QotW: Why do stars twinkle?

Twinkle twinkle little star, how we all wonder what you are.
24 January 2022



Listener Michael asks "What makes stars twinkle and what can their colours tell us about them?"


James Tykto interviews the bright Dr Jenifer Millard from the Awesome Astronomy Podcast to find the answer to this question...

James -  Well Michael, if ‘Twinkle Twinkle Little Star’ is anything to go by, you’re not the only one wondering what they are. Fortunately, Dr Jenifer Millard from the ‘Awesome Astronomy Podcast’ is here to help us figure that out.

Jenifer -  Stars twinkle only to those looking at them from Earth – in space, their light would be steadfast. They twinkle due to turbulence in our atmosphere which can be caused by wind, hot air rising or cool air sinking. This turbulence changes the starlight’s path from a straight line, causing it to bounce and bump around the atmosphere.

James -  Though we might enjoy gazing at shining stars, our atmosphere can make it difficult for astronomers to observe them. This problem can be solved by using equipment located in high altitudes, or even in space. For grounded telescopes, however, astronomers have a clever way of countering the turbulence.

Jeni -  By using lasers, we can create fake stars in the sky & compare them to the real stars in the sky. We can then use this information to try and account for all the atmospheric turbulence causing stars to wobble about in real time and give us much sharper, crisper images.

James -  So that explains how they twinkle, but what about the colour of a given star? Well, according to Jeni, that’s caused by their respective surface temperatures.

Jeni -  Most common stars in the night sky exist by fusing hydrogen into helium, we call these ‘main sequence’ stars. Just like when we heat metal, and we see it go from red hot, to orange, to white, the fusion inside the star heats up the surface area and produces the colours we see. Redder stars are comparatively cool, yellow are warmer, and blue the hottest.

James -  So the stars that have lots of these fusion reactions within them produce a blue colour on their surface area, whereas the cooler stars that have fewer fusion reactions glow red.

Jeni -  It’s true that more massive stars have greater gravitational forces trying to crush the molecules together, and so must burn through their hydrogen fuel more ferociously, making them hotter. However, once a star runs out of hydrogen fuel, this mass-temperature relationship breaks. Very big stars like Betelgeuse appear red because they have used up all the hydrogen to react with. Their new fuel source makes them puff up and cool down.

James -  There you have it. Stars twinkle because of our atmosphere getting in the way of the light they emit, and the colours are based on the reactions going on inside them. Michael, hopefully, we’ve put that lullabye to bed for you. Next week, we’ll be tackling this question, from listener Daniel.

Daniel -  'How do they stop the mirror on the Hubble telescope from getting dirty?'


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