The Stars are so far away from us that they look like a pin point of a light to us. We can calculate the distance of a Star by observing the light of a Star or the colour of light coming from the Star. The distance to a Star is calculated in the units of Light Years . Light Year can be defined as “The Amount Of The Distance Travelled By The Light In One Year. It Is An Astronomical Unit” . The nearest Star System to our beautiful planet Earth or to our Solar System is the “Alpha Centauri System” . In Alpha Centauri System the Star closest to the Earth is “Proxima Centauri”. The distance to the Proxima Centauri is 4.243 Light Years from Earth . If we calculate these figures in Kilo Meters, Then we will get the distance about 41,000,000,000,000 Kilo Meters . In other words we can say that, If we somehow travel at Speed Of Light ( 1,86,000 Miles Per Second ) , Even then it will take 4.243 Years of continuous journey for us to reach Proxima Centauri.
The above illustration showing the position of Planet Earth, Sun, Alpha Centauri A, Alpha Centauri B
Sir Issac Newton discovered that when the light of the Sun passes from a Prism It breaks up into it's components colour known as it's Spectrum as in a Rainbow.
Similarly, We can observe the light coming from other Stars by focusing a Telescope on a Galaxy or a Star. Different Stars have different Spectra but there Relative Brightness of different colour is always exactly the same. One would expect to find in the light emitted by an object that is glowing red hot. The light emitted by a glowing red hot object has a characteristic Spectrum that will depend only on it's Temperature , which is also called as Thermal Spectrum.
This means that we can measure the Temperature of Star from the Spectrum of it's light. We also find that there are certain Very Specific Colours missing from the Spectra. This can be varied from Star to Star. We know that each chemical element absorbs very specific colours, For example,
When a light is passed through a discharge tube containing Hydrogen Gas , Then this emitted light from discharge tube is again passed through the vapours or chemical substance and then analysed by the Spectroscope , Then we get a spectra consisting of Dark Lines.
The Hydrogen Absorption Spectra
In the figure above, We can see that there are Dark Lines in the Continuous Spectra, These dark lines represents the missing Wavelengths or Colour Of Light absorbed by the Hydrogen Atoms. Similarly, All other elements absorbs some amount of Wavelengths or the Colour Of Light.
By matching the Spectra of the known Elements on Earth and the Spectra received from a Star and by pointing out the missing Spectra from Star Spectrum’s , We can determine exactly about the Elements present in the Star Atmosphere .