Why do stars twinkle?

Why do stars twinkle?

Twinkle, twinkle little star,

How I wonder what you are…

Have you even wondered why the stars twinkle? And…do they, really?

It turns out, the stars themselves do not twinkle at all. They shine steadily! But as their light travels through the Earth’s atmosphere, it bounces off different layers and changes – ever so slightly – direction on the way. As a result, the stars APPEAR to twinkle. 

Interestingly enough, the ‘naked eye planets’ (that is the planets you can see with just your eyes, without a telescope),  look like stars in the sky, but do not seem to twinkle at all. Read on and you will find out why!

The magic of the starry sky…as seen through the Earth’ atmosphere

When you look up at the sky at night, you see stars, planets, shooting stars and other celestial objects – all through an invisible veil that covers up our whole planet, the atmosphere. While very useful for breathing and keeping the planet’s temperature stable, the atmosphere becomes rather a nuisance when we try to observe space from the Earth’s surface. You see, our atmosphere is made of different layers with different properties, such as temperature and density. Moreover, the atmosphere is not still, it keeps moving all the time. Warmer air rises, cooler air settles down, the winds blow…there is a constant turmoil up there that pushes and bends the starlight causing the ‘twinkling effect’ of stars.

Ok, but what about the planets?  Why don’t they twinkle just like the stars? 

The planets you can spot in the sky are Jupiter, Saturn, Venus, Mars and Mercury. In other words, the 5 nearest to us Solar System planets. The farthest of them is 1.4 billion kilometers away. That might seem far, but the nearest naked eye star (other than the Sun, of course) is 40 208 billion km away. Or 14 400 times further out! That means the light from the naked eye planets comes from a much closer distance and in a much bigger beam than the light from the stars and is therefore much more difficult to bend and distort. That’s why the planets don’t twinkle! Don’t believe it? Why not go out in the evening for a little stargazing session and check? Here is October 2021 Stargazer’s Calendar to help you plan your observing night! 

So…what can we do to minimize ‘twinkling’ as we study stars? 

OK, we’ve just learned that when we watch the stars through the Earth’s atmosphere, we end up with distorted, blurry, twinkly images. But luckily, there are a number of clever techniques scientists came up with to minimize the effect of the atmosphere on their scientific observations. 

For example, astronomers can send telescopes up into space and observe stars, planets and far-away galaxies from above the Earth’s turbulent air. If you are curious to find out more about the World’s best space telescopes and some of the mind bending discoveries they made, check out our post Do we really need to put telescopes into Space?. Unfortunately, launching things into space is difficult, not to mention expensive.

The other solution astronomers came up with is to use the so-called adaptive optics. The idea here is simple. If we know how exactly the atmosphere behaves [when and where we observe] and how it distorts our images, we can ‘bend’ a telescope’s deformable mirror to compensate for this effect. In order to find out how much to deform mirror, astronomers create ‘fake stars’ (they actually call them reference stars) by shooting a laser beam into the atmosphere and then observing the laser glow as if it was a star. They then compare the image of the fake star they observed with this ‘star’s’ ideal image they would have seen if it wasn’t for the atmosphere…and bend their mirrors accordingly. How clever is that? 

Author: Irina Vladimirova

Irina Vladimorova is a presenter at WonderDome Planetarium and a regular contributor to WonderDome Astronomy Blog. She holds a degree in Astrophysics and a special place in her heart for planetary science.