Why Do Stars Twinkle? The Atmosphere Behind the Flicker

Look up on a clear night and you will see the stars flicker gently, brightening and dimming from moment to moment. The sight has inspired countless nursery rhymes and poems. But stars do not actually twinkle. The flicker we see happens in the last few kilometres of starlight’s journey — in our own atmosphere.

Starlight’s long journey

Light leaving a star travels through the almost empty vacuum of space for years, sometimes thousands of years, before it reaches us. Along the way it moves in a straight line, undisturbed.

The trouble starts at the very end. As soon as the light enters Earth’s atmosphere, it has to pass through around 100 kilometres of constantly moving, mixing air.

How does the atmosphere bend light?

Light changes direction when it passes from one medium into another of different density. This is called refraction — it is why a spoon in a glass of water looks bent.

Air is not uniform either. Warm air is less dense than cold air and bends light slightly less. And the atmosphere is full of constantly moving pockets of air at different temperatures:

• warm air rising from ground heated during the day;
• strong winds and jet streams high up;
• eddies at the boundaries between layers of air moving at different speeds.

Each of these pockets acts like a weak lens, nudging the light slightly off course. As the pockets drift with the wind, the lenses keep moving. As a result, the light reaching your eye:

• brightens one moment and dims the next;
• shifts its position ever so slightly;
• sometimes seems to change colour.

Scientists call this scintillation.

Why do planets shine more steadily?

If you have spotted Venus, Jupiter or Mars in the night sky, you may have noticed that they shine far more calmly than the stars. The reason is distance and apparent size.

• Stars are so far away that even through large telescopes most appear as single points of light. When the atmosphere shifts that point, all the star’s light is affected at once.
• Planets are much closer and cover a real, if tiny, disc in the sky — too small to notice with the naked eye. Light from each part of the disc passes through different air pockets and is deflected in different directions. On average these deflections cancel out, so the planet looks steadier.

That difference is a handy way to tell stars from planets: a bright point that does not twinkle is probably a planet. Planets can twinkle too, though, when they are very low near the horizon.

Stars near the horizon twinkle more

Light from a star directly overhead takes the shortest path through the atmosphere. Light from a star near the horizon travels much further through the air. Passing through more air pockets, it is bent more — which is why stars low in the sky twinkle far more noticeably.

Bright stars near the horizon show another effect. The atmosphere bends different colours of light by slightly different amounts, so a star’s red, green and blue light can reach your eye at different moments. Sirius, the brightest star in the night sky, can look like a light flashing in many colours when it is low on winter evenings — a sight that has even led to UFO reports.

A big problem for astronomers

Twinkling stars may look romantic, but for astronomers they are a headache. The same turbulence blurs telescope images. Astronomers call this atmospheric blurring “seeing”: the better the seeing — the calmer the atmosphere — the sharper the images.

Three main solutions have been developed.

1. Going higher

Major observatories are usually built on the summits of high, dry and calm mountains, where the layer of air the light must cross is thinner and there is less moisture and turbulence. That is why Chile’s Atacama Desert, Mauna Kea in Hawaii and the Canary Islands are among the world’s most important observing sites.

2. Adaptive optics

Modern large telescopes use a technology called adaptive optics. It works like this:

1. The telescope watches a bright reference star, or an artificial “star” created by shining a laser into the upper atmosphere.
2. Computers measure how that light is distorted by the atmosphere hundreds or even thousands of times a second.
3. A flexible mirror inside the telescope constantly changes shape using hundreds of tiny actuators, applying the exact opposite distortion.

Thanks to this, ground-based telescopes can, for some observations, approach the sharpness of space telescopes.

3. Going to space

The ultimate solution is to leave the atmosphere entirely. Launched in 1990, the Hubble Space Telescope escaped atmospheric blurring and captured images of the universe with unprecedented clarity. Astronauts in space likewise see stars as steady, sharp, non-twinkling points.

What does twinkling say about the weather?

Sailors and farmers once tried to predict the weather by watching how much the stars twinkled — and there is some truth in it. Nights of strong twinkling are usually nights with strong winds high up and a turbulent atmosphere, which can sometimes herald a change in the weather. On its own, though, it is not a reliable forecasting method.

In short

Stars do not really twinkle. Their light travels straight through space but is bent randomly by moving pockets of air at different temperatures once it enters Earth’s atmosphere. Because stars appear as single points, those bends show up as changes in brightness and position; planets, being tiny discs, shine more steadily. To escape the twinkle, astronomers climb mountains, bend mirrors and send telescopes into space.