28: The Dark Side of the Sun

Episode 28: The Dark Side of the Sun

When we began our grand adventure in Episode 4, our very first topic, the first stop on the tour, was the birth of the sun. 

We haven’t talked about the sun since those early days, but it’s been there all the same. The sun has lit up the Earth every day of your life, and every day of the planet’s life. It’s easy to take the sun for granted- an unchanging light in the sky. 

Today, we’ll learn that’s not exactly true. Just like the Earth, just like you, the sun has changed over time. The sun 4 billion years ago was noticeably different than the sun today. We’ll learn how the sun has changed, and how the Earth has had to change alongside it. 

Our story starts with a titanic project: classifying every star in the sky.

Part 1: The Harvard Computers

When you look at the night sky, some stars are brighter than others, but they all basically look like tiny white pinpricks. However, if we gathered these stars together into a giant stellar ball pit, we would quickly see a wild menagerie of different sizes, colors, and brightnesses.

The smallest stars in the universe are only two miles wide, while the largest stars are more than a billion miles across. For a slightly more manageable frame of reference: if the smallest star was a single grain of sand, the largest star would be as wide as Alaska or Algeria. That’s an intimidating thought. On this shrunk down scale, the sun would be a few city blocks wide- the smaller end of medium.

We would also see a variety of colors: red, orange, yellow, blue, and white. Some would be blazing beacons, much brighter than our sun, while others would be dim bulbs, nearly burnt out. This cosmic ball pit is a pretty fun visual, but how do we know all this information about stars light years away?

We’ve actually seen these techniques before, in Episode 18. There, we were peering into deep space, searching for life’s first building blocks. We found those building blocks by sifting through starlight. Here’s how: light changes as it interacts with different elements and objects- sometimes a color is absorbed, sometimes it’s reflected. If we split pure white light into a rainbow, we can see black gaps where elements have stolen colors for themselves. The end result looks like a technicolor bar code- each element has a different code we can ID. 

This technique is called spectroscopy, since it uses a rainbow or a spectrum. In Episode 18, we used spectroscopy to search between the stars for carbon. Today, we’ll look directly at the stars themselves. 

 The history of stellar spectroscopy, of classifying the stars using light, was pioneered in the mid-1800s. Even these earliest observations showed the universe was far more diverse than previously thought. But there was a problem- there were a lot of stars in the sky, and cataloguing every single one would take a lot of time. A huge leap forward came with the invention of photography. The camera not only revolutionized art and media, it was a huge tool for science. Instead of hand-drawn sketches, researchers could directly show their results to each other and the world. You could now take photos of stars and their spectra and save them for later. But this surplus created a new problem: overabundance. There was now too much data coming in all at once, far more than individual researchers could handle. 

The year is 1882, and one of these astronomers drowning in data is Edward Charles Pickering. Born and raised in Massachusetts, Pickering was the director of the Harvard College Observatory. His goal was to photograph and classify all the stars in the sky.

As Pickering took up this monumental task, he knew he needed help. His first assistant came from an unlikely source at the time: his housemaid. Williamina Fleming was a 25-year-old single mother who had emigrated from Scotland just four years earlier. Williamina’s husband had abandoned her, forcing her to work as a maid for Pickering, but she clearly showed skills beyond housework. Pickering showed her how to read spectroscopic data- the rainbow barcodes, Williamina would catalogue more than 10,000 stars. 

And Williamina was just the beginning. Over the next four decades, Pickering would hire more than eighty women at the Harvard Observatory. This was an unprecedented leap forward for women scientists, but was not entirely altruistic. Most assignments were brutally repetitive and dull, six days a week, and while it paid more than a factory job, it was still half a man’s salary. This group of extraordinary women earned the nickname The Harvard Computers, which I think sums up the positive and negative aspects of their work very well.

Still, many of the computers would publish their research on the stars: Antonia Maury, Henrietta Leavitt, Florence Cushman, to name a few. Perhaps the most famous researcher was Annie Jump Cannon. When Cannon was a child, her mother encouraged her to be an astronomer. Cannon had a bubbly, outgoing personality, but was nearly deaf when she joined the Computers at age 33, which made it hard to socialize. Diving into her research, Cannon published her own star classification system in 1901, which was eventually adopted by the International Astronomical Union, and is still used today. During her lifetime, Cannon would receive an honorary doctorate from Oxford and would be a prominent suffragist for women’s right to vote.

I hadn’t heard of the Harvard Computers before researching this episode, and I highly recommend looking into them for more fascinating stories. But let’s return to our episode today. Now that we’ve sorted the stars, what do they tell us about the ancient Sun? 

Part 2: The Dimmer Switch

In the decades after the Harvard Computers, later astronomers discovered that different stars- white dwarfs, red giants, and all the rest, were different chapters in a much longer story. 

By this point, scientists knew that stars had life cycles- they were born, grew old, and then died. The problem was, the lifetimes of stars take billions of years to pass, much longer than the life of an astronomer. How could you tell what a star looked like when it was young, or how it would look in the far future?

The first step was figuring out the ages of the stars, lining them up in order from cradle to grave. Some stars are very young, just peeking out of their nebulas, the “cradles of stardust” from Episode 4. Others are exploding apart as we speak. Think of the night sky like a family photo album scattered into a thousand pieces. If you could figure out when the photos were taken, you could figure out how grandma looked as a child, or who that baby would grow up to be. 

In the same way, astronomers realized that most stars follow similar patterns of life and death- patterns that could lead to predictions. For example, there are many yellow stars similar to the sun in the Universe- some younger, some older. When astronomers lined all these siblings up by age, they found a clear trend. All the younger yellow stars were dimmer than the Sun. In contrast, all the older yellow stars were brighter. There was a clear relationship between age and brightness: the older a star gets, the brighter it becomes. 

So why is this happening? You might expect the light to become dimmer over time, like an old campfire. The answer is found in the heart of the sun.

The sun is mostly made of hydrogen, the simplest element on the periodic table. Normally, when two hydrogen atoms meet in space, they avoid each other. But in the intense heat of a star, hydrogen atoms are moving too fast to swerve away and they crash into each other. Collisions between hydrogen eventually form larger atoms of helium- the second simplest element, the gas inside party balloons. 

These crashes release energy- a little bit of light, a little bit of heat. Making one helium atom is barely noticeable, but as we speak, the sun is fusing 600 million tons of hydrogen every second. 600 million tons now, another 600 million, and so on. 

In other words, the sun is running out of fuel. The sun’s core is shrinking very slowly, getting smaller and hotter. The hotter the sun, the faster the hydrogen atoms speed around inside, crashing and making more helium. As you can see, it’s a vicious cycle, one that makes the sun a little brighter every second. 

On human timescales, we don’t notice this brightening. The sun is just as bright today as it wasto your grandparents, or the ancient Sumerians, or even the dinosaurs millions of years ago. But over billions of years, the change becomes noticeable. 

Even though no one was on the Earth back then, scientists have fairly confident estimates of the sun’s brightness over time. We have thousands of stars to compare with our own.

So how bright was the sun 4 billion years ago, February on the Earth Calendar?

The Eoarchean sun was only 75% of its current strength, three-quarters as bright as today. 

For reference, visualize the sun just after daybreak. I know that means different things for different people at different latitudes, but just go with me here. Look at the land around you as the sun begins to rise. You can still see things just fine, but it will get brighter. This is the sun at 75% of its total strength.

Four billion years ago, high noon would be as bright as the early morning you see today. It was a fainter, dimmer, more alien world back then, mostly shrouded in twilight. As the Sun and the Earth aged together, the brightness would slowly increase to the comfortable levels we know today.

Let’s back up and recap: there are many stars just like our sun. All of them are slowly getting brighter over time. On Earth, this means that our earliest days were dimmer, and our far future days will get brighter. This idea of a faint young sun is very well established, a cornerstone of modern astronomy. 

But in 1972, a new professor at Cornell University would compare this solar data with hard geological evidence from the Earth and discover one of largest paradoxes in science. If the astronomers were right, the sun had to be dimmer in the past. If the geologists were right, that idea was absolutely impossible. This professor’s name was Carl Sagan. 

Part 3: The Twilight Zone

For most folks, Carl Sagan is known for his public science outreach: non-fiction books such as Pale Blue Dot, sci-fi novels like Contact which became a feature film, and most famously, the 1980 TV series Cosmos. Carl Sagan was an incredible influence in my early life: I watched Cosmos on VHS tapes in my parent’s basement over and over again. If any of you have watched Cosmos, you can probably see the major influences on this podcast, from my speaking cadence to the music choices to the Earth Calendar itself. 

But today, I’m here to talk about Carl Sagan the researcher, the man who published more than 600 peer-reviewed papers. Carl’s research covered many broad bases, from the origins of life to planetary atmospheres. A full biography of Carl’s life would take at least one episode, so we’ll just cover a few stories here, ones that bring us back around to the sun. 

Carl Sagan was born in Brooklyn, New York, in 1938. His parents were not scientists, but to paragraph Sagan himself, they instilled the twin ideas of wonder at the universe and skepticism towards explanations that didn’t make sense. As a child, Sagan was inspired by museums, planetariums, science fiction and world’s fairs. 

Following his passion, he attended the University of Chicago as a bachelor’s student. In 1952, Sagan watched a seminar on a fascinating new experiment that made the building blocks of life using just water, thin air, and an electric spark. The presenter was Stanley Miller, who we met back in Episode 20, along with his incredible Miller-Urey experiment. Sagan was hooked, and would eventually demonstrate the experiment in his famous Cosmos TV series. 

As a student, Sagan took an honors class with Miller’s advisor, Harold Urey, hoping to learn more about the origins of life. Urey was unimpressed by Sagan, who he considered too speculative and fanciful, not rigorous enough with his science. As loved as Sagan is with the public, these were common criticisms in the scientific community throughout Sagan’s life. Such critiques would come to haunt him much later. 

By 1968, Sagan had been a Harvard professor for 5 years and was up for tenure. He was shocked when the position was denied. There were whispers about Sagan being too generalized and too focused on public outreach, stealing the spotlight from everyone else. A more tangible cause was an outside letter strongly advising against Sagan’s tenure. That letter was written by Harold Urey from Chicago, Sagan’s honors advisor. Urey was still not a fan of Sagan’s work, but eventually came around to Sagan’s popular science, giving a backhanded compliment to a book:

“I like it very much, and am amazed that someone like you has such an intimate knowledge of the various features of the problem… You are a man of many talents.”

Sagan quickly found a new home at Cornell University in upstate New York. It was here, in 1972, just 4 years after the tenure debacle, that he began to compare the early history of the Sun and the Earth.

Sagan agreed that the Sun had to be dimmer in the past- there were too many modern stars that confirmed this idea. But there was a major problem: if the sun was dimmer, then the Earth should have been frozen for the first half of its’ history, from Seasons 1-5 of this podcast. The idea initially makes sense, especially for anyone who lives in a temperate or polar climate today. The world is warmer when there is more sunlight. The darker the planet is, the closer we get to the cold void of space. 

But even in 1972, the idea that Earth was frozen for half its’ history was simply impossible. On this show, we’ve already seen evidence for warm liquid water in Seasons 1 and 2, and the evidence will only grow stronger in the future. We’ll see ancient beaches, calm seafloors, giant lakes, and in all of them, fossil bacteria playing in the waves. Ice will show up, but it is not the main player. 

Sagan was caught in a paradox: the ancient sun was dimmer, but the Earth was still warm. In time, this idea was called the Faint Young Sun Paradox, which I think would be a good prog rock album. 

Carl Sagan and his colleague George Mullen would propose a solution to this paradox, one that would be tweaked over the next fifty years but holds firm today. We’ll hear about this big fix next episode. For now, let’s review what we’ve learned today. 

Summary:

Our sun is just one yellow dot out of 100 billion stars in the galaxy. There are many stars just like the Sun, slightly older, slightly younger. Many were first catalogued by a team of incredible women- the Harvard Computers- at a time when men dominated astronomy. Looking at thesesolar cousins, later astronomers learned that yellow stars like the Sun slowly grow brighter over time. 

Four billion years ago, around the time of the Acasta Gneiss, the Sun was only 75% of its’ current strength. By all rights, the ancient Earth should have been an ice planet. But ancient rocks tell us there was liquid water. This Faint Young Sun Paradox has haunted geologists for decades, but there is a solution.

Next episode, we’ll take to the skies and learn how one of the greatest threats to the modern Earth once helped save the planet from a frigid death.