INTRODUCTION
The starry sky is not only delight and eternal mystery, but also a source of fundamental knowledge about the world in which we exist. Numerous people consider studying various galaxies, black holes, moons, planets, comets and asteroids to be extremely fascinating. However, the author is especially interested in researching stars: their formation and evolution, structure, characteristics, classification, distribution, radiation.
Since it was first described in 1842, the Doppler Effect has found various applications. For example, sirens, radar, satellite communication and
The astronomers did not ignore this effect either: the Doppler shift of the spectra of stars and other celestial bodies determines their speed of movement relative to the Earth. For example, it led to the discovery of the expansion of the Universe.
The object of this work is the application of the Doppler Effect for studying stars.
However, the author is particularly interested in the not very well known application of the Doppler Effect in astronomy, namely that with its help one can measure the temperature of a star.
The subject of this research is the relation between the temperature of the photosphere and the broadening of the spectrum lines caused by the Doppler Effect.
The purpose of this research is to study the principles of measuring the temperature of a star`s photosphere using the Doppler broadening of the spectrum lines. To achieve the goal, the following tasks are identified:
· 1) to explore the essence of the Doppler Effect,
· 2) to study the general principles of emission and absorption of light by atoms,
· 3) to describe the dependence of the broadening of spectrum lines on the temperature.
The practical value of the research performed is reflected in the fact that its results can find application in popular science lectures, be a part of an astronomy lesson with an in-depth study of the relevant sections of the course, as additional materials for self-study
Structure of the work: the work consists of an introduction, three chapters dedicated to the features of the Doppler Effect, absorption and emission of light by atoms; including the properties of the Bohr model, absorption and emission spectra and thermal radiation; Doppler broadening and conclusion.
CHAPTER I. THE DOPPLER EFFECT
A change in frequency or wavelength in relation to the observer`s movement relative to the wave source was described by Christian Doppler in 1842 and subsequently was named after him.
The common example of the Doppler Effect is the change of pitch of a sound produced by a car driving past the observer (e.g. a horn signal or engine noise). While the vehicle is approaching the observer, the frequency of the received sound is higher than of the emitted one and it is lower while it is receding.
The Doppler Effect occurs not only for sound waves, but also for electromagnetic ones. Let the distance to the source be ct (c is the speed of light, t is time it takes for the light the reach the receiver) and 𝜈0 - the source radiation frequency. During time t, the source emits 𝜈0t waves. In the absence of movement 𝜈0t waves fit on the segment ct but if the source moves (for example, it moves away with the velocity 𝑣r), then 𝜈0t waves will be laid on the segment of length ct + 𝑣rt.
We proceed to the wavelengths. In the absence of movement the wavelength λ0, called the source’s own wavelength is 
. However, if the source moves away, then 
. Thus, the increase in wavelength is:
Then the velocity of the source moving away is:
This method is used to determine the velocity of approach or recess of various objects: from stars and galaxies to cars exceeding the speed limit. This method plays a significant role in astronomy and astrophysics, where there is often no other way to find out how fast distant objects move. According to the resulting equation, we can determine the radial velocity of the source (star) relative to the Earth if we know source`s own wavelength 
and the received wavelength λ. The ways to determine the wavelength of the light emitted by a distant star will be considered in the following chapter.