A star that blinks in different colors. Why do stars twinkle but there are no planets?
Twinkling stars
Stars don't twinkle by themselves. This impression is created by an observer on earth when he perceives the light of a star after it has passed through the atmosphere. This is an indispensable condition for flickering. If you observe even a very distant star from space, it will not twinkle.
Astronauts observing the stars from the Moon, where there is no atmosphere, saw the sky dotted with stars that shone with an even, unblinking light. But here on Earth, covered with a thick “blanket” of atmosphere, rays of starlight are refracted many times in different directions before reaching the surface.
When do stars start to twinkle?
The light from a star becomes flickering as it leaves the atmosphere With high density into a layer with lower density. Why? The air masses around us do not stand still. They are constantly moving relative to each other. Warm air goes up, cold one goes down. Air refracts light differently depending on temperature. When light passes from a layer of air of lower density to a layer of higher density, the light begins to flicker. At the same time, the outlines of the stars become blurry, their images become larger. The intensity of radiation from stars, that is, their brightness, changes. Either the star is visible very well, or it has dimmed. But again it is visible very clearly. These changes in light intensity are scientifically called “scintillation.” But we'll call it " flickering».
Not all stars twinkle
Planets, for example, glow by reflection sunlight and don't flicker. Venus and Mars look like large, bright stars in the sky, but they differ from them in that they do not twinkle. Why? Planets are closer to Earth, and we perceive them as small disks rather than tiny dots. Light is reflected from different parts of the disks. Although it is refracted in exactly the same way, it is refracted differently. Bright light is reflected from some parts of the disk, and dimmer light from others. A second later they switch places. The average intensity of radiation from the entire surface of the disk remains constant. Therefore, the planet’s disk glows with an even, unblinking light.
How to distinguish a star from a planet?
A planet can be distinguished from a star by the nature of its radiation: the stars twinkle, but there is no planet. Indeed, this is not a bad way to distinguish a planet from a star. But if there is great excitement in the earth’s atmosphere, for example a hurricane, then the planets may begin to flicker. Our Sun is also a star. But it is much closer to Earth than the stars we see at night. The sun is not a point in the sky. We perceive the Sun as a large, uniformly shining disk. If the Sun were trillions of kilometers away from the Earth, it would be lost among many other stars and would twinkle just like them. The twinkling of a star is very beautiful and can inspire a poet. But for an astronomer this is truly a headache. Even if the sky is very clear, large movements of air masses occur in the atmosphere, so-called disturbances, which make observing and photographing stars very difficult.
The best time for astronomical observations is clear nights and a calm, undisturbed atmosphere. When the atmosphere above the telescope is calm, astronomers observe with good visibility and almost no flicker. With the development of the space age, powerful telescopes were launched into orbit, through which scientists observe the true picture of cosmic silence and examine the stars shining with a calm, eternal light.
Even without being an astronomer, you can easily distinguish stars from planets in the night sky. The planets shine with an even light and from the Earth look like tiny circles with smooth edges.
Stars do not give such a glow - they seem to twinkle and shimmer, and can take on different shades. Why is this happening?
Starlight and the Earth's Atmosphere
Stellar twinkling visible to the human eye is not a property of stars, but a feature of visual perception from Earth. You've probably noticed that the twinkling of stars is especially colorful on frosty nights or immediately after rain?
The fact is that the reason for the twinkling of stars is the atmosphere. Stars emit light, which on its way to the Earth passes through layers of the atmosphere, and it is known to be heterogeneous.
Starlight rays need to penetrate areas of the atmosphere with different densities and temperatures, and this directly affects the refraction of light rays. Sections of gas layers of different densities make this refraction multidirectional. 
We should not forget that air masses are moving: warm currents rise upward, cold currents descend to the surface of the Earth. Depending on its temperature, air refracts light differently. When a star's light moves from a high-density layer of the atmosphere to a lower-density layer and vice versa, it becomes flickering. The brightness of the stars itself also changes: they dim, then shine again.
Scientists call this process scintillation. In addition, the process of light emission from stars is influenced by turbulent vortices that move in different directions at different altitudes.
Different parts of the atmosphere act on a beam of light, like lenses with constantly changing curvature. The rays, passing through these peculiar “lenses,” are either scattered or concentrated again. This is also accompanied by color scattering, so stars located low above the horizon can change their hue.
The higher you are from the Earth, the less noticeable the stellar twinkling is - the layer of the atmosphere becomes thinner, the optical effect on the light rays decreases. It is for this reason that scientific observatories are usually set up as high as possible in the mountains - from there it is easier to observe the stars without being distracted by strong twinkling. 
There is no atmosphere in space, and, according to astronauts and available images from space telescopes, the stars there shine with an even and calm light.
Why don't planets twinkle?
Planets shine with uniform light primarily because they are located much closer to the Earth's surface than stars. We see stars as twinkling points, while planets are perceived by the eye as small disks that, due to their brightness, appear absolutely round. The fact is that planets, by their nature, differ from stars in that they do not emit their own light, but reflect extraneous light.
Light is reflected more intensely from some parts of the planet, weaker from others, and after just a second the intensity of the reflection changes. At the same time, the average intensity of reflection of light rays from the planet remains unchanged, and from a human point of view, the light from celestial body remains calm and calm.
In other words, the planets also flicker, but with different, constantly changing intensities at different points, and these changes in the brightness of the reflection at different points in time make up for each other. The overall reflection of light from the planet remains constant.
The brightest planets solar system, visible from Earth with the naked eye, are Venus and Jupiter. Venus is clearly visible in the morning and evening sky, against the background of dawn; it glows with an even yellow light. Venus is the third brightest in the sky (as seen from Earth) and the Moon. Jupiter's brightness is slightly fainter, and this planet also has a yellow tint. 
In recent decades, Mars has periodically become very noticeable in the sky. Mercury, the planet closest to the Sun, is also quite bright, but it can only be recognized with certain knowledge.
Due to the fact that Mercury is as close as possible to the Sun, it is hidden in its rays, and it is easy to see the planet only when it moves away from the star at a certain distance. This usually happens at dawn or dusk.
Admiring the myriads of stars twinkling peacefully in the night sky, we see that they are not only of different brilliance, but also of different colors. It is not difficult to guess that the color of a star is an indicator of its temperature. And just as an experienced steelmaker can easily determine the temperature and quality of metal molten in a blast furnace by color, so an astronomer can determine the temperature of a distant celestial body by the color of a star.
The glow of stars comes in all shades of heat, and each color - each temperature level - has its own spectrum. In the accepted spectral classification, stellar spectra are arranged in descending order of the surface temperature of stars, which is accompanied by a smooth transition of their color from bluish to white, from white to yellow, from yellow to orange and from orange to red.
So, the hottest stars are bluish ones. They are heated to 50,000 K (the temperature of stars is measured on the absolute temperature scale in Kelvin). But there are also dazzling blue stars that have an absolutely monstrous temperature - 100,000 K! In the spectral classification, all these stars are designated by the Latin letter “O”. There are many such stars in the constellation Orion. O-stars are adjacent to stars of spectral class B, whose temperature is close to 20,000 K. Their light also has a bluish tint. The constellations Scorpio and Taurus are rich in B-stars. Of the most bright stars This class includes Spica, the main star of the constellation Virgo, Regulus, the main luminary of Leo, and Rigel from Orion. Next come the hot stars of spectral class A, pure white, with a temperature of about 10,000 K. Among them are Bega and Sirius, as well as other brightest lights of the northern sky: Castor from Gemini, Deneb from Cygnus, Altair from Eagle.
Following them in order of priority (descending temperature) are light yellow class F stars with an average temperature of about 7000 K. Here we meet the famous Polaris and Procyon, the main luminary of Canis Minor.
Golden-yellow stars like our Sun and Capella from Auriga, incandescent up to 6000 K, form spectral class G. It is adjacent to orange stars of class K with a temperature of 4500 K. Orange stars also have their own celebrities. This is Aldebaran from Taurus, Arcturus from Bootes, Pollux from Gemini.
And the last spectral type M is populated by red stars such as Antares from Scorpius and Betelgeuses from Orion. They are heated to 3500 K.
In 1965, stars with lower temperatures were discovered - about 1000 K. Most of their radiation falls in the invisible part of the spectrum. Therefore, the unusual dark red luminaries were called infrared stars.
Just as dark lines of varying intensity (absorption lines) are visible in the spectrum of the Sun against the background of a continuous rainbow stripe, so the spectra of stars are intersected by absorption lines. Each gas produces lines in strictly defined places in the spectrum. This allows astronomers to determine the chemical composition of stellar atmospheres with great accuracy.
One might think that the differences in the spectra of stars are due to differences in their chemical composition. However, the reason is different - the difference in their temperatures.
The chemical composition of stellar atmospheres is close to the chemical composition of the earth's crust. There is only one difference: on our planet there are no noticeable amounts of volatile gases - hydrogen and helium, while the atmospheres of stars are 80% hydrogen. Therefore, hydrogen lines are visible in the spectra of stars of all classes. They are most intense in stars of spectral class A. In the spectra of stars like our Sun, numerous absorption lines of metals are presented, and in the spectra of stars starting from class K, whose temperature is already quite low, chemical compounds of titanium oxide, zirconium oxide and other molecular compounds are discovered. This is how the difference in the temperatures of stars entails a different state of the atoms of chemical elements in their atmospheres. This determines the diversity of stellar spectra.
You can make a table showing what temperature corresponds to each spectral class and their subclasses (within each class, the spectra are divided into 10 subclasses; the Sun, for example, is a star of the G2 spectral subclass). Sometimes the temperature of a star can be estimated from just one type of spectrum. By measuring the distribution of energy in the spectrum, the astronomer determines the temperature of a distant celestial body with the highest possible accuracy. It must be borne in mind that since the light emitted by stars comes from their outermost layers, we are talking exclusively about the temperature of their surfaces. The inner layers of stars are hotter, and in the central regions of stars the temperature is in the millions of Kelvin. Likewise, the chemical composition of stars relates only to their atmospheres. About the structure and chemical composition researchers can only guess about the stellar interior, based on the results of complex theoretical calculations.