Why do stars twinkle?

By Fred Watson 7 August 2017
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Have you ever wondered why stars twinkle? Well it has a lot to do with something called ‘scintillation.’

ONCE AGAIN, scientists have a fancy name for a common phenomenon that everyone is familiar with. The reason stars twinkle can be explained through scintillation, which in this context refers to the effect that our continuously moving atmosphere has on light.

If the atmosphere was at exactly the same temperature throughout, the stars wouldn’t twinkle and the night sky would lose much of its charm. In reality, however, the atmosphere is made up of countless bubbles of warmer and cooler air, jostling together as they are carried along by the wind. This turbulent motion takes place at all heights above the ground, and is often very rapid in the high-altitude jet stream. 

Because the temperature of air governs its density – and that, in turn, dictates the extent to which it will bend light – these moving atmospheric cells act like invisible lenses. Admittedly they are very weak lenses, because the refractive (light-bending) power of air is feeble, but they are enough to cause the twinkling of stars – and to compromise observations made with giant telescopes. 

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The light of Capella travels for 45 light-years before entering Earth’s turnbulent atmosphere. (Image Credit: Tragoolchitr Jittasaiyapan)

Twinkling arises because the moving blobs of air act to alternately focus and defocus the starlight arriving at our eyes. The effect is that the star gets rapidly brighter and dimmer – that is, it twinkles, or scintillates. Looking through a telescope, you can detect this focusing and defocusing as the star image seems to contract and expand. It is also in frenzied random motion, like a moth fluttering around a lamp, as the light rays are bent this way and that in the moving air. 

For 300 years, astronomers have referred to the degree of turbulence in the atmosphere as “seeing”. When the seeing is good, the star images in their telescopes are stable points of light. When it’s not, they become inflated, trembling blobs, and the exquisite detail that the telescope is capable of revealing is lost altogether in the shaking of the air. 

Generally, the seeing gets better as you look higher in the sky – and thus the twinkling seen with the unaided eye is reduced. That is because the path of starlight through the atmosphere is shortest for stars that are overhead. For the same reason, high-altitude sites like mountain tops have better image quality for astronomers than sea-level locations – there is less turbulent air above them. The seeing is also strongly dependent on local weather conditions. It is often depressingly poor after the passage of a cold front, for example. But the twinkling on such occasions is, well…charming.