“Twinkle, twinkle, little star,” is a lullaby many of us remember from our childhood. Twinkling stars may be a fanciful subject for a child’s lullaby or entrancing to look at, but astronomers despise them. It seriously affects the quality of their observations.
What Causes Twinkling Stars?
Twinkling stars occur because of the turbulent and inhomogeneous nature of our atmosphere. This motion makes it very difficult for the light of distant stars and galaxies to pass through unaltered. Two effects of this muddled atmosphere are scintillation (twinkling), which is the apparent change in brightness of the star. The other is seeing, which is the apparent movement of the star caused by the atmosphere. See if you can tell the difference in the pictures below.
How Do Astronomers Reduce Twinkling?
First, they put their telescopes and observatories high on mountain tops. The higher you go, the less atmosphere starlight has to pass through to reach the telescope’s optics and instrumentation. Ninety percent of our atmosphere is within 10 miles above sea level. That’s why you see observatories such as the Kitt Peak National Observatory perched high (6,750 ft) on a mountain top.
Astronomers also observe objects as far above the horizon as possible. As the earth rotates stars, as does our sun, rise in the east. As it rotates, they get higher in the sky till they culminate, or reach their highest point above the horizon. It’s there that starlight passes through the least amount of atmosphere. For comparison, light from a star fifteen degrees above the horizon passes through four times more atmosphere than one straight overhead. Stars just above the horizon pass through 38 times more atmosphere.
De-Twinkling The Stars
Science also has a high tech solution to the problem of these distortions.—adaptive optics. Adaptive optics actually de-twinkles star light after it enters the telescope. The magic is done by rapidly deforming the reflecting surface of one of its mirrors to null it out. Done at the same rate as the atmospheric distortions, “de-twinkled” starlight falls on the telescope’s sensors giving pristine images.
Is Space the Place?
Poor seeing and scintillation are only a few of the issues the atmosphere creates for astronomers. Space telescopes do not deal with atmospheric distortions, but are very expensive, complicated, and take years of planning and testing. Until that cost comes down significantly, we will continue to make the best of our earth bound telescopes such as those found here on Kitt Peak.