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Stargazing: Why stars twinkle

December 24, 2011 at 11:31 AM

Gatehouse News Service

The other night seemed promising at first. Skies were clear and there was no moon. Perhaps the stars would present themselves in abundance, their light cascading into my telescope.

It was a cold night. I let the reflecting telescope sit outside a while to let warm air currents flow out of the open end. That way, the air in the tube and out can equalize, and the views will be less turbulent. This is important to enjoy anything through the telescope at more than low power. Highly magnified views of the planets or moon will distort, and the less detail will be seen.       

Alas, once I joined the telescope in the cold air, I found that high clouds were thickening across the sky. Few stars were visible. Brilliant Jupiter, high in the south, still was easy to see, so I turned the tube that way and slipped in an eyepiece.

The view of Jupiter and the four big moons were excellent- much better than normal! It was obvious the air was unusually steady. It wasn’t just because I had let the telescope sit out a while; the earth’s atmosphere had one of those rare moments when the air currents high above us settled down.

Air turbulence is not only annoying to jet airline passengers. Shimmering air high up also blurs images at high magnification.

Planets are especially difficult in a telescope. Their disc is tiny in the telescope; pockets of moving air easily jumble and erase the ability to discern and resolve fine details, whether it be the light and dark belts and spots on Jupiter’s cloud deck, the rings of Saturn or dark shading on the Red Planet, Mars.

The moon, while appearing large, is loaded with tiny things- craters and mountain peaks, cracks and ridge lines - all of which are harder to see well if the air keeps moving.

If you have the patience, keep looking. Every now and then the air tends to settle and you might see amazing detail. Like in most everything else we do in life, patience is a tremendous virtue in enjoying astronomy, the science of the universe. Patience can mean not giving up if the sky is rarely clear to not quitting if shimmering air gets you down.

This night, there was no need to wait. The sky was practically stable, at least when looking at Jupiter. Multiple bands and spots were easily seen at high magnification. Even more astounding, the largest of the four moons, named Ganeymede, showed more than a point of light. The moon showed a tiny but discernible disc.

The stars, too, were more of a joy to behold in the telescope as their images were not “boiling” due to air turbulence. They were much more like points of light. Such a night is described as one with “good seeing.”

But shimmering air isn’t all bad.

It’s what make the stars seem to delightfully twinkle.

From childhood we likely have known the nursery rhyme, “Twinkle, twinkle little star… .”

They seem to twinkle the most when low on the horizon, where the air is thicker. Bright stars like Sirius, can even show momentary colors not normal to the star, when low on the horizon. Sirius, which normally appears bluish-white to the lower left of Orion, can spark red twinkling low on the horizon- the same way the sunset or sunrise is red.

You can normally tell a bright planet from a star because the planet is less likely to twinkle to the unaided eye than the star. This is because the star, vastly farther away, is essentially a point of light and more subject to the disturbance in our atmosphere. The planet, such as Jupiter, shows a much wider face (as seen in a telescope), which we can think of as many, many “points” of light together. The reflected light from each “point” on the planet may be twinkling, but they tend to cancel each other and smooth out when we look up with unaided eyes.

Professional observatories utilize “adaptive optics” to counter air turbulance. Tiny areas of the large telescope mirror constantly adjust to make up for the shimmer. An artificial guide star is made with a powerful laser beam, allowing a comparison with the real stars.

Another way of course is to send a telescope into space, above the air (such as the Hubble Space Telescope). Now there’s an idea.

New moon is on Christmas Eve; first-quarter moon is on New Year’s Day.

Merry Christmas and Hanukkah greetings to readers of this column. Your notes are always welcome at .(JavaScript must be enabled to view this email address).

Keep looking up!

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