Giving Vega a spin

Contributed by
Mar 21, 2006
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'If you live in the northern hemisphere and go outside in the late summer, you'll see a bright blue jewel of a star high in the sky. Named Vega, it's one of the brightest stars (if it sounds familiar, it was the star Ellie Arroway, Jodie Foster's character in Contact, detected alien signals from).

Vega is critically important to astronomers. Being bright and high overhead for many observers, it's become a "standard star", a target you can use to calibrate your instruments. It was used in such a manner for years by astronomers around the world. I don't think it's used directly any more, but many astronomical brightness measurements are in some way based on Vega.

It therefore surprised astronomers years ago when it was discovered that Vega had way too much infrared light coming from it. It was quickly realized that the star was surrounded by a disk of dust. Heated by the star, the disk was warm, and emitted infrared light (just like you, a warm human, emit IR, which can be detected using heat-sensitive cameras).

But there have been some problems. Compared to similar stars, Vega appears to be too bright. Worse, high-resolution spectra seem to show anomalous features, what you might expect from a rapidly rotating star. But Vega shows no signs of rapid rotation.

Now a new paper puts all that into a tail spin. Literally.

Using interferometry, an amazing technique that allows incredibly high-resolution data to be taken, astronomers have discovered that Vega indeed spins quickly-- very quickly. They took advantage of the fact that a star spinning really quickly will flatten out near the equator due to centripetal force; the same force that keeps water in a bucket as you swing it around. In a sense, this force acts against gravity, so if you were to stand on the equator of a spinning object, you'd feel like you'd weigh less (this is true on the Earth, too-- you weigh about 0.3% less on the Equator due to the Earth's spin).

In a star, this balance of forces makes the star cooler at its equator than at its poles, so in optical light its not as bright at the equator. Normally, stars are way too far away to detect this difference, but interferometry can make extremely high-resolution observations, and the astronomers were actually able to see this difference in Vega.

They determined we see Vega nearly pole-on-- like we're looking right down over its north (or is it south?) pole. The polar region is hot, while the equator is cooler. You can see that in this graphic:

The orange "plus" marks

Vega's pole the &subsolar point" * , the center of Vega's disk as seen from Earth (incidentally, it's known that the debris disk around Vega is circular in appearance, which matches the idea that we are "looking down" on an actual circle-shaped disk; if we saw it at an angle it would appear elliptical, like the rim of a glass as seen form an angle) , and hotter regions are in blue while cooler are in red. They also determined that to give this degree of uneven heating, Vega must be spinning really fast: about 275 kilometers/second at its equator-- 620,000 miles per hour! If the Earth spun that fast, our days would be 90 146 seconds long! * Incredible. In fact, if Vega spun much faster, the centripetal force would be stronger than gravity, and the gas on the equator would fly off. Vega would tear itself apart.

This affects a lot of calculations astronomers use, and it will be interesting to see how this new data will be assimilated into the body of knowledge. After reading the paper, my first thought, oddly enough, was not so much the impact on astronomy, but on the movie "Contact"-- for a brief moment, we see Vega as Ellie Arroway stops there on her way to meet the aliens. We see Vega as a beautiful spherical blue star... but if it's spinning as fast as the observations indicate, it would not be spherical at all: it will be highly flattened, like a basketball someone is sitting on. It will be 25% wider through the equator than through the poles.

It figures: a new observation comes along that affects almost all of observational astronomy, and I wonder how it'll affect how I watch a movie.

Tip o' the dew shield to Larry Klaes for the heads-up on this one.

* I made a couple of unrelated errors in this entry for some reason, which is annoying. The period error was because I didn't convert from miles to kilometers! The thing with the orange plus was just misreading the plot. Duh.'