Twinkling stars

(astronomy) Rapid fluctuations of the brightness and size of the images of stars caused by turbulence in the earth's atmosphere.

A phenomenon by which light from the stars, as it passes through fluctuations in the Earth's atmosphere, is rapidly modulated and redirected to make the starlight appear to flicker. The twinkling phenomenon affects all wavelengths that manage to penetrate the Earth's atmosphere, from the visible to the radio wavelengths. At visible wavelengths, atmospheric fluctuations are caused predominantly by temperature irregularities along the line of sight. Such irregularities introduce slight changes in the index of refraction of air, and these changes affect light waves in two ways: they modulate the intensity of the light, and they deflect the light waves in one direction and then another. At radio wavelengths, electron density irregularities in the ionosphere modulate and redirect radio waves. See also Refraction of waves.

The twinkling phenomenon is of utmost interest to astronomers who view the skies from ground-based telescopes. While modulation variations are present, it is the deflection of light that causes the most serious problems. The composite star image produced by a large telescope is a blurry circle that results when the randomly deflected light waves are added together in an extended time exposure. To diminish atmospheric effects, telescopes are built on high mountains, and are placed at least 30–45 m (100– 150 ft) above the ground. See also Astronomical observatory.

To completely remove the twinkling effects of the atmosphere, there are two alternatives. The first is to place a telescope in orbit above the atmosphere, as with the Hubble Space Telescope. The second alternative is to monitor the random deflections of the atmosphere and, within the telescope, to bend the deflected light back onto its original path. This optical technique is given the name adaptive optics. See also Adaptive optics; Satellite astronomy.