Twinkling of stars

The idea

Stars are point-sized sources, so as changing atmospheric conditions vary the path of starlight, the star's apparent position fluctuates and the light entering the eye flickers, making it look brighter and fainter by turns.

A star at night

Stars are so far away that each looks like a tiny point of light. The air above us is never completely still.

What do you think the changing air does to starlight?

• It makes the star flicker
• It makes the star turn red
• It makes the star move across the sky

Follow the rays

Open the tabs one by one. The picture is only an illustration.

Twinkling

The path of the starlight keeps changing because the air is not stationary. The apparent position of the star fluctuates, and the amount of starlight entering the eye flickers. The star looks brighter at one time and fainter at another. That is twinkling. Stars are very distant, so they are almost point-sized sources of light.

Stars are almost point-sized sources; as the atmosphere changes, the path of the rays varies, the apparent position fluctuates and the amount of light entering the eye flickers: twinkling.

Notes

A star is a point-sized source. As the atmosphere changes, its apparent position fluctuates and the light entering the eye flickers, so it looks brighter and fainter by turns: twinkling.

Check yourself

Why are stars treated as point-sized sources of light?

The twinkling of a star is due to…

The star seems brighter at one time and fainter at another because…

The apparent position of a star is…

• They are very distant — correct. Yes!
• They are very small compared with a planet. Since the stars are very distant, they approximate point-sized sources of light.
• They are very close to the earth. Planets are much closer to the earth. Stars are very distant.
• the star switching on and off. The twinkling of a star is due to atmospheric refraction of starlight.
• atmospheric refraction of starlight — correct. Yes!
• dispersion of starlight by a prism. The twinkling is due to atmospheric refraction of starlight.
• the star really changes size. The path of the rays keeps varying, so the amount of starlight entering the eye flickers.
• the eye changes the colour of the star. The amount of starlight entering the eye flickers.
• the amount of starlight entering the eye keeps changing — correct. Yes!
• not stationary, it keeps changing slightly — correct. Yes!
• fixed for ever. The apparent position of the star is not stationary, since the physical conditions of the earth's atmosphere are not stationary.
• always the same as its actual position. Atmospheric refraction makes the apparent position differ from the actual position, and it keeps changing slightly.