Twinkling stars, steady planets: what's the difference?

The journey of light to your eyes

A light that comes from very far away

Stars are suns, but so distant that they appear to us only as tiny points. The closest star to the Solar System, Proxima Centauri, is about 40,000 billion kilometers away. Its light, which travels at 300,000 kilometers per second, takes more than 4 years to reach us!

Even with the largest telescopes, a star remains a simple point of light: it is too far away for us to see its shape.

The last step: crossing the air

After a journey of several years, a star's light finally arrives near Earth. But it still has one last ordeal: crossing the atmosphere, that great layer of air that surrounds us and is several dozen kilometers thick.

And the air is never completely calm.

Moving air bends light

Bubbles of hot and cold air

In the atmosphere, there are air currents, wind, places that are warmer and others that are colder. Hot air is less dense than cold air. These differences form sorts of big invisible air bubbles that constantly move and mix.

Why light bends

When light passes from one zone of air to another of different density, it slows down just a tiny bit and changes direction slightly. We say it is refracted. It's the same phenomenon that makes a straw look "broken" in a glass of water.

Since the air bubbles move constantly, the direction of light changes dozens of times per second. To your eyes, the star's point of light then seems to tremble: sometimes brighter, sometimes fainter, sometimes even a bit shifted. This is twinkling.

N.B.:
Twinkling is also called "atmospheric turbulence". Astronomers measure it with a unit called "seeing" (or "image quality"): the calmer the air, the sharper and clearer the star images.

Star versus planet: a point versus a disk

A star: a single thread of light

Imagine that a star's light is a very thin thread. When a bubble of air passes in its path, the whole thread is shaken at the same time. Your eye then sees the star blink.

A planet: a thick rope of light

A planet is much closer than the stars. Seen from Earth, it is no longer a simple point, but a tiny disk. Its light is like a thick rope made of thousands of little threads. Each thread passes through a slightly different part of the atmosphere: one is bent to the left, another to the right, another barely moves. In the end, all these little movements mix and cancel out. Result: the planet's light appears much more stable.

N.B.:
The Sun is also a star, but it doesn't twinkle. It is so close to us (150 million kilometers) that it appears as a big disk in the sky, not as a point. The "point versus disk" rule works for it too!

Colors that change near the horizon

Twinkling is stronger for stars near the horizon. Their light then has to cross a greater thickness of air, and therefore many more bubbles. Very bright stars, like Sirius, can even seem to change color: red, blue, green… like a little blinking light. The air acts a bit like a prism and spreads out the colors of light, which are bent slightly differently.

And in space?

Stars no longer twinkle

Astronauts on the International Space Station, who are above the atmosphere, see stars shining with a perfectly steady light. This is proof that twinkling comes from Earth's air and not from the stars.

Telescopes that correct the air

To avoid this problem, the Hubble Space Telescope observes from space. Large ground-based telescopes use adaptive optics: a deformable mirror adjusts its shape hundreds of times per second to cancel out the shaking of the air, a bit like glasses that adjust themselves.

Your turn: recognize a planet

The blinking test

Planets are "wandering" bodies: the word "planet" comes from a Greek word meaning "wanderer". Over the weeks, they move among the stars. You can spot one very simply:

1. Choose a clear evening and go outside at nightfall, with an adult.
2. Look for a very bright point of light, low in the sky, after sunset or before sunrise.
3. Look at it carefully for a few seconds: if it shines with a calm light, like a little lighthouse, it's probably a planet (Venus or Jupiter, for example). If it blinks and changes color a bit, it's probably a star.

N.B.:
This test isn't perfect. A planet very low on the horizon can also twinkle a bit, because its light then crosses a lot of air. To be sure, you can use a sky map app, which shows the position of the planets.

An experiment to see the air tremble

To observe the effect of hot air on light, look at an object through the hot air above a radiator, a summer road, or a candle (without touching it!). The object seems to waver and tremble. This is exactly what happens to starlight, but with the entire atmosphere.

Why do stars look pointed, with branches?

In drawings, stars are often shown with points. This is a drawer's habit. In reality, a star is an immense ball of burning gas, like the Sun. The "branches" that are sometimes seen in photos come from the equipment, for example the internal supports of a telescope, which diffract light, and not from the star itself.