The first stars were nothing like the relatively cool, long-lived stars that mostly populate the universe today. At the time, more than 13 and a half billion years ago, almost all the visible matter in the universe was comprised of hydrogen with some helium.
Without heavier elements, the first stars, once lit by nuclear fusion, furiously churned through their hydrogen stores and then burst in supernovae. These behemoths swelled to some hundred times the mass of the sun, and they lived for only a few million years. For comparison, our home star is about 4.6 billion years old, and it will continue living for at least that long.
Yet astronomers have never seen these early stars. They sparked to life at the end of a period called the cosmic dark ages, when the universe was suffused with opaque hydrogen gas. The light from these stars is not bright enough to be detected individually, even by the most powerful telescopes. To peer into the hearts of these monsters, scientists are turning to supercomputer simulations, such as this recent look at a primordial star-forming cloud from the early universe.
“What’s beautiful for us is that we actually know the physics and the equations of how matter behaves and how gravity works,” says Tom Abel, a computational astrophysicist at Stanford’s Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) who made the simulation along with software developer Ralf Kaehler, also of KIPAC. “It gives you a framework in which to think about how one thing could have turned into the other thing.”
This process of transformation, as stars fused lighter elements into heavier metals, drove the evolution of the universe. Everything heavier than helium is considered a “metal” in astronomy, and these new elements were generated for the first time as the earliest stars erupted in supernovae and scattered their contents across the cosmos.
At some point, assemblages of stars swirled together to form the first galaxies, including the earliest structures of the Milky Way. Metals accumulated, and new generations of stars formed from these heavier elements, many evolving to be smaller, cooler, and longer-lasting. Around some of these stars, leftover dust—material made during supernovae—clumped together into the first planets.
The birth of the first stars represents the beginning of a sequence that produced all the worlds and living beings of the universe, and simulations can be used to study the critical first steps that telescopes cannot yet see.
Layers of a cosmic cloud
Scientists can simulate the universe with ever-growing capability thanks to advances in both physics and computing. Inspired by the launch of the James Webb Space Telescope, which quickly began discovering earlier galaxies than ever seen before, Abel runs new simulations of the early universe for months at a time with almost a thousand times more resolution than was possible when he started working on cosmological computer models more than 20 years ago.
It allows for experimentation, Abel says. “If I change this a little bit, you know, what happens then? And so you can build up an intuition of the how the universe works and how the pieces fit together.”
For the first stars to ignite, gas had to accumulate in dense enough pockets to force hydrogen atoms to fuse into helium, releasing heat and energy. This occurred due to the gravitational forces of an invisible hand: dark matter. Before the first stars blazed to life, this unseen matter, which astronomers believe accounts for about 85 percent of all matter in the universe, clumped together in structures called dark matter halos.
These immense orbs—named for the way dark matter surrounds visible material and creates rings of blackness encircling light—form the scaffolding of the universe. Within them, turbid pockets of gas were forced ever inward, kindling the fires that would end the cosmic dark ages.
One of the benefits of simulating the first stars, Abel says, is gaining an appreciation for how the fundamental physics of hydrogen, the tiniest and lightest element, dictated the formation of giant stars that would transform the universe.
During the dark ages, most of these atoms were in the form of neutral hydrogen—that is, individual atoms flying freely through space. At the centers of large dark matter halos, where much of this neutral hydrogen amassed, the temperatures rose and individual atoms would sometimes collide and stick together, forming molecules of two hydrogen atoms.
At this point, things started to change. As the Stanford simulation shows, a cloud forms—about a thousand light-years across—where molecules of hydrogen accumulate. The outer layers of this cloud began to cool because the newly formed hydrogen molecules occasionally release photons of light, bleeding away energy and heat. As temperatures drop, the infalling gas slows down, and material behind it piles up, sending shock waves through the cloud.
“There is so much structure in here,” Abel says of the simulation’s different layers of a star-forming cloud. “It’s so much fun.”
Deeper within the cloud, additional layers are heated or cooled, causing more turbulent collisions. The cooling processes also reduce the pressure of the gas pushing outward—the primary thing fighting against gravity. Inexorably, bit by bit, the cloud collapses ever inward.
“Essentially what will happen is that there’s an about 10-Jupiter-mass object that will form, and then that will accrete very rapidly,” Abel says.
Scientists don’t know exactly how big these earliest stars got as gas continued to pile on, but they may have grown to hundreds of times the mass of the sun.
Supercharging the universe
The intense energy released by the first stars not only scattered metals in supernovae, but also blasted the cosmos with ultraviolet light. This radiation stripped the neutral hydrogen atoms of their electrons and made the gas more transparent, a key time in cosmic history known as reionization.
While we may never find the very first star to shine out in the abyss, our ability to simulate the cosmos is providing an ever-clearer picture of what this key time must have been like. Such simulations could also reveal parts of the universe’s future.
“You can study the very first thing that we haven’t seen yet,” Abel says, “and you can study the very last thing that people could ever see.”