The clear night sky is full of stars, and each star is a distant star, and their distance to the earth must be measured by "light years".
Some of these stars are dozens or hundreds of light years away from the earth, while some are calculated in millions or tens of millions of light years. So the question is, these little highlights that seem similar, how do we know how far it is from us? Actually, it is not difficult, only two simple calculations are required. First of all, we know that the universe is constantly expanding, and the expansion of this space is uniform and has various homogeneity, which means that in all directions, the properties of space expansion are the same. Because of the uniform expansion of the universe, the farther away from us, the faster it will be from us. Astronomically, this is called the " degeneration speed" of the celestial body.
Since the distance from us will cause the celestial bodies to regress differently, we can use this to calculate the distance between a star and us. How to calculate
? You need to use Hubble's Law , which is very simple, that is V=H*D. In this formula, V represents the regression speed of the celestial body, H is the Hubble constant, and D is the distance of the target star relative to the earth. Now we want to calculate the distance between the astral body and the earth, we only need to transform this formula into D=V/H. H is the Hubble constant. After more than ten years of hard work, scientists have determined the Hubble constant to be between 67.8km/(s·Mpc) and 82.48km/(s·Mpc). So now you only need to know V, that is, the regression speed of a celestial body, and you can calculate the distance of a celestial body relative to the earth.
How can we know the speed of a celestial body's regression?
Here we first want to explain a phenomenon, that is, Doppler effect . Austrian Physicist Doppler discovered an interesting phenomenon while waiting for the train at the train station. The train will whistle sound when entering and leaving the station. The same whistle sounds completely different when the train enters and leaves the station. The former sounds high-pitched, while the latter sounds low. Why is this? After research, Doppler found that this phenomenon occurs due to changes in wavelength. Sound propagates in the form of waves, and since it is a wave, it has wavelengths. The sound waves emitted from a stationary sound source have a fixed wavelength. But the train is moving, which will cause changes in wavelength.
When the train enters the station, the train moving forward at a high speed will compress the wavelength of the sound wave in front. When the wavelength becomes shorter, the sound will naturally sound high.
When the train leaves the station, it is getting farther and farther away from us. This movement will lengthen the wavelength of the sound wave. As the wavelength becomes longer, the sound will naturally sound low. If you still don’t understand it very well, you can think of the sound wave as a spring, which is compressed when entering the station and lengthened when leaving the station. This phenomenon is called the "Doppler effect". The Doppler effect is not unique to sound waves, and it is also used for light. Light has the wave-particle duality , so it can naturally be regarded as a wave. If it is a wave, there will be a Doppler effect. When a star stays away from the earth due to the expansion of the universe, its wavelength will become longer. The visible light is composed of seven different colors of light, among which the red light has the longest wavelength, so stars far away from the earth will show red light. This phenomenon is called " redshift ".
Redshift phenomenon is relatively common in the universe. In areas relatively close to the earth, due to the lack of obvious expansion effect of the universe, the movement of some celestial bodies shows a tendency to be close to the earth, so blue light will appear, which is called " blue shift ".
Two celestial bodies that are moving differently from the earth, their relative movement speeds are different, and different velocities will lead to different wavelengths. Therefore, we can obtain the unique spectrum of this star by passing through the prism dispersion, and then compare the spectrum of this star with the spectrum obtained under the laboratory stationary light source to obtain the redshift of this star.
Knowing the redshift amount, you can get the regression speed of a star. The specific formula is: V=ZC, where Z represents the redshift amount, while C is the speed of light, the speed of light is constant, and the value is 299792458m/s.
Get the regression speed of the star, and you can use Hubble's law to calculate the distance between a star and the earth, that is, use the formula D=V/H mentioned earlier. It is worth mentioning that the distance calculated in this way is a rough data, which is not very accurate. It is very normal to have a few light years or even dozens of light years apart from the actual distance. However, for distant stars with hundreds or tens of millions of light years apart, such an error is completely acceptable. As for the cosmic celestial bodies at close range, there are simpler calculation methods. For example, we know that the distance between the earth and the moon is 380,000 kilometers. Now if we want to know the distance between Mars and , we only need to draw a triangle with the three stars as the vertices. When we know the length and angle of one side, we can easily find the length of the other two sides, one of which is the distance between us and Mars.
