by Prasad Ganti

The hunt for the first stars formed in the universe is on. There have been some promising candidates but no confirmations yet. The first batch of stars called the population III stars were formed when the universe was in its infancy. They consist of the primordial hydrogen and helium soon after the big bang. 

Population III stars are metal free. In astronomy anything other than hydrogen and helium are called metals. These stars are posited to be very massive, very hot and also short lived. Heavier stars, 10-1000 solar masses, burn fuel very rapidly, generating a lot of luminosity. At the end of their short lives, with all the hydrogen converted to helium, they churn out heavier elements like carbon, oxygen and nitrogen in their burnt out cores. These form the seeds for the next generation of stars – the population II stars.    

Population II stars are low  metallicity stars. Recent stars which are the population I stars are high in metals. Our Sun is considered as a population I star. Our discoveries to date have centred around population I and II stars. Population III stars are the real challenge. 

Firstly, all of them died within a few million years of their formation, which is in the distant past. There would be none in our vicinity for sure. The distant ones are dead as well. But the light from those stars could still be travelling towards us. The further we look into space, the older the objects are. Watching distant objects is like going into the past on a journey using time travel. We could be seeing objects which do not exist “now”.   

Secondly, we cannot see objects further away without any intervening objects blocking our line of sight. For example, assuming that the earth were flat and we had a powerful telescope, we still cannot see Los Angeles from Jacksonville. Because of some intervening town or city blocking our view. However, if we go higher into space, we can see. In astronomy, we don’t have the luxury of flying higher into space. Our solar system is essentially in a plane and so is our galaxy the Milky Way confined to the galactic plane. Any attempt to go beyond the solar and galactic planes needs a lot of rocket fuel. It is not a practical proposition to move out to a vantage point to make observations. All the spacecraft and telescopes launched to date are confined to our planes with very little deviation in the vertical dimension.      

Fortunately in astronomy, even if there is an obstructing object in between, we still can see objects behind using a concept called gravitational lensing. If the obstructing object is massive enough, the radiation (including light) coming from the hidden object gets bent by the gravitation force exerted by the obstructing object. As a result of this bending, the image of the hidden object appears as a distorted ring or as part of a ring around the obstructing object.  

Another factor is that the earlier universe was very compact and dense. As the universe expanded, objects moved away from each other very rapidly. The population III stars moved away as well, resulting in redshifting of the radiation coming from those stars. A promising candidate named “Earendel” (named for a character in  JRR Tolkien’s novel meaning “morning star”) is the closest we have come to observing population III stars. It was spotted by the Hubble  telescope in 2022. It later proved to be a cluster of stars, but not a population III star.

One of James Webb Space Telescope’s goals is to detect population III stars. And the hunt goes on.