Explanation of the phenomenon of stars twinkling in the night sky, preventing planets from twinkling, and the effect of the atmosphere on the path of light.
Omer Azran, Davidson Institute, the educational arm of the Weizmann Institute of Science
Since the dawn of history, humans have looked up to the sky and have come up with many theories about the origin of the stars: from myths about gods or souls, to the understanding that they are very distant, hot balls of gas. When you look at the stars, you can't miss their mesmerizing twinkle, but have you ever wondered why this happens? Before we answer this question, let's take a few steps back.
Who are you, stars?
If we look at the night sky from within the city, we will see dozens of points of light, and if we go far enough to a dark place, we can even see thousands of twinkling points. Some of the light comes from planets, galaxies, or nebulae, but most of the points originate from light coming from stars or star clusters. Stars, also called suns in the astronomical context, are enormous balls of gas – like our sun. Almost all the stars we see are suns that are far away from us. They differ from each other in their size, the materials they are made of, their age, and their temperature. Stars are made mainly of hydrogen gas, which is the lightest substance in nature. At the core of a star, hydrogen is so hot and dense that quartets of hydrogen atoms tend to combine together to form another gas, helium. This process is called nuclear fusion, and during its occurrence, a tremendous amount of energy is released. The starlight we see is the result of this energy emission.
Unlike stars, planets do not undergo nuclear fusion. They do not produce energy on their own, but receive it from their stars. The Earth, for example, receives energy from the Sun, in the form of light and heat, and does not produce energy through an internal process in its core.
Light coming from stars or star clusters. The night sky is dotted with twinkling stars | Design: Liat Peli, Image: KK.KICKIN, Shutterstock
And why are you sparkling?
Because the stars are very far away from us, the light from them also travels a long way. The light beams from them enter the Earth's atmosphere, where the air is constantly moving and shifting due to the interaction between warm and cold air, winds, and other factorsThe incoming light is "broken" between the layers of the atmosphere, and as a result of the path it takes, which consists of broken lines (in a zigzag shape), the stars appear to us to twinkle.
If the Earth had no atmosphere, the stars would not twinkle; and when astronauts observe them in space, outside the Earth's atmosphere, or when they are photographed with a telescope in space – they do not twinkle.
And what about planets?
The planets we see are in our solar system, so they are closer to us than the stars. Unlike stars, which are so far away that we see them as dots even when using magnifying glasses, planets appear to us through a telescope as disks, and sometimes we can even see details about them, such as the red storm on Jupiter or the rings of Saturn. Because the light from each point on the disk takes a slightly different path through the atmosphere, the changes in the brightness of all the points together cancel each other out to a good approximation, and the planets hardly appear to twinkle to us. They also appear much more prominent and brighter compared to the stars. When you look up at the sky and see bright stars that barely twinkle, it is probably one of the planets closest to us: Venus, Jupiter, Saturn, and Mars.
You can use software or an app. Stellarium To find out exactly which star or planet we are looking at.
Distances in space
Because distances in space are vast, describing them in the usual units of distance, such as kilometers, would produce astronomical numbers—literally—that are difficult for us to grasp. Therefore, the unit of measurement used to describe distances in space is called light year, and means the distance that light travels in one year.
Light is electromagnetic radiation, and its speed of travel in a vacuum is about 3×108 (3 followed by eight zeros, or three hundred million) meters per second. There are 365 days in a year, each of which is 24 hours long; each hour is sixty minutes long and each minute is sixty seconds long. Therefore, a full year has approximately 365×24×60×60 seconds, or approximately 31,536,000 seconds. The approximation is due to the fact that for the purpose of calculating a light year, the length of a year is measured with extreme precision, and includes small deviations in the number of days in a year or hours in a day.
If we multiply the speed of light per second by the number of seconds in a year, we find that the distance light travels in a year is approximately 9.5×1015 Meters – that’s how a light year is defined. One light year is equal to about 63,240 astronomical units, which is the average distance of the Earth from the Sun – the distance varies throughout the year, depending on the Earth’s orbit around the Sun. The distance between the Milky Way Galaxy, our galaxy, and the Andromeda Galaxy, the nearest galaxy, is about 2.5 million light years.
So when we look at a star one light-year away from us, we are actually seeing the light that came out of it a year ago, and are essentially "going back in time" to the moment when the light came out of its source.
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