On the first stars

The Big Bang basically produced only Hydrogen and Helium (and the Universe is still full of Hydrogen and Helium), which means the first stars would have solely had Hydrogen and Helium to work with when they formed. These so-called “Population III stars” have been predicted for years, but no one has ever seen evidence of their actual existence… until possibly now, when a team of astronomers at ESO have discovered proof that this first generation of stars actually existed. (Ignore the title of that article, there are no ‘spectacular photographs’ yet, it’s just an artist’s interpretation).

It’s not that anyone doubts the existence of Population III: If there are stars, there must have been a first batch of stars… we just haven’t seen any yet. In theory, by only having hydrogen and helium, these stars should have been very different from normal stars, because the heavier elements are needed to catalyze some of the common fusion reactions that turn hydrogen into helium. Yeah, I know catalyze is a chemistry term, but the process is kinda similar. Without heavier elements, the stars would have had to rely on more inefficient fusion reactions and would have been able to be larger than most stars today.

A quick explanation: The concept of “populations” came out of work done in the 1950s by Dr. Nancy Roman* and others. Population III stars are the hypothetical first generation, with NO metals, produced before ANY supernovae had a chance to enrich the gas in the universe. Population II stars are noticeably deficient in “metals” (all elements heavier than hydrogen and helium) as if they formed before many supernovae had exploded and enriched clouds of gas in the Galaxy; as Dr. Roman noticed, they also had much more inclined orbits that swung above and below the disk of the Galaxy, making a “thick disk” and globular clusters. Population I was made up of the stars we know well, that form the “thin disk” of the Galaxy. Presumably the next highly-enriched stars will be the awkward-sounding Population 0, but everyone responsible for this nomenclature will have passed into myth and legend by the time it’s a problem.

In the past, astronomers have proposed that extremely low metallicity stars (for instance, the low mass star SMSS J0313 has 1/10000000th the amount of iron the Sun does) are Population III stars that have managed to gain material after they formed, through a variety of processes. Those claims have not been terribly convincing.

What this new team of researchers have done is identify a distant galaxy named CR7 that, when examined with spectroscopy using some of the world’s most powerful telescopes, shows signs of hydrogen and helium lines, but nothing else. That’s what we hoped to see.

So have they finally identified the signs of actual Population III stars, in a galaxy 13 billion light years away (and back in time)? There are definitely some tricky parts of this analysis. The galaxy CR7 is at a cosmological redshift of 6.6, which means that it’s extremely far away and therefore a.) extremely faint and b.) extremely tiny. And c.) the redshift of the expanding universe has moved the spectral lines very far away from where we usually expect to find them.

The authors of the paper have very well established brightnessess for CR7 (25th magnitude, which is faint but still definitely possible for modern instruments). Even at the extreme distance, it’s a 3″ diameter object, which is also well within the reach of modern equipment. And they have a good spectrum: there’s a clear line that would be the brightest line of hydrogen in galaxies, and then a line corresponding to helium emission, and then nothing where carbon or oxygen should be. Of course, their spectrum isn’t quite high enough precision to actually say there’s NO carbon (so this could still be a case of Population II stars like SMSS J0313) but it’s consistent with zero. It COULD be a population of Pop III stars, less than 100 Myr after the Big Bang.

Ultimately, all the paper actually says is that to get the really large hydrogen emission, you probably need two components, of which the larger one is Population III stars, and the smaller one is Population II stars forming in their wake. As with all things scientific, we’ll need better and more detailed observations to learn more and confirm that hypothesis over the other potential explanations. It’s really cool they’ve gotten this far.

* Roman spent most of her career at NASA designing and supporting space missions, including the Hubble Space Telescope, back when it was just called “Space Telescope”. She was also involved in digitizing a lot of early astronomical catalogs we still rely on today, and is currently (at the age of 90) working on education and public outreach.

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