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.
Additional layers of the Sun, which burn at much higher temperatures, became visible. And ever since, the dynamic life of our Sun –
and all stars – has been coming into focus.
SOLAR SPACE TELESCOPES
NARRATOR: As any space scientist will tell you, we live in a golden age of solar research. Thanks to a series of new telescopes
launched in the last two decades, we now have more – and better – ways to look at the Sun than ever before. The list of missions
responsible for this new wave of solar research is long.
Here’s a quick introduction to three key ones that you’ll be using in our Sun Lab.
The first is the SOHO satellite, which launched in December, 1995. A collaboration between NASA and the European Space Agency,
SOHO bristles with a dozen telescopes. With each one tuned to a distinct part of the electromagnetic spectrum, SOHO brought new
regions of the Sun into focus for the first time.
For example, one of its telescopes uses relatively long wavelengths coming from the surface to indirectly model the Sun’s core.
But SOHO is probably best known for revolutionizing our understanding of space weather. Within its first year, SOHO scientists had
learned how to track Coronal Mass Ejections and figure out which ones might be headed our way. Here, SOHO captures a period in
2011 when the Sun produced six of these storms in a single day.
The next groundbreaking mission, called STEREO, consists of two satellites. Since its launch in 2006, Stereo A has raced ahead of
Earth in its solar orbit, while Stereo B has fallen further behind. As the telescopes have drifted farther apart, they’ve provided
increasingly complementary views of the Sun, each day bringing more of it into view.
This led to a dramatic moment in February, 2011. For the first time in human history, we had a view of the entire Sun, not just the part
facing Earth.
Because the Sun takes 27 days to rotate once, there used to be plenty of time for solar activity to develop without us noticing. Now, we
can see active regions as soon as they form, leading to much better forecasting.
But the newest and most powerful telescope ever pointed at the Sun is the Solar Dynamics Observatory, or SDO for short. Think of it
like SOHO on steroids. From its position near Earth, SDO also looks at many different wavelengths of solar radiation. But more than
once per second, it delivers images ten times more detailed than HDTV.
Some estimates say this digital fire hose will transmit 50 times more science data than any mission in NASA history.
In our Sun Lab, you’ll see images from two of SDO’s three main instruments.
The first is AIA, a battery of four telescopes that can look at ten different wavelengths of light. These range from the Sun’s surface up
to the highest reaches of the super-hot corona, the key to modeling space weather.
The second SDO instrument featured in the lab is called HMI. The “H” stands for “helioseismology,” because it uses sound waves
moving through the surface to model changing magnetic fields generated in the convective zone below. By mapping these complex
fields across the entire Sun, HMI helps scientists more quickly spot the conditions that can lead to solar storms.
Of course, these are not the only solar missions, and there are more in the planning stages – including one that will fly right into the
Sun’s corona.
So keep looking – not at the Sun itself, but at the images coming from our solar telescopes. Not only is it safer for your eyes, but you’ll
see a lot more.
HOW CAN I STUDY THE SUN?
NARRATOR: When you think of space exploration, what comes to mind? For many of us, it’s images of powerful rockets and hi-tech satellites.
Or maybe it’s the people who operate all that expensive equipment–brilliant and dedicated astronauts, scientists, and engineers.
But don’t assume you need advanced degrees or specialized training to experience some key NASA’s missions firsthand.
In fact, with nothing more than a computer and a good internet connection, you can use the Helioviewer, a tool built by NASA’s
Goddard Space Flight Center.
The Helioviewer serves up raw solar images, constantly updated, from space telescopes like STEREO, SOHO, and the incredible new SDO.
With it, you can investigate a big solar event in the past, study a particular region of the Sun, or just see what’s happening on our local
star today. Who knows, you might be the first to spot a solar storm as it’s forming…
Whatever your interest, the Helioviewer lets you choose when and how to observe the Sun–with the same solar telescopes the
professionals use every day.
Of course, a tool this powerful can seem a little complicated at first. So the Sun Lab starts with two guided research projects that will
help you get familiar with the Helioviewer while learning to read what’s happening on the Sun.
The first one, focusing on the solar cycle, shows you how to identify sunspots, early indications that storms might be brewing.
We’ve selected the dates, times, and instruments for you – pulling up great examples of sunspot activity from the recent past. Zooming
in, you’ll count both groups and individuals spots, then see how your estimates compare with what the scientists got.
In the second investigation, you’ll learn to predict which sunspot regions are most likely to generate solar storms.
Once again, we’ve picked the date, time and the instruments you’ll see. Your job is to analyze two regions as they evolve, predict
which one will erupt, and then find out what really happened.
In the third investigation, you’re in command. This is your chance to put your curiosity and powers of scientific observation to work.
Come up with a question, figure out what you need to see, and go get it.
First, pick the date and time range of the event you’re interested in.
Next, choose your instrument. Here’s an example of using SDO’s AIA telescope to see what’s happening in the super-hot corona, or
atmosphere of the Sun.
Finally, select a wavelength of light to observe. This will determine which layer of the Sun you see, because hotter regions emit light
with more energy than cooler ones.
But remember that the higher the wavelength number, the cooler the region. That’s because light waves carrying more energy are
more tightly packed, with a smaller distance between each peak.
To document what you’ve done, you can capture pictures, make movies, and share them with fellow researchers in our Social Feed.
So give the Sun Lab a try. Right now there are billions of dollars worth of solar telescopes are hard at work, gathering images for your
own personal solar exploration.