The Sun Lab
Anatomy of the Sun
NARRATOR: At 100 times the diameter of Earth and more than 5,000 degrees Celsius at its surface, it's safe to say the Sun is both huge and hot.
But, what else do we know about its basic anatomy? What lies beneath the Sun's fiery surface?
Chemically speaking, the Sun is pretty simple. 91.2 percent hydrogen, 8.7 percent helium, with 65 other elements making up just a tiny fraction of the total number of atoms in the Sun. But that’s where the simplicity ends.
LUC PETERSON (Princeton Plasma Physics Lab): The Sun's a crazy place, right? It's far too hot to be a solid, we know that, heat it up it's far too hot to be a liquid, and so you think, well, it's a gas, right? Well, not really. It is this gaseous soup of charged particles that we call plasma.
NARRATOR: The thing is, the composition of this soup of particles is different depending on the part of the sun you are looking at.
Like a complex dessert, the Sun is made up of multiple layers, or regions—six in all—three beneath the visible surface layer and two above. And each has its own set of physical and behavioral properties.
Peeling back the layers beneath the visible surface, we arrive at the Sun’s core. This is the nuclear reactor where, under the crushing force of gravity, hydrogen atoms fuse to form helium. This process releases tremendous amounts of energy, making this by far the hottest part of the Sun, burning at more than 15 million degrees Celsius.
Above the core is the radiative zone. As its name implies, energy produced in the core is radiated through this layer, but not as quickly as you might think. On average, it takes more than 100,000 years for a photon, a packet of energy released by the core, to escape the dense radiative zone.
Above that is the convective zone, a layer known for the swirling currents of plasma that carry energy from the Sun’s interior towards its bubbling surface. Because energy is actively transported by these currents, it takes on average only a month for a photon to move through and out of this region.
Which brings us to the visible surface itself, the photosphere. This layer often appears smooth and calm, its only blemish the occasional sunspot.
Only the dramatic increase in the number, size, and complexity of these spots would suggest any unrest beneath.
Until recently, that’s all we knew of the Sun—the visible surface and theoretical models of its interior.
But now that we can see the Sun in high-frequency wavelengths of light, two more regions—the chromosphere and the corona—have revealed themselves. These layers, which collectively make up the Sun's atmosphere, burn far hotter than its surface— in the case of the corona, about 1.6 million degrees Celsius.
Now that we can finally see these layers, they tell us more about what goes on inside the Sun than we ever knew before. And they hold the key to understanding and predicting violent solar storms.
The Sun’s Energy
NARRATOR: The Sun has been shining brightly for more than 4 billion years. So, where does all that energy come from?
The answer starts with the Sun’s formation from a swirling cloud of gas and dust.
As gravity pulled matter together, intense heat and pressure began to break hydrogen atoms apart into protons and electrons—creating a high-energy mix of charged particles, called plasma.
As the Sun grew, the heat and pressure intensified to unimaginable levels.
LUC PETERSON (Princeton Plasma Physic Lab): And it’s under these extreme conditions that something really, really cool happens: nuclear fusion.
NARRATOR: Under the crushing power of gravity, protons in the plasma fuse together to form helium atoms, releasing a staggering amount of energy in the process.
The ongoing nuclear reaction inside the Sun is the same process that takes place inside a hydrogen bomb, only on a tremendous scale. We're talking 10 billion hydrogen bombs every second… for more than 4 billion years, and counting.
These nuclear fusion reactions, driven by heat and pressure, are the source of the Sun’s seemingly limitless energy.
But, with all that explosive force driving everything apart, how can the Sun possibly stay together?
LUC PETERSON: In the core of the Sun you’ve got this pressure from all of this fusion pushing outwards. And the Sun is huge so you have all this gravitational pressure pushing downwards. And so you’ve got gravity pushing down and the Sun trying to blow itself apart from the inside. And it is this beautiful balancing act between the two that keeps the Sun in one piece.
NARRATOR: If that were the end of the story, the Sun might really be the predictable glowing orb we once thought it was.
But, there’s another major force at play—one that makes our star far more dynamic and hard to predict: magnetism.
The Dynamic Sun
NARRATOR: From relatively minor interruptions that create sunspots, to massive surges that drive solar storms, change is one of the few constants on the Sun's surface.
So, what’s behind all this variability? The short answer is, magnetism. More precisely, it’s the shifting and twisting magnetic fields, seething below the Sun’s surface.
Like Earth, the Sun has north and south magnetic poles, with magnetic field lines connecting the two.
At times, these field lines follow neat vertical paths between the northern and southern hemispheres—but that’s the exception more than the rule.
More often, the motion of plasma inside the Sun mixes up the neatly organized field lines.
This is because plasma spins faster at the equator than near the poles, a difference that causes the lines to stretch.
At the same time, swirling currents in the convective zone twist the field lines and cause them to kink upward.
What’s more, all the stress and strain generated by these two forces actually strengthens the magnetic field, rather than weakening it.
SARAH GIBSON (High Altitude Observatory): Imagine this spring is a magnetic field line. The magnetic field inside the Sun is amplified, is strengthened by the rotating motions, and the shearing motions, and the churning motions inside the Sun. It wants to expand upwards, and it does, until it pokes out through the surface of the Sun.
NARRATOR: As magnetic field lines emerge, they form loops. Where they break through the surface, they temporarily divert the upward flow of hot plasma and create the relatively cool, dark regions that we know as sunspots.
But the effects can be more than cosmetic. Often extending far above the Sun's surface, the magnetic loops continue to be twisted by the currents beneath.
Sometimes, the field lines twist enough to cross. The resulting magnetic reconnection—similar to a short circuit—heats the surrounding plasma to tens of millions of degrees.
This unleashes a solar flare—a powerful burst of energy and particles that can have disastrous results if directed towards Earth.
Sometimes, the energy released by these magnetic reconnections produces another type of solar storm: coronal mass ejections, or CMEs, which can propel huge amounts of matter away from the Sun's surface.
A single CME might blast 10 billion tons of material out into the solar system.
While all this solar activity sounds chaotic, it actually follows a very regular cycle—once again, driven by magnetism. About every 11 years, the Sun’s magnetic field undergoes a dramatic shift.
Just when the twisting of the field lines reaches its peak—which we call the solar maximum—a realignment occurs, flipping magnetic north and south. Once again, the field forms neat vertical lines and solar activity drops to near zero.
That is, until movement of the plasma inside the Sun begins to act on them, and the march toward the next solar maximum resumes.
Solar Wind and Storms
NARRATOR: From our home on Earth, roughly 93 million miles away, the Sun appears to glow gently, sending a steady stream of heat and light our way.
But how much, and exactly what kind of energy and matter the Sun releases changes all the time, depending on what’s going on beneath its surface.
This means the environment of our solar system is constantly changing, creating “space weather” around our planet.
Like weather on Earth, solar conditions are moderate most of the time. Even though the Sun belches out a million tons of energetic particles every second, this “solar wind” is so spread out, it’s like a warm, steady breeze.
But sometimes, space weather can take a sudden turn for the worse. Violent storms erupt, with the potential to cause serious damage.
There are two main kinds of storms: solar flares and coronal mass ejections, or CMEs for short.
Flares and CMEs are closely related, and start the same way: with fluctuations in the Sun’s magnetic fields. These magnetic lines themselves are invisible, but we can see them light up as they channel bright, hot flows of solar plasma.
The most common storms, solar flares, tend to be quick, powerful, and localized–like intense thunderstorms.
They shoot high-energy particles, as well as x-rays and gamma rays, away from the Sun at incredible speeds. A single flare can release the energy equivalent of 10 million volcanic eruptions or more than a billion hydrogen bombs.
CMEs are bigger, slower, and more spread out–more like a hurricane. These huge eruptions of plasma from the corona start out narrow, but they soon expand to about 30 million miles across. And traveling at speeds of up to 4 million miles per hour.
Like gathering clouds, sunspots offer a clue that storms might be brewing.
SARAH GIBSON (High Altitude Observatory): A sunspot is a massive region, several times the size of the Earth, which appears on the Sun as a dark spot. It’s dark because it’s relatively cool compared to its surroundings. And it’s cool because the magnetic fields are so strong that they’re suppressing the flow of heat from below.
NARRATOR: The Sun sends storms out in every direction, and Earth is small and far away, so we’re largely oblivious to all this solar activity.
But a small percentage of solar storms do hit Earth. And, when they do, they can cause serious damage.
Earth’s Magnetic Shield
NARRATOR: Our Sun is constantly blasting huge amounts of hot plasma out into space. All those charged particles have a big effect on everything in their path. So what’s protecting Earth from the solar wind and solar storms?
Earth’s thick atmosphere provides some defense, scattering and absorbing solar particles before they reach the surface. But, our planet also has a secret weapon-- a strong magnetic field it projects into space, mostly generated by molten iron alloys moving in Earth’s outer core.
The area of space where the magnetic field interacts with the solar wind is called Earth’s magnetosphere. Its shape constantly changes, as it is bombarded by solar particles.
As positively charged protons and negatively charged electrons enter the magnetosphere, most of them are deflected around Earth, long before they reach the atmosphere.
The magnetosphere really gets tested when big solar storms, carrying more plasma and traveling at higher speeds, are on a collision course with Earth. While most of the plasma is deflected, some of it gets trapped in the magnetosphere, and funnels back toward Earth along field lines emanating from the poles.
As charged particles from the Sun collide with nitrogen and oxygen molecules in our atmosphere, they create the cosmic light shows known as auroras.
The bigger the storm, the further from the poles auroras can be seen. Some storms are so big they interfere with satellites, cause planes to alter their routes, and can create other problems. The vast majority of the time, these inconveniences are pretty minor. But the next time the solar version of a “perfect storm” overwhelms Earth’s magnetic shield, we might not be so lucky.
The Threat to Earth
NARRATOR: Even though the solar wind and storms are hitting Earth all the time, we usually don’t notice them. Thanks to our planet’s strong magnetic shield and thick atmosphere, most of the harmful rays and particles the Sun emits never make it to Earth’s life-friendly surface.
But most experts believe it’s only a matter of time before a huge solar storm overwhelms Earth’s natural defenses. After all, it’s happened before…
In 1859, newspapers in the U.S. reported widespread aurora sightings, some so strong they turned night into day. Based on these accounts and other evidence, scientists believe that back-to-back storms gave Earth the biggest solar blast ever recorded.
The storms created huge variations in Earth’s magnetic field; inducing currents in telegraph lines so strong they burned operators and started fires in offices.
What would happen if a similar storm hit us today? With societies so reliant on modern technology, the damage would be far worse. Blown transformers could take months to repair, leaving millions without electricity. And the damage would not be limited to our power grid.
J. TODD HOEKSEMA (Stanford University): More and more, we rely on technology that could be affected by the Sun: global positioning satellite, long distance communication, airplane tracking, astronauts in space. So there is an urgency in understanding what it is that the Sun is doing, what it’s gonna do next, and how can we prepare for that and respond to it?
NARRATOR: There’s nothing we can do to prevent huge storms from happening. But if we can get better at predicting and spotting them, we’ll buy time to take preventative measures and reduce the amount of damage they create.
So, will we be able to predict when the next megastorm will come?
COLLECTION OF SCIENTISTS: No… Maybe… No… Maybe… No…”
JIM GREEN (NASA Planetary Science): Maybe. And the reason why is we’ve learned so much about the Sun, we’re getting better at it, but we have a long way to go. And the more we look at some of these historic events, the more we get a deeper appreciation for what we need to know.
The Electromagnetic Spectrum
NARRATOR:For countless generations, humans have felt the Sun’s warmth and watched it rise and set. Some societies have literally worshiped it.
And yet, until quite recently, the Sun was a mystery. No one really knew where it came from, what it was made of, or why it gave off light at all.
Today, the situation is different. Because we’ve learned to read something called the electromagnetic spectrum, we know a lot about the Sun, and are learning more every day. Here’s how it works.
Like most stars, our Sun is basically a big nuclear furnace. Deep inside its core, immense gravitational pressure fuses hydrogen into helium.
These reactions release a tremendous amount of energy, in the form of electromagnetic radiation.
Because our Sun huge and dense, the particles that carry this energy, called photons, can take thousands of years to reach the surface.
But once they break free, it’s about an 8 and a half minute journey to Earth.
The photons that carry energy from the Sun travel in the form of waves. Alternating electric and magnetic fields push each other forward at the constant speed of light.
But, even though they travel at the same speed, not all photons pack the same punch. Those that carry more energy oscillate more quickly, with a shorter distance between the crest of each wave.
This distance between one crest and another is known as the light’s “wavelength.” And the shorter it is, the more energy the light carries.
Together, the entire range of possible wavelengths is known as the electromagnetic spectrum.
Every second, the Sun emits light across different parts of the spectrum, from low-energy radio and microwaves to high-energy x-rays and gamma rays.
The problem is, our eyes are tuned only to a narrow sliver in the middle – the so-called “visible light” range.
In the early 1600s, scientists first learned how to magnify this light from the skies with glass lenses. Continued improvements helped astronomers see the Sun’s surface in more detail.
But no matter how large the telescope, there’s only so much we can learn from looking at visible light.
The real revolution in astronomy has come with our ability in recent decades to see a much wider range of wavelengths.