Have you ever gazed up at the night sky and wondered why do stars twinkle? This beautiful phenomenon has captivated humans for centuries. Luckily we understand why they do, and it has to do with our atmosphere.
The answer lies in two complex atmospheric phenomena: air turbulence and the refraction of light. As light interacts with the atmosphere starlight dances across the night sky in a dazzling display of sparkles. This article will explore how these phenomena work together to produce this stunning effect.
Why do Stars Twinkle: Refraction
Before looking at the question directly, we should understand what refraction is. Refraction occurs when light passes from one medium to another with different optical properties. A common situation that can be observed is partially submerging a pen in some water and holding it at an angle. The pen will appear to bend at the water’s surface. It appears bent because light is refracted at the boundary between the water and air.
We can determine which direction the light will bend if we know the properties of the two mediums. However, we must first understand what ‘the normal’ is. The normal is an imaginary line drawn at 90 degrees to the boundary between the two mediums.
The important media property for refraction is the speed at which light travels in it. If light travels more slowly, the media is said to be more dense, and if faster rarer.
I wish to dispel a common misconception just to go off track a bit. Many people think that the speed of light is fixed. This is not correct because its speed is different in different media. What you normally see quoted is the maximum speed of light, which is what it is when in a vacuum.
If the light enters a media in which it travels slower, it will bend towards the normal. Conversely, if it moves into a rarer medium, it will bend away from the normal.
You must remember that light can be refracted in different directions many times as it travels through the atmosphere. Earth’s Atmosphere.
The Earth’s atmosphere is a complex system of different layers. Within layers, there are smaller-scale variations. The variations between and within layers are caused by changing temperature, humidity and/or pressure. Wind causes turbulence that ensures that at any particular point, these parameters can change quickly.
Technically the twinkling viewed in stars is called scintillation.
So, Why Do Stars Twinkle?
Stars twinkle because as their light moves through the atmosphere it travels through media with different properties. Each time the light enters air with different properties it is refracted. The amount of refraction is dictated by how much the light’s speed changes. As an added complexity wind moves around pockets of air with differing properties.
The location of a star in the sky affects how much it will twinkle. A star directly overhead will appear to twinkle a lot less than one near the horizon. This is because the latter has a much greater thickness of atmosphere to travel through. A star 10 degrees above the horizon must travel more than five times the distance through the atmosphere than one overhead.
I have glossed over an important fact and that is that not all light refracts by the some amount. White light is composed of a continuous light spectrum from blue to red. When white light is refracted higher frequencies (for example blue light) bend more than lower frequencies (red light). This is the same effect as observed when light is passed through a prism. Due to this twinkling stars appear to change color. Other than color changes the star’s brightness and position appear to change for the same reasons.
Below is a video illustrating a twinkling star.
Atmospheric Conditions
You need to remember that star twinkling does not always occur to the same degree. Some nights it may be very easy to see while on other nights it may be difficult to detect. This is because atmospheric conditions can change from night to night. In astronomy, this is called seeing conditions, or just simply seeing. On some nights the seeing may be so atrocious that using a telescope brings very poor viewing.
Your maximum magnification on nights of poor seeing will be lower than on nights of better seeing. You may also like How To Find Total Magnification.
Seeing forecasts are made available. You can use this to ensure the conditions are good enough for your observation session. Below is an example atmospheric seeing forecast. In the lower right is the meaning of the shades used for seeing. On the chart there are other useful forecasts. You can use these charts to plan your observation sessions and maximize your time.
If you have a telescope and wish to observe the effects of seeing point it towards the Moon. When you look through the telescope the Moon will seem to shimmer.
Why Don’t Planets Twinkle
Stars are so far from us that they are point sources of light. This means that no matter how much they are magnified they will always be a point. You will never see them as a disk.
Conversely, planets are not point sources and viewing them as a disk is easy. Because of this light from planets is averaged over a small area. This averaging negates some of the effects of atmospheric refraction.
Conclusion
The next time you look up at the night sky, take a moment to appreciate the delicate beauty of twinkling stars. The science behind this phenomenon explains why some stars appear brighter than others, and how conditions in our atmosphere can affect starlight.
I found astronomy while working in dark rural locations. Initially, I explored the night sky and learnt the constellations before purchasing a pair of binoculars to further my knowledge of the sky.
My first telescope was a 200 mm Newtonian reflector on an equatorial mount. I found that this telescope had a steep learning curve but was a rewarding experience.
As time progressed, I became interested in astrophotography. This resulted in purchasing a 110 mm refracting telescope and a dedicated monochrome-cooled astronomical camera. This resulted in another very rewarding steep learning curve that far surpassed the experience with my first telescope.
I have joined Telescope Guru to share my knowledge of telescopes and astronomy.
