Astronomers have caught a glimpse of what could be the earliest stars ever spotted. Found in a very distant galaxy, which has been nicknamed “CR7” in reference to the Portuguese footballer Christiano Ronaldo, the stars would have lived shortly after the Big Bang. They belong to a class of stars that has possibly never been seen before – stars that are uncontaminated with the metals that appeared later in the evolution of the universe.
The discovery was made using the European Southern Observatory’s Very Large Telescope.
The galaxy was observed at a redshift of ~7, which means it has taken the light from it well over 12bn years to reach the Earth. This means that the galaxy was in existence a mere ~800m years after the Big Bang.
The oldest stars are the purest
Arguably, our own Sun is an unremarkable and fairly young star situated in the spiral arms of the Milky Way galaxy. But for it to appear exactly like it does – a yellowish orb – the universe must have evolved significantly from its initial state at the Big Bang.
When we perform a spectroscopic analysis of the Sun, we find that not only do we have very common elements such as hydrogen and helium present within, but there are also heavier elements such as oxygen and carbon.
This may not sound surprising at first, but in contrast to the very early universe, elements that are heavier than helium are comparatively rare. Just how did the Sun become “dirty” with these extra elements? It must have been through nuclear evolution in the cores of stars from the elements present in the early universe to those that are observed today.
When dealing with cosmic chemistry, astronomers like to have a full working knowledge of the periodic table. Due to the comparative rarity of elements other than hydrogen and helium in the universe, the astronomer’s periodic table is remarkably simple, labelling most elements as “metals”. There are but three elements in it: hydrogen, helium and “metals”.
It is the presence of these “metals”, such as oxygen and carbon, that make the Sun “dirty” or “polluted” compared to much earlier stars in the universe. And the Sun is not alone. There are many stars in the disk of the Milky Way that are similarly polluted with heavier elements. All of these polluted stars appear to have formed relatively recently.
Past research has suggested that the stars that make up the Milky Way were composed of at least two fundamentally different populations. Stars that contained an appreciable fraction of “metals” were labelled as Population I stars. This includes our very own Sun. On the other hand, stars that were unpolluted with metals, were termed Population II stars.
Given that the universe is becoming more “metallic” with time due to the production of more “metals” inside stars with every generation, we can conclude that Population II stars must have formed much earlier in the universe than Population I stars such as our Sun.
Interestingly, this cannot be the entire story. Although they are very old, Population II stars can not have been the first stars in the Universe. Astronomers have long predicted that a third class must exist.
The very earliest stars – the first generation – must have formed from pristine hydrogen and helium. These stars would have had only very trace amounts of metals (and mostly lithium at that), in line with the elemental composition of the universe shortly after the Big Bang. All of these stars were therefore made of the same materials and would have had approximately the same mass and nearly the same life expectancy. This is because without the availability of molecules containing elements such as oxygen, small stars could not have formed.
First-generation stars were therefore very, very massive, bright, and exceptionally short-lived – a life expectancy of mere millions of years. However, proof has been exceptionally hard to come by.
The observations of galaxy CR7 showed strongly ionized helium was present, but no heavier elements. Critically, it is this lack of heavier metallic elements that indicates that the stars within this bright galaxy are Population III.
Not only is this a success in confirming the theory behind Population III stars, but it validates that we can track down the very earliest stars in the Universe – stars that were very different to our own, unremarkable, Sun. And that is no small feat. It is within these Population III stars that the heavier elements that make up human beings, such as carbon, were created.