Why do stars twinkle?15th Mar 2023
The beauty of a clear night sky cannot compare to anything. The myriads of luminous dots on a blue-black canvas are fascinating, and this masterpiece, which the Universe has given us, can be admired for hours. But the longer we peer into this amazing landscape, the more questions we have. For example, why do stars twinkle and planets don’t? How to distinguish a star from a planet in the night sky, and why are they not visible in the daylight? Let’s find out.
Why do only stars twinkle in the night sky?
To answer this question, let’s first understand why we see starlight at all.
Scientists have long figured out that stars are giant spheres of gas, held together by their own gravity, in which thermonuclear fusion reactions are constantly taking place. As a result of these reactions, a large amount of energy is released. Part of it reaches the Earth in the form of electromagnetic radiation, including visible light, which flies towards us at a speed of 300,000 km per second.
That is, stars radiate energy for as long as they live, but even after they die, we can still see their light for a long time because it needs to travel billions of kilometers to reach us.
But during the day, the stars disappear from the sky because the light of our star, the Sun, outshines them. Sunlight is scattered in the Earth’s atmosphere, creating an effect of a blue glow in the daytime sky. It is this brightness that prevents us from seeing the stars during the day.
If you want to know — do stars twinkle in space, prepare for a surprise. In space, starlight is absolutely stable and unblinking. Then why do stars seem to twinkle when we look at them from the Earth’s surface? The reason is the perturbations of the Earth’s atmosphere, which cause a specific kind of refractions called atmospheric refractions. What exactly are we talking about here?
Air is not static; it is constantly moving in different directions under the influence of various forces. Warm air masses emanating from the soil, heated by the Sun, rise into the cold upper layers, hit rocks and mountains, and stretch over plains and seas, creating atmospheric eddies that we call turbulence. Starlight passing through these currents is refracted and creates a twinkling effect.
Our atmosphere acts like a lens between us and the open space. This means the light from celestial bodies, falling into our field of vision, is repeatedly refracted at different angles, giving us a distorted picture of the star, including its flickering glow. Scientists call these distortions the effect of stellar scintillation.
Different climates and weather conditions affect not only the number of visible stars but also the intensity of their twinkling. When passing through humid, dense air, the light from a star will appear to shimmer strongly; but as this light passes through still air in a dry area, it will appear more stable. This, by the way, partially explains why do some stars twinkle more than others. The twinkling of stars is especially noticeable on windy and frosty nights, and in summer, a strong twinkling indicates the approach of a strong storm.
Who sorted out ‘why do stars look like they twinkle?’
The phenomenon was first explained by the French astronomer and physicist Dominique François Jean Arago. In 1805-1806, together with Jean-Baptiste Biot, Arago studied the reasons why light is refracted in the atmosphere and explained why do stars twinkle in the sky as the phenomenon of interference and the asymmetry of the atmospheric layers relative to a person on Earth.
Why do stars twinkle in different colours?
You have probably noticed that different stars at different times or at certain viewing angles can twinkle in red, yellow, white, or blue light. The reason for this phenomenon lies both in the scintillation already mentioned above and in the intensity and temperature of the thermonuclear reaction of the luminaries.
The hotter the star, the closer its hue to white or blue. Blue stars have a temperature of 20,000K and higher. Red stars are the coldest. By the way, astronomers have learned to determine star age by their temperature and colour. The colder the star, the closer it is to the end of its life cycle. Our Sun is a yellow star with an intermediate temperature of about 6000 K, that is, around its middle age — so to say, in its prime.
In fact, stars radiate energy over a wide range of wavelengths, but the human eye doesn’t recognize all of them (only at wavelengths of about 380 to about 750 nanometres). Medium-temperature stars, like our Sun, emit both blue and green photons, but they appear white and yellow to our eye. Besides, one should keep in mind that the Earth’s atmosphere absorbs short wavelengths of the visible spectrum (violet, blue, green), with a greater effect than long wavelengths (red, yellow, orange).
And yet, one can see green stars in the sky: they are part of the visual double stars. However, this is only an optical effect that has nothing to do with the real colour. A white star will seem greenish if there is a red star next to it in the sky. Our vision tries to average these colours, so the actual hues of the stars may differ from the ones we see.
Why do some stars twinkle more than others?
It depends on several factors at once:
- celestial body size,
- surface temperature,
- distance from Earth,
- location in the sky relative to the observer.
Large hot stars closer to us will twinkle brighter. The same applies to the luminaries that can be observed near the horizon. Closer to the ground, air currents are much denser, and refractions are more pronounced. And vice versa, the higher above the Earth’s surface, the less noticeable star flickering is — the atmosphere layer becomes thinner, and the optical effect on light fluxes decreases. This is why observatories are usually located high in the mountains so that strong flicker does not interfere with observations.
Why do stars twinkle, but planets don’t?
The planets are much closer to Earth than the stars, so they appear to us not as small dots in the sky but as luminous disks. The same logic applies to the Sun, even though it is a star. But the planets do not undergo thermonuclear reactions and do not emit their own light; they only reflect the light of stars strong enough to pass through their atmosphere without refraction. Some parts of the planet reflect light more intensely, while others reflect it weaker, but even this intensity is constantly changing. In other words, a drop in brightness at one point is compensated by an increase in brightness at another. This is why this glow seems smooth and static to us. That explains why the planets do not twinkle like stars at night, and is the simplest way to distinguish a star from a planet.
So, let’s sum up. The main reason why do stars twinkle is the passage of their light through Earth’s atmosphere. Fluctuations in air currents cause multiple refractions of light, which, from Earth, look like flickering. The Sun and planets shine more evenly because they are closer to the Earth, and their light is less distorted by our atmosphere. There is no atmosphere in space, so you won’t see twinkling stars there.