Stars are classified into two types by astronomers. Population I stars - like the Sun, Betelgeuse, Deneb and many others - are found mostly in the spiral arms of the Galaxy. They are hot, luminous and formed in open clusters. Population II stars are older, less luminous and found in globular clusters and in the bulge at the centre of the Milky Way. Population II stars have fewer heavy elements in them that Population I stars. Since heavy elements are created within stars and scattered across the Galaxy by supernovae, it makes sense that Population I stars which have been around longest were formed in conditions where there was a lot less heavy elements.
The very first stars in the universe - born perhaps 400 million years after the Big Bang (when the universe was about 2% of its current age) would have formed from the raw ingredients created in the Big Bang. Those ingredients were hydrogen, helium and a dash of lithium. This first generation of stars is part of what astronomers call Population III.
The universe was a lot warmer when it was 2% of its current age. That coupled with collapsing regions that were almost entirely hydrogen and helium means that Population III stars could be much more massive than modern day stars. There is an upper limit to stars forming today - thought to be around 150 solar masses. By contrast Population III stars were monsters - with perhaps thousands or tens of thousands of solar masses.
Those first stars not only shone brightly it is thought the intense UV radiation from them shaped the early universe by reionising hydrogen which had previously recombined as the universe expanded and cooled after the Big Bang.
Population III stars would have lived fast and died young. Current theories suggest they would have exploded as violent supernovae within a few million years of forming. Perhaps those stars created black holes which were the seeds for the supermassive black holes in the centres of many galaxies today. Perhaps they didn't and we still need another explanation. There are no Population III stars in our universe now - conditions necessary for their formation don't exist any more.
At present there is no direct evidence of Population III stars. Hardly surprising really. To see them today we'd have to look deep into the universe to collect light which has been travelling from them for well over 13 billion years. During that time their light will have been redshifted so that they now shine in infrared and longer wavelengths. They must be very faint. However, there are hopes the James Webb Space Telescope will be able to see signs of our universe's first stars.
Dr Adrian Jannetta
Guitar strummin' explorer of the universe. Mild mannered maths teacher by day and astronomer by night.