Although the stars seem like an immutable fixture of the night sky, they haven’t always been around. Before there were stars, there was darkness. Four hundred thousand years after the Big Bang the universe had cooled so much that the hydrogen and helium plasma that then made up most of the visible matter began to combine to form neutral atoms and molecules. While plasmas constantly interact with, and emit, light, neutral atoms do so much less readily, and as a consequence, everything went utterly dark. It was as if someone pulled the plug on the entire universe. There was no visible light—not that anyone was there to see it, of course. Scientists call it the Dark Ages. The universe remained dark for 100 million to 400 million years—no one knows just how long.
“I study roughly the time from a few minutes after the Big Bang to about a billion years after the Big Bang,” Emma tells me, which is a period that, to say the least, puts scientists’ deductive skills to the test. She explains that we know a lot about planets, about our galaxy, and even about the moments right after the Big Bang. But there’s a gaping hole in our knowledge of the early universe.
“We’ve got one point of data at about four hundred thousand years after the Big Bang,” she says. “After that we’ve got nothing for about a billion years. If you compare that to a human lifetime, that’s the equivalent to missing everything from just after the point of conception to when the child first goes to school.”
It was during this period that an amazing event took place that would change the universe forever: the first stars formed and began to shine.
So how did nature build the stars from the materials available in the early universe? Gravity is the extra ingredient that turns a sprawling cloud of gas into a star. That’s how it happened then, and it’s how it happens today in patches of space that are stellar nurseries, like the Eagle Nebula. Because of gravity, a small cloud of gas attracts more gas to it, and that attracts yet more, clumping together. The only reason that gas clouds don’t collapse under gravity is because they’re supported by their own gas pressure, the kind of resistance you feel when you pump up a bicycle tire. Once a gas cloud gets big enough, the gas pressure is overwhelmed by gravity, and the matter collapses.
There’s something extra special about the first stars that Emma studies. Compressing a hot gas is harder than compressing a cold one. You may have heard that hot gases like steam can push hard enough to drive a turbine, so it’s no surprise that it takes more work to compress hot gases. The early universe was mostly composed of hydrogen and helium, which are less good at radiating away energy as light than other elements. As a result, they tended to form hotter gas clouds that were more resilient to gravitational collapse. Only when the gas clouds reached enormous sizes did gravity overcome gas pressure, triggering collapse.
“What you get,” Emma tells me, “are these giant gas clouds that condense into a star, [which results in] stars that have about one hundred times the mass of our Sun forming. Some simulations have stars with one thousand times the mass of the Sun forming, which is absolutely huge.” Those huge stars are gone now, but star formation today is still a dramatic process; in the interstellar medium, the collapses that occur are akin to taking an object the width of Australia and compressing it into something the size of a ball bearing.2
It was around 4.6 billion years ago that our own Sun formed from a cloud of interstellar dust, sucking up 99.9 percent of the mass of the solar system as it did so. The energy provided by gravitational collapse of that dust went into heating and compressing the matter into a dense ball. As the temperature of the core rose beyond two thousand degrees, its atoms began to be ripped apart and transformed into plasma. Over a few tens of millions of years, the temperature and density of the plasma rose, until particles began to crash together with enough speed for nuclear fusion reactions to begin: light, energy, heat—all unleashed as the solar system came alive.
Likewise, the universe’s Dark Ages ended as the tender young universe was flooded with light from fusion. That light swept through the clouds of molecular hydrogen, stripping the electrons from the nuclei, and creating a hydrogen plasma in the interstellar space. Before becoming plasma, the hydrogen emitted a telltale signal that still bounces around the universe today, faint but detectable. Gaps in this signal herald the first stars and it is these that Emma Chapman and her colleagues are trying to detect.3
Surprisingly, Emma does her scientific detective work by listening to the radio. It’s not a regular radio but a radio telescope—it can listen in on radio waves from deep space. The technology isn’t as sophisticated as you might think: “A radio telescope looks like an antenna that you used to have on your car, stuck in a field,” Emma says. She explains that although, ideally, astronomers would build a huge single telescope, it’s impossible to build one big enough. Instead, she’s part of a consortium putting up 130,000 antennas, each of which looks like a metal Christmas tree, in an isolated part of Western Australia. The trick is that they can all be connected together to act like a single giant telescope. “What we’ve done is built a telescope the size of Western Europe,” she says. It’s a big telescope, but it does more than just enable Emma and her colleagues to see extremely distant objects in space.
“With astronomy, the further you look, [the further you go] back in time,” she says. This is why we see the Sun as it was eight minutes ago; it takes that long for the light to reach us. The farther objects are away, the longer it takes for the light to reach us and—in effect—the farther back in time we see. When we look at Andromeda, a neighboring galaxy that’s on a collision course with the Milky Way, we see it as it was 2.5 million years ago.
What this means is that the farther away an object is through a telescope, the farther back in the past it is. Look far enough, and you can see right back to the start of the universe. Astronomers use the language of “seeing” with radio telescopes loosely: they don’t mean that they can literally see what happened. They simply mean that they can capture and measure the faint radio signals from billions of years ago.
The humungous new radio telescope that Emma and her colleagues are using is called the Square Kilometre Array. The physical scale is not the only challenge: raw data arrive at a terrifying 157 terabytes per second. “There are no hard disks that can store that amount of data quickly enough without exploding!” Emma says. And the signal that they’re looking for is just a small part of that data, a signal that’s one-ten-thousandth the strength of many other radio waves they pick up. These include radio waves generated by human activity as well as the noises of our own galaxy. If you want to hear the history of the universe, you better listen closely.
With this forest of radio antennas, Emma and her colleagues hope to better sift out the telltale signals from the period when the Dark Ages ended and stars and planets were formed from clouds of neutral hydrogen gas. “Planets and stars are formed from the same cloud, from the same stuff,” she tells me. “One is fusing, one is not, and that’s the difference.”
Since Emma is not a star builder but an early universe astronomer, I’m interested in what she thinks of nuclear fusion—though she’s quick to point out that it’s not her area of expertise. “If there’s one thing I would put my money in, it would be that, it really would, it would be nuclear fusion.”
Let the Sun Shine
Our closest star, the Sun, is both extraordinary and completely unremarkable. It’s unremarkable because there are more stars like it just in our local galaxy than there are people on the planet. It’s extraordinary because of its role in the human story. It’s a ball of plasma 109 times the radius of the Earth and 330,000 times Earth’s mass. So incomprehensible is the Sun’s power and scale that many civilizations have worshiped it as a god. Almost all life on Earth has been, in some way, dependent on energy from the Sun. Given this, those civilizations don’t seem too far off the mark.
The star builder who has come, literally, closest to our solar system’s only net-energy-gain-producing fusion reactor is Dr. Jeff Wisoff, NIF�
��s principal associate. He’s been to space four times. During those trips, he conducted three space walks, some to test the tools that would eventually be used to repair the Hubble Space Telescope. Remarkably, he’s not even the most experienced astronaut at Lawrence Livermore National Laboratory: he tells me that Dr. Tammy Jernigan, until recently a deputy associate director on the weapons program, has been up one more time than he has (Tammy is also his wife).
On a 4.9-million-mile journey into space and back in 2000, Jeff did one space walk in which he tested a jet pack. Remaining tethered to the space shuttle at all times, his jet pack squeezed nitrogen gas out in one direction, which had the effect of gently propelling him fifteen meters (approximately fifty feet) out into space’s vacuum in the opposite direction. In orbit far above the Earth, the days are much shorter, and astronauts may see sixteen sunrises for each Earth day. Out there in space, as he fast-forwarded through a day every ninety minutes, Jeff Wisoff was momentarily closer to the Sun than any human being alive.
“One of the perspectives you get from being an astronaut,” Jeff tells me, “is that the whole universe is powered by fusion energy; it’s the quintessential source of energy, and when you look out and see all those stars you think ‘wow’… if humankind can harness that in the laboratory, it’s as big a deal as the invention of flight, landing on the moon, the invention of steel; it will be one of those landmark points in human history.”
Like all star builders, Jeff Wisoff believes that the best way to achieve net energy gain is to re-create some of the conditions in stars that make them so good at fusion. NIF tries to create conditions that are similar to the cores of stars. Doing this with a machine is a tall order, and even Jeff describes NIF as being “more complex than the Space Station”—but, he adds, the commute is a lot easier than in his old job.
Despite the challenges, Jeff Wisoff and the others we’ve met are inspired by stars because they seem to effortlessly produce net energy from fusion reactions. But stars don’t primarily use the deuterium and tritium fusion that he and the others are using.
The Sun is powered by a series of other fusion reactions that fit together like the pieces of a jigsaw puzzle. The raw material is the nucleus of the most common isotope of hydrogen, also called a proton. There are three steps: In the first, the protons in the stellar plasma fuse to form a single deuterium.I Then, the deuterium nucleus fuses with another proton to form the more rare helium-3 nucleus (two protons and a neutron). Two of these fuse together to form a regular helium nucleus, and two protons. Start to finish, the cycle fuses protons to create helium, releasing energy as it goes.
In stars, these fusion reactions happen at scales so vast, so beyond the scale of individual atoms, that it’s scarcely possible to imagine them. From 600 billion kilograms of hydrogen reacting, the Sun yields 4 billion kilograms’ worth of pure energy. Every. Single. Second.
There’s a second chain of reactions that powers stars larger than the Sun, called the carbon-nitrogen-oxygen (CNO) cycle. It uses a carbon nucleus and four protons, and turns them into another carbon nucleus and a helium nucleus. Because these chains of fusion reactions result in new nuclei, the process is called stellar nucleosynthesis.4
It doesn’t matter that the Sun is doing a different set of fusion reactions from the one utilized by scientists on Earth. Star builders can still learn from what makes the Sun’s inner core, up to 25 percent of its radius, an effective fusion reactor. What this part of the Sun has is a high temperature, a high density, and strong confinement of particles and energy. So how is it achieved? The Sun and other stars have two tricks that help them with all three of these properties simultaneously: gravity and scale.
Gravity doesn’t just cause stars to form, it also keeps their mass compressed to a high density—so much that a single teacup of the plasma in the Sun’s core has a mass of twenty kilograms (approximately forty-four pounds). When fusion releases energy, a virtuous feedback loop is initiated—higher temperatures, more fusion; more fusion, higher temperatures. What this means in practice is that all fusion reactions in stars have an extremely sensitive dependence on temperature. For example, in bigger stars, the rate at which fusion reactions occur in the CNO cycle is proportional to the twentieth power of the plasma temperature. This means that for every doubling of temperature, the reaction rate increases by a whopping factor of 1 million. Why doesn’t the energy escape? For particles with mass, gravity also helps with confinement. The gravitational pull of the Sun is twenty-eight times that of the Earth, which helps keep all that hot plasma in place. Energy from fusion also ends up as light; in fact, it’s because of light from fusion reactions in the Sun that, as I write this, I can see everything in my garden. Light must be escaping the Sun eventually then, but it doesn’t do so easily, because of the Sun’s vast scale. Physicists often think of light in terms of packets of light energy called photons. The Sun is so dense that even photons can only make it millimeters to centimeters before they collide with charged particles in the plasma. If the Sun were a small ball of plasma, these photons would escape quickly. The Sun has a radius of 700,000 kilometers (approximately 435,000 miles). Photons are fungible—one is indistinguishable from another—so you can’t track them individually. But if you could, it would take the average photon hundreds of thousands of years to make it to the surface.
Being so big has other advantages. Earth-bound fusion reactors are plagued by badly behaving plasma that goes unstable at the drop of a hat, ruining confinement. Stars, however, have a built-in stability system that keeps them more or less the same size. Add more mass to a star, and gravity contracts it a bit, making it more dense, and prompting fusion reactions to occur at a higher rate. Because stars have excellent plasma confinement, the extra energy raises the temperature. The result is that the star pushes harder against the force of gravity, expands, and lowers its density again. Conversely, take a little mass away and gravity binds the star less tightly, the star expands, the density falls, and fusion happens at a slower rate. But this means that the temperature drops, and the star pushes back on gravity less firmly, so it contracts again. It’s a stable, self-correcting system that, like a pendulum, will return to its starting point if disturbed.
Of course, stars aren’t completely stable—this is plasma we’re dealing with, after all. Solar prominences, vast arms of plasma on the scale of the largest planets, frequently reach out of the Sun’s surface and into space. The most dramatic examples are coronal mass ejections (the corona is the outer layer of plasma of the Sun). Like the Earth, the Sun has magnetic fields that emanate out from the poles like so many strands of spaghetti. The Sun rotates around 11 percent faster at the equator, with a period of twenty-five days, compared to the mid-latitudes between the poles and the equator.5 This differential rotation stirs up the plasma, which gets caught up in the magnetic fields. Occasionally a strand of the magnetic spaghetti will develop a kink that extends beyond the surface of the Sun and breaks apart, flicking an arc of plasma into space.
Coronal mass ejections can throw plasma right at us at speeds of up to three thousand kilometers (approximately 1,864 miles) per second. Once it hits the Earth’s magnetic field, it’s guided to the poles, creating the Northern or Southern Lights. But when there are huge coronal mass ejections, the Earth’s magnetic field becomes heavily distorted and plasma rains down much closer to the equator, even as far as Cuba. Such ejections can cause serious damage to power grids.6
Ironically, the immense scale of the Sun permits it to be, in some ways, a lousy fusion reactor. The number of fusion reactions in a chunk of Sun plasma is 3 million times fewer than what is needed, or practical, for the star builders’ Earth-bound machines. A whole cubic meter (approximately thirty-five cubic feet) of stellar matter produces just 0.03 percent of the energy, per second, that an electric kettle uses. Fusion reactors on Earth need to be much more efficient with their space.
It’s because of gravity and their vast scales that stars have the three key elements that st
ar builders’ machines need: temperature, density, and confinement. So could star builders ever hope to create significant net-energy-gain fusion using the exact same tricks?
The answer is: no, because the scales needed are so vast. The exact ways that stars pull off their fusion tricks are impossible to replicate in miniature on Earth. Without gravity compressing everything, it’s hard to re-create the same densities as in the Sun for all but the briefest of moments. And a plasma that’s just millimeters or meters in size is easy for photons to get out of, and tends to be very unstable.
The only way to mimic the Sun’s way of doing fusion would be to use a star that already exists. This was the idea of the physicist Freeman Dyson, who died in 2020. He wrote about how civilizations—not necessarily our own—use energy, in an article published in 1960 called “Search for Artificial Stellar Sources of Infrared Radiation.”7 There, he began his argument with the idea that, at each phase of its development, a civilization will use more and more energy. Dyson imagined intelligent extraterrestrial species meeting their energy needs not by building a fusion reactor on their home planet but by enveloping their home star in a shell that absorbed every single joule of energy: the ultimate, full-scale fusion reactor. As Dyson himself put it, “Within a few thousand years of its entering the stage of industrial development, any intelligent species should be found occupying an artificial biosphere which completely surrounds its parent star.” These constructs are now known as Dyson spheres. The point of Dyson’s article was actually to propose how we might detect intelligent life elsewhere in the universe—e.g., a star that’s surrounded by an artificial sphere would create a telltale signature of infrared light that telescopes could search for.
Creating a Dyson sphere is implausible, so much so that even the wildest star builders aren’t contemplating it. There’s no chance, for now at least, of humanity being able to directly use a star as a fusion reactor (though we can benefit indirectly from the Sun’s fusion, using solar panels on Earth). So the scientists looking to save the planet with fusion need to think of other ways to achieve the conditions within stars.
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