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HOW COME? Why some stars twinkle, and others don't

Why do some stars twinkle and others don't? asks Howard Cohen's class at Francis Lewis High School in Fresh Meadows

Look up at the night sky, and you'll notice that most stars are glittering and sparkling. But a mysterious few are far less flashy, shining with a steady light. Meanwhile, stars near the horizon seem to be extra twinkly, as if signaling their arrival as they rise, or waving a frantic goodbye as they set.

But those "stars" that don't twinkle aren't actually stars at all. Instead, they're planets in our own solar system, such as Venus and Mars, shining with the reflected light of the sun. And even when it comes to distant stars, twinkling is an illusion, a trick of light.

After all, our sun is a star, but it doesn't twinkle as we bask in its light at the beach.

The stars in the night sky are also enormous suns, anchoring their own solar systems. And if there's anyone standing on planets circling those faraway stars, they may have noticed our sun, a twinkling point of light in their own night sky.

The secret to twinkling lies in the atmosphere, the blanket of gases, thin or thick, that can surround planets and some moons. At night, the surface of the Earth radiates stored heat into space. Air just above the ground warms and rises, mixing with cooler air. Starlight passes through this turbulent, unstable air and is bent this way and that by its run-ins with gas molecules.

Shifting pockets of air act like lenses. The result? As you look at a star, its light seems to shimmer, making the star appear larger and less focused. The star also seems to brighten and dim, as its light rays alternately bunch up and spread out. Presto: twinkling.

Stars sparkle most when near the horizon, as we look at them through extra-thick layers of air. A bright star, low in the sky, may also change color rapidly. As white starlight travels through the atmosphere, different wavelengths (colors) of light are bent more or less. Just as with a prism, we see a rainbow of colors, a star flashing red, orange, yellow, green blue, and violet.

But the sun and Earth's sibling planets are near enough that we see them as discs rather than as points of light. Our eyes receive light from many points on the disc; light rays bent one way by the atmosphere are canceled out by light rays bent the opposite way. And so we see Saturn or Jupiter, steadily shining. But when the atmosphere is very unstable, or a planet is close to the horizon, it may begin to twinkle, briefly masquerading as a bright star.

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