by Harry Cliff
If you take sunlight and break it into a rainbow spectrum using a prism and look very, very closely—you’ll need to use a microscope—you’ll discover that the rainbow band is littered with dark lines, not unlike a barcode. These dark lines correspond directly to chemical elements in the upper atmosphere of the Sun absorbing the light from the shining surface below. The discovery of spectroscopy was a revelation, transforming our understanding of the heavens in a way not seen since the invention of the telescope at the start of the seventeenth century and heralding the birth of astrophysics.
The Sloan Telescope at Apache Point uses just this technique to decipher the code hidden in starlight and reveal the makeup of the stars in our galaxy. Later that afternoon as the Sun began to sink toward the San Andres Mountains, Karen pointed me to a small room just below the telescope platform that houses an instrument known as the Apache Point Galaxy Evolution Experiment, or APOGEE to its pals. Linked directly to the telescope above by bundles of fiber-optic cables, APOGEE can analyze light from a thousand targets simultaneously, allowing scientists like Jennifer back in Columbus to figure out what they are made from.
In the control room, surrounded by a bank of screens displaying the local weather map, live feeds from the telescope, and various graphs monitoring its performance, Karen talked me through what to expect on the night shift. The Sloan is operated by two astronomers, known as the warm observer and, rather ominously, the cold observer. The warm observer’s job is to point the telescope at a list of target stars and galaxies while making sure that it’s performing as expected, all from the relative comfort of the heated control room. Meanwhile, the less fortunate cold observer has to make multiple dashes into the freezing darkness to change 150-kilogram cartridges that plug directly into the base of the telescope, each containing a metal disc that acts as a star map, with hundreds of holes drilled at the locations of target stars or galaxies, connected to fiber-optic cables that carry the starlight to the APOGEE instrument.
Tonight, Karen had the good fortune of being the warm observer, but working the night shift is a grueling task even if you’re wrapped up indoors. She had only made it to bed after the previous shift at seven a.m. and had been up again by one p.m. to start running checks for the coming night. She wouldn’t get back to her cabin for some more shut-eye until after sunrise. It was now late November, and as winter drew on, the shifts would only get longer, darker, and colder.
Back outside, as we walked down toward the telescope, I made the mistake of trying to ask a slightly long and convoluted question and soon found myself gasping for air. Karen chuckled wryly as I recovered my composure; Apache Point is almost 3,000 meters above sea level, where the air is 25 percent thinner than I’m used to. Up here you either walk or talk, but not both.
The Sun was now low above the distant mountains and there was a decided chill in the air. The clouds that had caused some concern earlier in the afternoon were dissolving, with the few remaining wisps glowing orange pink against the pale blue evening sky. It was shaping up to be a beautifully clear night, perfect conditions for stargazing.
After some checks of the telescope itself, it was time to open it up. With the push of a button and a short blast of a siren, the large white building that shelters the telescope began to slide back, running along a set of rails until the telescope was left alone and proud on the edge of the wide platform, nothing between it, the landscape, and the sky above. Then, slowly and silently, its huge barrel began to rise heavenward, and in a moment of unexpected drama, the protective cover burst open like the petals of a flower opening to greet the Sun.
I found myself almost overwhelmed by the scene. The vastness of the landscape and the vivid colors above, which faded from orange through pink to deep blue and inky black, the brilliant diamonds of Venus and Jupiter chasing the Sun on its path toward the horizon, and beneath it all the silent telescope, thrust out into the cold, thin air, gazing upward at the darkening sky. This has got to be science at its most romantic. Even Karen, who has watched this scene unfold countless times, told me that the magic never quite wears off.
As she headed back to the control room to prepare for the first observations, I stayed outside a little longer to watch the sunset. Gradually the stars started to appear, one by one, each with its own story, its own past, and its own future. By human standards, the lives of stars are unimaginably long, most changing imperceptibly as the years roll by. Fortunately, the Milky Way provides us with hundreds of millions of stars to study, allowing astronomers to discern how they live and die from a myriad of stars at different points in their evolution.
A star’s life is governed by the nuclear process occurring deep in its core. Take, for example, our Sun. As we’ve discovered, the Sun is currently fusing hydrogen together to make helium, and it will carry on doing this for another 5 billion years. However, this process cannot continue indefinitely. Slowly but surely, the Sun will burn through its hydrogen supplies as the core becomes increasingly rich in helium, which builds up like ash in a fireplace. Eventually, the Sun’s hydrogen supply will run out, and when this happens things start to get interesting.
With its source of internal heat removed, the core will start to collapse in on itself under the pressure of gravity, heating up as it does so until it becomes so hot that it causes the burning hydrogen to flare up in a thin spherical layer surrounding the helium-rich core. This releases a flood of light into the overbearing gassy bulk of the star, blowing it up to monstrous proportions and turning our Sun into a swollen red giant.
This will be bad news for the Earth, which will most likely be engulfed in the Sun’s scorching atmosphere.*6 Meanwhile, the inert helium core will continue to shrink and heat up, until it reaches a temperature of 100 million degrees, so hot that Salpeter and Hoyle’s triple-alpha process can kick in and start fusing helium into carbon. This will result in a violent helium flash, releasing as much energy as the Sun radiates in 200 million years in about the same time as it takes to boil an egg.
Now burning helium in the core and hydrogen in a layer above, the Sun will shrink again by a factor of fifty until it’s only ten times its current size, while it slowly manufactures carbon, some of which is then converted into oxygen by capturing another helium nucleus, producing two of the key ingredients in our apple pie.
However, this phase doesn’t last very long, at least compared to the life of the star. After another 100 million years the helium too will run out, causing the core to resume its inward collapse, while helium and hydrogen continue to burn in concentric layers above. Mixing between these layers will also allow some of the carbon to fuse with hydrogen to make nitrogen (another element we’ll need).
Now in its death throes, the Sun will undergo a final series of convulsions, gradually blowing its outer layers off into space and enriching the galaxy with carbon and nitrogen. Eventually, the last of its atmosphere will be wafted away, exposing a hot, dense core made almost entirely of carbon and oxygen—a white dwarf.
This is the end of the Sun. With the last nuclear reactions exhausted, all that remains is a glowing ember about the size of the Earth, surrounded by an expanding luminous cloud, the remnants of the Sun’s atmosphere. The white dwarf itself is fantastically dense—a sugar-cube-sized lump would weigh around a metric ton—and the only thing that stops it collapsing any further are the laws of quantum mechanics, which forbid all its atoms from being in the same place at the same time.
We know all this thanks in large part to spectroscopic studies carried out with instruments like APOGEE. The light from the surface of a Sun-like star can reveal information about the process going on deep within, particularly later in its life when some of the products of nuclear fusion are dredged up to the surface by swirling convection currents. However, astronomers have also learned a lot from good old-fashioned observations using ordinary visible light.
Later that night, with the other
worldly glow of the Milky Way dominating the dark, moonless sky, I got a rare and completely unexpected opportunity to look through the largest instrument at Apache Point. Housed in a towering observatory building a short walk from the Sloan Telescope is the huge 3.5m ARC telescope. Normally it’s controlled remotely over the internet, allowing observers to study the sky from wherever they are in the world. However, tonight it was being fitted with an eyepiece so that a visiting group of University of Virginia PhD students could get some firsthand experience of stargazing. All was quiet in the control room, and so Karen suggested I tag along and take a look myself.
It was dark and freezing in the observatory, which was lit only by the stars shining through the narrow opening in the front of the building. ARC telescope observer Candace Gray drove the huge instrument from a computer at the back of the room. As she selected the first target for inspection, I felt the entire building begin to rotate beneath me and saw the stars move across the opening ahead, while the telescope pivoted to target the precise coordinates given to it by the computer.
To keep the students in suspense Candace gave them a clue to the first object of the night. “Eleventh doctor,” she said teasingly, but was met with dumbfounded silence. Clearly these American twenty-somethings weren’t Doctor Who fans. “Matt Smith, of course,” I said, rather pleased with myself. “Bow ties!”
When my turn came, it took me a little while to get my eye adjusted, but when I did, a faint, delicate object came into view. I was looking at the remains of a Sun-like star, what astronomers call, rather misleadingly, a planetary nebula.*7 At the center was a shining white dwarf, surrounded by two bulging lobes of luminous gas. If you used your imagination it did look a little bit like a rather badly tied bow tie, hence its nickname, the Bow Tie Nebula. I looked transfixed for a few moments—I had never seen a dead star with my own eyes before, and what’s more here was exactly the type of object that produced most of the carbon in the world around us.
But what about the oxygen? Sun-like stars do indeed make oxygen at the end of their lives, but spectroscopic studies of planetary nebula show that almost all of it remains locked up in the dense white dwarf, never escaping into the wider universe. Consulting Jennifer Johnson’s color-coded periodic table reveals that we must look elsewhere for the oxygen in our apple pies.
Back outside in the cold night air the Moon had risen, its light almost dazzling compared to the previous darkness. The Milky Way had faded out of view, with only the most brilliant stars still visible. Rising in the eastern sky was Orion, instantly recognizable thanks to its distinctive belt of three bright stars. According to the ancient Greeks, Orion was a hunter who got up to various shenanigans, including walking on water, drunken assaults on princesses, and threatening to kill every animal on Earth (he sounds like a swell guy) before finally coming a cropper fighting an oversized scorpion and getting shoved up in the sky by Zeus. Anyway, the stars in Orion are supposed to look like the figure of said hunter, if you really use your imagination and are a bit sloppy about the details.
Tracing your way from Orion’s belt up to his left shoulder you’ll find a particularly bright star with a distinctly reddish glow. Its name is Betelgeuse and it’s an absolute monster, technically what astronomers call a red supergiant. If you were to pop Betelgeuse at the center of our solar system, its vast, gassy bulk would engulf all of the inner planets including the Earth and Mars and stretch all the way out to the orbit of Jupiter. Betelgeuse is nearing the end of its life, a supersized vision of our Sun 5 billion years from now. However, its eventual fate will be rather more spectacular.
All other things being equal, the life of a star is dictated by its mass. The larger a star’s mass, the more its core is crushed by gravity, and the more the core is crushed, the hotter it gets. As we’ve seen, at higher temperatures atomic nuclei whizz about at faster speeds, which means they can more easily overcome their electrical repulsion and fuse together. All this means that a heavier star burns through its nuclear fuel faster than a lighter star, and as the saying goes, “The star that shines twice as bright shines half as long.” The Sun is relatively small in stellar terms and so will take about 10 billion years in total to burn through its hydrogen supply. Betelgeuse, on the other hand, is between ten and twenty times more massive than the Sun, and despite only being around 8 million years old has already consumed its hydrogen, like some enormous, voracious toddler, and swollen up to become a red supergiant.
We can’t know for certain, but at best astronomers give Betelgeuse just another million years before all its helium is used up too, leaving a carbon-oxygen core. However, while this would be the end of the road for the Sun, Betelgeuse’s giant size allows something extraordinary to happen.
Once helium burning ceases, the carbon-oxygen core will start to collapse under its colossal weight, heating up to more than half a billion degrees. At these ferocious temperatures, carbon nuclei are moving about so fast that they can overcome their tremendous electrical repulsion and fuse together to make even heavier elements, including neon, magnesium, sodium, and oxygen.
This phase of carbon burning lasts a mere one thousand years, a blink of an eye in stellar terms. Once the carbon runs out, the core goes through a series of further collapses, heating up each time and igniting new nuclear fuels, first neon and then oxygen. During this short period the star resembles a thermonuclear onion, with concentric layers fusing increasingly heavy elements as you move toward the core. In the star’s last gasp, the core heats to a blistering 3 billion degrees, igniting the final nuclear reaction: fusing silicon into iron and nickel. This lasts for just a day.
Once the star’s core is converted into iron and nickel, it’s game over. These are the most stable nuclei in the periodic table, which means that fusing nickel and iron to make heavier elements actually uses up energy. The star has well and truly run out of juice. With no source of heat to fight against gravity, the core begins a final, unavoidable, and catastrophic collapse.
The core implodes, getting denser and denser as it falls inexorably toward oblivion. Nuclei are forced together until the entire heart of the star reaches the same density as an atomic nucleus. Now protons and neutrons don’t like to be closer together than in an atomic nucleus, and so when this happens, the strong nuclear force fights back and the infalling matter effectively bounces, sending a cataclysmic shockwave tearing back upward through the star. At the same time, electrons and protons are forced together to make neutrons, releasing a colossal wave of neutrinos so intense that it actually blasts the infalling bulk of the star back outward into space.
The consequence is one of the most powerful events in the universe, a supernova. As the star is torn apart, it briefly pumps more power into space than all the hundreds of billions of stars in a galaxy. When Betelgeuse goes supernova sometime in the next million years, it will outshine the full Moon and will be easily visible in daylight.*8 Fortunately, Betelgeuse is far enough away from the Earth not to pose any serious risk, but it certainly will be one hell of a show. Also, Orion will lose his left shoulder, though he probably deserves it.
Supernovae play a pivotal role in creating the elements crucial for the existence of life. The oxygen, sodium, magnesium, and iron in our apple pie were forged billions of years ago in the cataclysmic deaths of giant stars. Their violent ends enriched the universe in heavy elements, which mixed together with the remnants of smaller stars like our own Sun and eventually formed the planet on which we live. Carl Sagan, who had an almost unrivaled ability to convey science at its most lyrical, put it beautifully: “The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff.”
While a lot of the story I’ve just shared was laid out in the B2FH paper of 1957, there is still much we don’t understand about the origins of the chemical elements. “Sodium is a complete disaster,�
� Jennifer told me during our Skype call, “and we don’t know who to blame!” Theorists used to think it was all made in supernovae, but the trouble is that supernovae should make magnesium and sodium together, meaning the amounts you see across the galaxy should be closely related. Curiously, Jennifer and her colleagues don’t see as tight a correlation between these two elements as you’d expect, which seems to imply that at least some sodium must get made somewhere else.
However, perhaps the biggest challenge to how we think about the origin of the elements came just a few years ago. On August 17, 2017, the LIGO collaboration*9—a pair of observatories situated 3,000 kilometers apart, in Washington State and Louisiana—detected gravitational waves produced by an almighty smashup between two ultradense objects known as neutron stars. Admittedly, there’s rather a lot to unpack in that sentence. What’s a gravitational wave? you might reasonably ask. More on that to come—gravitational waves are just too big and important for an aside—but in short, they are ripples in the fabric of space and time produced when extremely massive objects bang into each other.
A neutron star, on the other hand, is a possible end result of a supernova. If the dying star is heavy but not too heavy (between around eight and twenty-nine Suns) then as the core collapses electrons will get forced into atomic nuclei, converting all the protons into neutrons in the process and ultimately resulting in a single, ginormous atomic nucleus made entirely of neutrons. When the supernova has ejected the rest of the star into space, what’s left is a tiny, incredibly dense neutron star with a mass of between one and two Suns but only around 20 kilometers across. If you thought a white dwarf was dense, a half a cup of neutron star matter would weigh as much as Mount Everest.
