Astronomical popular science: visual Doppler effect-red shift and blue shift

Red shift means that when an object is far away from the earth, the wavelength of the light it emits will increase. Blue shift is the antonym of red shift, which refers to the decrease of wavelength caused by the object approaching the earth.

Red shift and blue shift are visual versions of Doppler effect. You may have experienced the Doppler effect first hand. The best example is that when a car rings a siren, the tone of the siren is much higher than when it passes by and leaves you. The increase of this sound corresponds to the increase of frequency.

The Doppler effect also applies to light waves. When an object approaches us, the wavelength of light will move to the blue end of the spectrum; When an object is far away from us, the wavelength will move to the red end. This change can be observed on the spectral line.

Red shift and blue shift diagram

The absorption line (right) in the supercluster spectrum of distant galaxies is compared with that in the solar spectrum (left). The arrow indicates that the wavelength of red shift increases to red shift and above (frequency decreases).

The history of red shift and blue shift

The Doppler effect is named after the physicist Christian Andreas Doppler, who gave a physical explanation for this phenomenon for the first time in 1842. Subsequently, this hypothesis was confirmed by the experiment of Dutch scientist Christopher Balot in 1845.

Doppler redshift was first proposed by French physicist Armand Fizeau in 1848. He pointed out that the movement of the star spectral line position is related to the Doppler effect, so the Doppler redshift is also called the "Doppler-Fizeau effect". 1868, British astronomer william huggins used this theory to measure the speed of stars relative to the earth for the first time.

187 1 year, the Fraunhofer line and Fifi line in the solar spectrum have a red shift of 0. 1 angstrom, which confirms the optical red shift theory. 190 1 year, Aristak BeLoppol-sky proved the optical red shift with a set of rotating mirrors in the laboratory.

Looking for redshift

The spectrum of a light source from a distant object can be measured by spectroscopy. In order to measure the red shift, it is necessary to find out some characteristics in the spectrum, such as absorption line, emission line or other light intensity changes. After the red shift is discovered, spectra with similar characteristics need to be compared before measurement. Atomic spectra of hydrogen, a very common element in the universe, can be used.

In the picture above, you can see two spectra. One comes from sunlight with a known spectrum, and the other comes from a supercluster of a distant galaxy. When we compare the two, we can see that there is a correlation between the sun and the hydrogen lines of distant galaxies. The only difference between the two is that the absorption lines in the galaxy spectrum have moved to the red end. This shows that the red shift is happening, and this galaxy is moving away from us (or we are moving away from the galaxy).

Calculation methods of red shift and blue shift

When we find a known spectral line, we can calculate its wavelength in the spectrum. Then we can use this to calculate the value of redshift.

From the chart above, we can find the hydrogen alpha emission line at 656.2nm. Then we can calculate the wavelength according to the observed spectrum. For this example, the observed line is at 675 nanometers. In this way, we can use a simple equation to calculate the value of red shift.

Replace the wavelength data we observed:

Z is a dimensionless quantity, with positive values indicating red shift and negative values indicating blue shift.

Red shift example

The celestial body with the highest known redshift is a galaxy. The most reliable redshift comes from spectral data. At present, the galaxy with the highest spectral redshift is IOK- 1, and the redshift z=6.96.

(GRB 0809 13)

The farthest gamma ray burst observed is GRB 0809 13, and its red shift is z=6.7.

relevant knowledge

Doppler effect (English: Doppler effect) is a phenomenon that the frequency of the wave received by the observer is different from that of the wave emitted by the wave source when the wave source and the observer move relatively. The train whistle in the distance becomes sharp (that is, the frequency becomes higher and the wavelength becomes shorter), while the train whistle leaving us becomes lower (that is, the frequency becomes lower and the wavelength becomes longer), which is the Doppler effect phenomenon. The same phenomenon happens to the ringtones of private cars and trains.

This phenomenon was first discovered by Austrian physicist Doppler in 1842. 1845, Dutch meteorologist Buys Balot stood a group of trumpeters on an open-top train that sped by near Utrecht, the Netherlands, and he noticed the change of the tone on the platform. This is one of the most interesting experiments in the history of science.

Since the second half of19th century, astronomers have been using Doppler effect to measure the apparent velocity of stars. Now it has been widely used to prove the observation of celestial bodies and artificial satellites.

Author: Tim trotter

Bill booth

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