What Were the First Stars Like?

Massive Blue Monster Stars

An artist's concept of what the first stars in the universe might have looked like. NASA/WMAP

What Was the Early Universe Like?

The infant universe was nothing like the universe we know today. More than 13.7 billion years ago, things were very different. There were no planets, no stars, no galaxies. The earliest epochs of the universe occurred in a dense fog of hydrogen and dark matter. 

It's tough to imagine a time when there were NO stars because we live in a time when we can see thousands of stars in our night sky. When you step outside and look up, you're looking at stars in a tiny portion of a much larger stellar city—the Milky Way Galaxy. If you look at the sky with a telescope, you can see more of them. The largest, most powerful telescopes can extend our view out more than 13 billion years, to see more and more galaxies (or shreds of galaxies) out to the limits of the observable universe. With them, astronomers are seeking to answer questions about how and when the first stars and galaxies formed. 

Which Came First? Galaxies or Stars? Or Both?

Galaxies are made of stars, primarily, plus clouds of gas and dust. If stars are the basic building blocks of the galaxies, how did they start forming? To answer that question, we have to think about how the universe began, and what the earliest cosmic times were like.

We've all heard of the Big Bang, the event that began the expansion of the universe. It is widely accepted that this pivotal event took place about 13.8 billion years ago. We can't see back that far, but we can learn about conditions in the very early universe by studying what is called the cosmic microwave background radiation (CMBR). This radiation was emitted about 400,000 years after the Big Bang, and it comes from light-emitting matter that was distributed throughout the young and rapidly expanding universe.

Think of the universe as being filled with a fog that was giving off high-energy radiation. This fog, sometimes called a "primordial cosmic soup" was filled with atoms of gas that were cooling as the universe expanded. It was so dense that if stars existed, they couldn't be detected through the fog, which took several hundred million years to clear as the universe expanded and cooled. That period when no light could work its way through the fog is called the "cosmic dark ages".  

The First Stars Form

Astronomers using such satellites as the Planck mission (which looks for the "fossil light" from the early universe) have found that the first stars formed a few hundred million years after the Big Bang. They were born in batches that became "proto-galaxies". Eventually, matter in the universe began to arrange into structures called "filaments", stellar and galaxy evolution began. As more stars formed, they heated the cosmic soup, a process called "reionization", which "lit up" the universe and it emerged from the cosmic dark ages.

So, that brings us to the question "What were the first stars like?" Imagine a cloud of hydrogen gas. In the current view, such clouds were constrained (shaped) by the presence of dark matter. The gas would get compressed into very small regions and temperatures would rise. Molecular hydrogen would form (that is, atoms of hydrogen would combine to form molecules), and the gas clouds would cool enough to form clumps of matter. Inside those clumps, stars would form—stars made only of hydrogen. Since there was a lot of hydrogen, many of these early stars could have grown very large and massive. They would have been very hot, emitting a lot of ultraviolet light (making them appear blueish.) Like every other star in the universe, they would have nuclear furnaces at their cores, converting hydrogen to helium and eventually to heavier elements. 

As is the case with very massive stars, however, they probably lived for perhaps only a few tens of millions of years. Eventually, most of these first stars died in catastrophic explosions. All the materials they cooked up in their cores would rush to interstellar space, contributing heavier elements (helium, carbon, nitrogen, oxygen, silicon, calcium, iron, gold, and so on) to the universe. Those elements would mix with the rest of the hydrogen clouds, to create nebulae that became the birthplaces of the next generations of stars. 

The galaxies formed as stars did, and over time, the galaxies themselves were enriched by the cycles of starbirth and stardeath taking place. Our own galaxy, the Milky Way, likely began as a group of smaller protogalaxies containing later generations of stars created from the outburst materials from the first stars. The Milky Way began forming some 10 billion years ago, and today is still ingesting other dwarf galaxies. We see galaxy collisions across the universe, so the mixing and mingling of stars and star-forming "stuff" has continued from the early universe to the present time.

If it hadn't been for the first stars, none of the magnificence that we see in the Milky Way and other galaxies would exist. Hopefully, in the near future, astronomers will actually find a way to "see" these first stars and the galaxies they formed. That's one of the jobs of the upcoming James Webb Space Telescope.