Meghnad Saha and The mysterious story of the Sun God

Sun has been a subject of great interest for astrophysicists since time immemorial.

Since Newton proved that white light can be split into different colors, the idea of the spectrum was used for studying heavenly objects. But the spectrum of the sun posed some mysterious problems.

Basically, there are two types of spectrum 

1) emission spectrum

2)absorption spectrum

Suppose we have a gas and we pass light through the gas and observe the spectrum of light that comes out through it on a screen. Then we will observe some dark lines called Fraunhofer lines. These lines correspond to the range of wavelengths absorbed by the gas and form the absorption spectrum. This energy will excite the atoms of the gas and send them to higher orbits, but since the excited state will be unstable the electrons will transit back to lower orbits releasing energy this will form the emission spectrum.

When during a solar eclipse the spectrum of the sun was absorbed it was rather a curious case as some spectral lines were enhanced and these lines came from the outer surface of the sun known as the chromosphere.

Scientists of that time believed that the enhanced lines were due to more stimulus in the outer layer of the sun, what was interesting is that they believed that the stimulus was temperature.

This theory defies many established laws of physics as the temperature should be more at the core than at the surface. This became the major concern of many scientists including the Indian genius Meghnad Saha.

Also, there were some more questions about the sun:

1)There were no fraunhofer lines corresponding to Rb and Cs .

2) in the sun some heavy elements were found near the surface but why haven't they gotten near the center due to the immense gravity?

Saha was in search of an answer to the above questions and didn't believe that temperature can be the stimulus for enhanced lines.

He proposed that the enhanced lines were due to the ionization of an element.

But why the ionization is more in the outer layer of the sun. He was stuck with this problem.

One fine day as he was teaching thermodynamics in his lecture, a thought occurred to his mind.

Suppose we have a chemical reaction:

                     

                                        

In this rxn, the products formed can also react back to give CO+ h2O and such reactions are called reversible reactions.

Then the ionization of an element can also be treated as a reversible rxn 

for example-   

                                                                          

                        Ca ⇋ Ca+  +  e-

in this case Ca+e- can also form Ca so this is also a reversible rxn.

Using his knowledge of thermochemistry he came up with the formula which is now known as the Saha ionization formula

                 

here,

x= fraction of elements ionized

U = energy needed for ionization called

ionization potential

T = temperature

P= pressure

It was known at that time that as we go towards the surface of the sun the pressure falls much more sharply than the temperature. 

Making use of this fact he argued that if we kept the temperature constant then the fraction of atoms ionized increases with the fall in pressure and hence the ionization is more at the surface.

This was a major breakthrough in the field of astrophysics. 

Using this Saha solved many problems

1) Spectral lines of rubidium and cesium:

As discussed above scientists were not able to find spectral lines of Rb and Cs on the sun which should be present there as they are present on earth and it is believed that the earth has come from the earth so the elements of the earth should be present at the sun.

At that time ionization potential of different elements was known. The ionization potentials of Rb and Cs were very low.

Saha made use of this fact and argued that due to the low ionization potential, Rb and Cs are expected to be completely ionized which he calculated by his formula and so the spectral lines corresponding to the normal Rb and Cs cannot be found. He suggested looking at the sunspots as the temperature is low there so they won't be 100% ionized. And to his delight, the spectral lines of Rb and Cs were duly detected at the sunspots.

2)Why heavy elements are at the surface:

The most perplexing question of that time was why many heavy elements are at the surface despite the immense gravity or in other words why doesn't the sun collapse under its own gravity? It seems like Saha has got an answer for this one too. He theorized that the photons that are released at the core of the sun will have some momentum which is given by 

p = h/λ

now when this radiation collided with atoms of mass M this momentum will be transferred and the atom will attain a velocity 

  v = h/Mλ

Now this continuous supply of momentum by striking radiation pushed the atom to the surface. This concept is known as Radiation pressure and since atoms absorb only the photons which correspond to the difference between two energy levels so it is also selective and is called Selective Radiation Pressure.

Hence the mystery of the sun is solved by the Indian genius - "Meghnad Saha"

THANKS FOR READING

            -Arvin Gupta