Sky watch: The science in a star's twinkle

It began as a five-verse poem called "The Star" that appeared in the 1806 publication "Rhymes for the Nursery." More than three decades later, this poem by Ann and Jane Taylor was set to music -- a 1761 French folk tune that, coincidentally, seemed to fit perfectly:

Twinkle, twinkle, little star

How I wonder what you are

Up above the world so high,

Like a diamond in the sky . . .

We've all grown up singing this magical song and believing that stars actually twinkle. But do they? Often our childhood beliefs are at odds with reality, and this is a perfect case in point.

Twinkling -- or "scintillation" -- originates not with the stars but with the air through which their light must travel before reaching our eyes. We've all seen this effect on a hot summer day, when our view of a terrestrial scene over a sizzling roadway or barbecue is distorted by rising heat waves. Similarly, a star's light is bounced around thousands of times each second as it makes its way from the still vacuum of space into our thick and turbulent atmosphere.

The amount of twinkling we see depends on the steadiness of the air; the more turbulence there is, the more a star will appear to twinkle. That's why stars lower in the sky flicker and flash more than those overhead. Their light must pass through a longer column of unstable air on its way to us.

Right now, the most dramatic stellar scintillation appears low in the east-southeastern sky after dark. It's the brightest star in all the heavens: Sirius, the Dog Star. You can find it by following a line eastward from the three bright "belt" stars of Orion, the hunter.

Watch Sirius and it doesn't just appear to twinkle wildly; it fires off colorful sparks. But this is a natural phenomenon. Since a star's white color is a blending of every color of the spectrum, and each is bent rapidly in different directions at different times by moving air currents, Sirius appears as a colorful celestial sparkler.

As amazing as this looks to the unaided eye, you'll find it more stunning by aiming binoculars or a small telescope in its direction, and throwing it slightly out of focus.Yes, twinkling stars have always been a magical part of our night sky, and an inspiration for that timeless poem that we learned as a children's song long ago. But what if "The Star" had been written not in the 19th century but more recently?