When you look at the night sky, you can probably see the billions of stars that make up the firmament. One of the curious things about stars, unlike planets and other satellites, is that they twinkle. That is, they appear to be shining intermittently. Many people wonder why stars twinkle and planets don't.
For this reason, we are going to dedicate this article to telling you why the stars blink and why they do so.
why do the stars twinkle
Everything outside the atmosphere twinkles (yes, that includes the sun, the moon, and the planets in our solar system). This effect occurs when starlight interacts with air masses. In our case, that air mass is the atmosphere, which is full of turbulence . This causes the light to constantly refract in different ways, so that starlight appears to be in one place from our perspective on the surface and, after a few milliseconds, appears to change slightly.
Why don't we notice the twinkling of the planets, the sun, and the moon? It's easy to explain. Because of our distance from them (the nearest star, Proxima Centauri, is just over 4 light-years away), these stars appear as mere points of light. Since only a point of light reaches the atmosphere, it can be greatly affected by air turbulence and will therefore continue to twinkle. In addition to being closer , the planets appear as disks (though not to the naked eye), which makes their light more stable (whereas the moon and the sun are much larger, so the effect is imperceptible).
Some stars seem to change color
On some nights, around midnight, the Quintuple Star (one of the brightest stars visible in the sky) is above the horizon (in a north-northeast direction), but close enough that it appears to twinkle and fade across a wide range of colors (red, blue, green, etc.). This is a fairly common phenomenon, easily observed in stars near the horizon, but also seen in other stars.
The explanation is the same as for twinkling, but we add that the amount of air the light has to travel through to reach us is much greater, so the refraction is more pronounced , which also makes it seem as if stars are constantly changing color. Likewise, although they don't usually twinkle, planets can also emit this changing light if they are very close to the horizon.
How to avoid flicker
While the twinkling of stars poses no problem for us, for astronomers things can be very different. We have many observatories on Earth, so we must eliminate this distortion to see the stars . To do this, some of Earth's most advanced telescopes use adaptive optics, rotating the telescope's mirrors many times per second to compensate for turbulence in the atmosphere. In some ways, it's similar to the correction systems used when observing the night sky.
Astronomers project a laser into the sky, creating an artificial star within the telescope's field of view. Now that they know what the artificial star should look like and what color it should be, all they have to do is adjust the mirror's distortion with a piston to eliminate the effects of atmospheric distortion. It's not as efficient as launching a telescope into space, but it's much cheaper and seems to suit our needs well.
Another option, as you have seen, is to launch the telescope directly into outer space. Without the intervening atmosphere, the flicker disappears completely. Probably the two most famous space telescopes are Hubble and Kepler.
In size, Hubble is much smaller than the telescopes we have on Earth (it's actually about a quarter the size of a telescope mirror at a large observatory), but without the effects of atmospheric distortion, it's able to capture images of galaxies billions of light-years away —in just a few years. You just have to look in that direction long enough to receive light from it.
Also, some telescopes have a small secondary mirror that corrects for this atmospheric turbulence, but this is not common. That is, the process is as I told you, but the distortion does not occur in the main mirror, but in the small mirror that is part of the tool that we use to see.
The stars change intensity
You may have heard that stars twinkle because they emit different amounts of light. While this is true, the change isn't noticeable enough to cause a flicker in the night sky, and it occurs over a longer period rather than just a few seconds. In fact, some of these stars are known to vary in brightness and size, and we use this information to help us better explore the universe. Simply put, stars twinkle because the Earth's atmosphere distorts their light before it reaches us.
Since they are so far away, we can only see tiny droplets of light, so this distortion occurs, and the closer you get to the horizon, the more pronounced this distortion will be. In the case of the planets, although they appear larger to the naked eye, they appear to us as small discs of light, and enough light reaches the atmosphere so that the distortion of the light caused by the atmosphere is imperceptible.
Why the stars twinkle: the atmosphere
The light that leaves the star and travels far to Earth is barely deflected. It travels in a straight line. When it has to pass through the atmosphere, its path changes. Although the atmosphere is transparent, it is not a layer of uniform density. The parts closer to the surface are denser than the upper layers. In addition, during the day, warm air rises, which is less dense than cold air. All of this causes the atmosphere to become turbulent gas. We emphasize, even though it is transparent.
When starlight is about to reach us, it has to pass through the atmosphere. It is slightly deflected each time it encounters layers of air of different densities. It is refracted when it changes from one medium of one density to another. And so on, continuously. Since the air is constantly moving, we perceive the tiny dance the stars perform as also constant, giving the impression that they are twinkling. These small deflections can also cause them to change color, just as the sun does when it sets over the horizon.
I hope this information helps you learn more about why stars twinkle but planets don't.
