Spectroscopy Technique for determination of composition of Sun

Composition of Sun
The sun consists of mainly
hydrogen and Helium.
(source: NASA)

Sky is full of beautiful objects which always attract human and give rise to curiosities. In your childhood, you must have been wondering what are these twinkling objects and what are they made up of? You must have heard or read that the researchers have discovered Hydrogen and Helium in Sun. So what do you think how do they know that? Have they gone to the Sun and brought the sample and then studied them to know about the composition of Sun? The answer is no because no one can go to the Sun and collect the samples. So how do they detect the composition of Sun or any other star?

The researchers observe the light coming from the stars. This light contains information about the composition of star and the elements present in the star. The method is called spectroscopy.

You must have seen the rainbow during rain. Do you know what is it and how does it form? The answer can be given by simple activity which Newton had also performed. He passed the white sunlight from the prism and obtained the colourful bands resembling the rainbow. Newton concluded that the white light is made up of seven different colours. The droplets of water act as prism during rain and split the white sunlight into seven different colours to give rise to the beautiful rainbow.

Visbile Light Spectrum
Visible light Spectrum after passing white light through prism

Emission Spectrum

When any element is heated or electrically ionised, the electrons of the atom get excited and jump to higher energy states. When the electron jump back from the higher energy states to lower energy states they emit light.  For example, when Hydrogen gas is heated in the test tube, it emits light. When this light is passed through the prism 4 different lines of various colours are observed.

Emission Spectrum of Hydrogen
Emission Spectrum of Hydrogen

Similarly, the heated sodium vapours produced light consists of only two yellow lines.

Emission Spectrum of Sodium
Emission Spectrum of Sodium

The spectrum obtained after heating an element is called emission spectrum.

Absorption Spectrum

When the white light is passed through some element and observed through the spectrometer. Some dark lines are obtained in the spectrum. For example, the spectrum of white light passed through the Hydrogen gas has 4 different dark lines.

Absorption Spectrum of Hydrogen
Absorption Spectrum of Hydrogen

The spectrum obtained after passing the white light through a medium is called absorption spectrum and the absent lines are called dark lines.
You must be wondering to see that the 4 lines absent in the above picture, are corresponding to the 4 lines present in the emission spectrum of Hydrogen gas. This is because the lines present in emission spectrum or absent in absorption spectrum correspond to the specific energy levels of a particular element. So the element emits particular lines when heated and it absorbs the same lines when a white is passed through it. This indicates that by identifying the lines present in emission spectra or dark lines in absorption spectra we can detect which element is present in the sample. The same spectroscopy technique is used in the detection of elements present in the stars.

Solar spectrum

When sunlight is passed through the spectrometer, many dark lines are observed. These dark lines are due to the presence of different elements in the sun. The white light emitted from the sun when passed through the atmosphere of Sun, it gets absorbed by the different elements present in the atmosphere. The same absorbed lines appear as dark lines in the spectrum we observe. Now we can detect the elements present in the Sun by identifying the dark line.

Solar spectrum
Solar spectrum having dark lines due to absorption by elements present in the atmosphere.

You can see many dark lines in the solar spectrum which indicate the presence of different elements in the Sun.

Elements in Sun

The Helium was discovered by Norman Lockyer in 1868. He observed the solar spectrum and obtained a dark line at 587.6 nm. He thought this must be a new element and named it as Helium. The Hydrogen in the sun was also discovered by Cecilia Helena Payne through spectroscopy. The Hydrogen and Helium are most abundant elements found in all the stars. The trace amount of oxygen, carbon, neon, nitrogen, magnesium, iron and silicon are also detected in the sun.

Other application of Spectroscopy

Spectroscopy can also be used for measurement of the speed of galaxy, rotation of star and temperature of stars. The motion of stars and galaxy can be studied by the Doppler shift. Spectroscopy is very powerful tool for measurements in the present astronomy.

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