Do stars really twinkle?

September 13, 2023

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We all know how the nursery rhyme goes and we all see that stars ‘twinkle’ if we catch a glimpse of them on a clear night, but do the stars themselves actually twinkle?

In short, no.

Distant stars appear to twinkle when we observe them from Earth. This is because the stars are so far away that when their light arrives here, it covers such a small area on the sky that it appears as a single point of light. When this light travels through the atmosphere, it’s direction is changed by the atmosphere itself making its path a random zig-zag rather than it’s usual direct path. This creates the famous twinkling effect that we see. The more atmosphere a star passes through, the more twinkly it appears. So, rather boringly, stars do not actually twinkle.

So, do they always just stay the same brightness?

In short, no! Some of them do change brightness before our eyes and these stars are actually very important in Astronomy. They are called variable stars because their magnitude varies with time. One example of a variable star is called a Cepheid variable. These stars themselves expand and contract in regular patterns. Their change in magnitude is all linked to their changing size and surface temperature. These stars are more massive than our Sun and they are in an unstable part of their lives while pulsating as Cepheids. They are very luminous and easy to spot in this stage. There is a well-known establish relationship between the time between maxima of these stars and their actual intrinsic brightness. This is a major tool for astronomers to work out distances to objects. Cepheid variables can be observed both in our Galaxy and others, therefore if we know how long it is between ‘flashes’ we can use that to determine it’s actual absolute magnitude. If we compare that to its apparent magnitude as observed from Earth, we can work out how far away other Galaxies are using a formula. How useful is that?!

So in conclusion, no twinkling but definitely some ‘flashing’!

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