50% brighter than averaged over the surface.   The rapid rotation makes Vega somewhat brighter than a similar slowly rotating star from every direction.