Stellar Population

Last Wednesday (4th July) at Sion we discussed about the Population of stars. There were around 10-15 people present for the discussion. Sukhada started the topic and gave some information and then we discussed about few things.

Stars are divided into three populations according to metallicity. Metallicity is the proportion of the elements present in the star other than hydrogen and helium to the mass of the star. The population 3 (or generation 1) stars were formed just after the big bang and were very massive i.e. about 100 Solar masses. They lived for very short period of few million years and died off. They did not have any metal content (here metal means elements other than hydrogen and helium) and there is no star found to be population 3 in today’s universe. They formed little amount of elements upto iron in their core which increased the metallicity of further stars.

Population 2 (and generation 2) stars were formed after the death of the population 3 stars and hence they contain very little amount of metallicity (about 0.1%). Population 2 stars are mainly seen near the centre of Galaxies and hence most of the globular clusters are formed by these stars. Population 1 (or generation 3) stars are metal rich stars which show metallicity of about 2-3 %. The metallicity is also responsible to form the planetary systems to form around these stars. Our Sun is population 1 star and it’s metallicity is around 2 %.

Unsolved questions:
1. Metallicity of stars is calculated with respect to Sun’s metallicity, then how Sun’s metallicity is calculated?
2. How to calculate the mass of particular element in a star or say Sun by analyzing the spectrum?

Topics for next week:
1. Initial Mass Function.
2. Scattering of light.
3. How the number of electrons for the particular element is determined or basically in History how they determined the number of electrons present in the atom of a particular element.