Tracing Chemical Elements in the First Stars

I continue to be fascinated by the existence of the first stars in the universe. What were they like? How long did they live? How can we spot evidence of them? These are all questions astronomers who study the universe continue to ask and try to answer.

At this meeting, we heard from astronomers who are searching out evidence of those first stellar objects. They’re not exactly easy to spot. You can’t really just stare hard across more than 13 billion light-years and see these things. They’re too dim, too far away, and hidden behind clouds of gas and dust. It seems like that would be the kiss of death for learning anything about those stars, but instead, astronomers found a way to turn that “bug” into a feature by performing spectroscopy on the cloud (studying the characteristics of star light after it passed through the cloud).

Neil Crighton and Professor Michael Murphy from Swinburne University of Technology in Melbourne, Australia, and Associate Professor John O’Meara from Saint Michael’s College in Colchester, Vermont, U.S., studied the light from distant stars after it filtered through a distant cloud of gas that lies in the line of sight between us and the stars. It turns out the cloud may bear the signature of those very first stars and tell astronomers something about them and the lives they led.

What Light Filtered through a Cloud Reveals

The distant gas cloud is billions of light-years away from us, and appears as it looked 1.8 billion years after the Big Bang. That’s about the time the first stars were forming.  The cloud has an extremely small percentage of heavy elements. These are carbon, oxygen and iron. It turns out that’s less than a thousandth the fraction of the same elements observed in the Sun, which was born a few generations AFTER the first stars, in a universe that was significantly richer in heavy elements.

So, what’s the deal about the lack of heavy elements in a distant cloud in the early universe? Crighton explained it. “Heavy elements weren’t manufactured during the Big Bang, they were made later by stars. The first stars were made from completely pristine gas, and astronomers think they formed quite differently from stars today,” he said. We can thank those stars for their role in the enrichment of the universe by heavier elements.

The first stars — which astronomers call “Population III stars” — were born as quite massive stars out of the original hydrogen supplies in the infant universe. Then, they did what stars do — fuse hydrogen in their cores to form helium. Then, they went on and fused heavier and heavier elements — the first stars to do so.

Then, those stars began to die in supernova explosions, which spread their heavy elements into surrounding pristine clouds of gas, infusing them with an unmistakeable chemical record of the first stars and the “star stuff”  their deaths hurled into space. Astronomers read that record like detectives study a fingerprint at a crime scene.

The way they do it is to study the light that passes through the clouds. Different elements absorb different wavelengths of light, and we see those as “dropouts” in the light. Those dropouts tell you what elements are in the cloud, and since they’re heavy elements, they had to have come from the nearby stars that lived, died, and spread their carcass materials through space.

“Previous gas clouds found by astronomers show a higher enrichment level of heavy elements, so they were probably polluted by more recent generations of stars, obscuring any signature from the first stars,” Dr. Crighton said. That makes it the first cloud to show the tiny heavy element fraction you would expect to see in a cloud enriched by the first stars.

What’s Next?

AS I mentioned in a previous article, the best science requires more than one example of something to study. That would be true with these distant clouds and the story they tell of the first stars. So, astronomers want to find more of these systems, where they can measure the ratios of several different kinds of elements. By finding new clouds with traces of the more heavy elements, astronomers can trace the existence of more early stars and track how they began to enrich the infant universe with elements that got used later to make the next generations of stars, form planets, and seed Earth (and hopefully other worlds) with life.