Why Do Stars Twinkle?

And why don't planets twinkle too?

If you look up at the stars on a dark night, you might notice that these bright points of light appear to twinkle. In actual fact, stars do not actually twinkle: they just appear to do so from our perspective on Earth.

Our atmosphere reaches about 10,000km up from the surface of the Earth, and within the atmosphere air gets blown around, while hot air rises and mixes with cooler air. Stars appear to twinkle because as light from those stars passes through our atmosphere, it is bent and distorted by winds, as well as varying temperatures and densities of air.

There is even a scientific term for stars’ twinkling, and that’s ‘atmospheric scintillation’. This is the astronomical term for those quick changes in the apparent brightness of a star or even the colour of a star produced by the aforementioned atmospheric irregularities.

For most people, the concept of a twinkling star is quite romantic, and conjures up memories of one of the most famous nursery rhymes of all time. But for astronomers, the effect of twinkling stars can be a nuisance as it can cause the image seen through a telescope to shake and jump around.

The extent to which astronomical objects appear to shake and jump around is known by astronomers as ‘seeing’. You will often hear practical astronomers complain about ‘bad seeing’ or praising ‘good seeing’, for example.

Stars twinkle while planets don’t because stars are so much further away from Earth. This makes them appear as concentrated points of light, and that light is more easily disturbed by the effects of Earth’s atmosphere. Planets, on the other hand, are much closer, and the sunlight reflected off them comes back through Earth’s atmosphere in a much thicker beam of light than starlight, so it is not as noticeably affected by the distorting effects of the atmosphere.