Sunset, by Alvesgaspar

Every now and then, it is worth taking a look outside during a sunny day, or during a sunset, and remembering what an incredible thing our Sun is. Even if the Sun is the most familiar object in our sky, the Sun is also a star, and stars are some of the most mysterious and fascinating objects in the Universe.

The first thing that is amazing about stars is just how big they really are. You could fit over 5 million planets like the Earth inside the Sun! It's the biggest object in our solar system. If you put together all the planets, moons, rocks, dust, ice, and everything in between, you'd still only have about 0.5% as much stuff as is in the Sun.

It is actually very hard to imagine how big this really is. To do this, we are going to do something called a thought experiment. This means using your imagination to try to understand something real that is otherwise very hard to measure, do, or see.

The first step in our thought experiment to understand the size of the Sun is to imagine if you were the only person on the Earth. Imagine walking to the next town and not seeing a single person. Or going to the next State, even the next country, and not seeing a single other person. Imagine walking to the ocean, or to a nearby mountain, to a river miles away, like people used to do before airplanes and cars. Now see if you can imagine walking all the way around the Earth like this, and still not seeing a single person! You would quickly realize how big the Earth really is. Now imagine if you had a whole second Earth to yourself and had to do the same thing. Now see if you can imagine putting together more and more Earths like this, all floating in space, getting bigger and bigger as they all came together. Imagine a thousand of these Earths, all floating together in space, right now, a few miles away. It would be an awesome and amazing sight! Well, the Sun is not only more massive than this ball of a thousand Earths, but it is as massive as a thousand of these balls! Now you can start to really wrap your mind around how big this object that we see shining in our sky every day is.

Close up of the Sun, dimmed so that you can see the bubbling features on its surface.
You could fit dozens of Earths inside the bright patches you see in this picture!


Credit: NASA

It turns out that the fact that the Sun is really, really massive is also the reason for the other thing we immediately notice about it: the Sun is very hot and very bright. If you think about it, it is pretty amazing that the Sun can light up the entire Earth from this far away. On average, the Sun is about 92,935,700 miles away from the Earth (that’s more than 92 million miles)! Think of how many light bulbs you would need in the sky to make it as bright as it is during the daytime. Now imagine if these light bulbs were over 90 million miles away!

In fact, the Sun is so far away that it takes light, the fastest thing in the Universe, a whole eight minutes to reach us from there! A fun way to think about this is to look at your watch right now. As you’re reading this, the Sun just pumped out a whole bunch of light and heat that is rocketing across space at 186,000 miles every second! Check when 8 minutes from now will be, and remember to look outside when that time comes. You are now looking at sunlight that has blasted across over 90 million miles of space! That means that when you look at the Sun (don’t look straight at it though!), you are actually looking at the Sun as it was eight minutes ago, and not right now!

Think this is weird? Well then look up at the stars at night. Those stars are so far away that it has actually taken millions of years for some of their light to reach us. That means that when some of the light blasted off those stars, there were still dinosaurs on the Earth! Can you imagine what the Earth will look like when the light leaving those stars right now finally arrives? Maybe people will have discovered ways to travel between the stars by then!


Both heat and light are forms of electromagnetic energy, and the Sun produces huge quantities of both every second. The process that makes the Sun so hot and so bright is a process we call nuclear fusion. Fusion simply means “binding together” or “sticking together,” so “nuclear fusion” means the binding together of the “nucleus,” or centers, of atoms. It so happens that when these centers of atoms get stuck together, they release energy. It is the process of trillions and trillions of atomic centers sticking together and releasing these tiny bits of energy every second that makes the Sun continuously shine so bright. And it is because of the enormous size of the Sun that all the atoms in the center of the Sun end up getting smashed together. Imagine how tired you would get holding up a bag full of heavy books for an hour. With all the trillions of tons of stuff in the Sun, there are trillions of trillions of tons of material pressing down on the atoms in the center. It is this pressure that squashes the atomic centers together, heating them up to incredible temperatures. It's so hot that these centers, or nuclei, can fuse together, releasing energy and making the Sun burn so brightly every day.

Atoms and the Sun

Atoms are the microscopic building blocks that make up all the matter around us. Atoms are incredibly small. In fact, they are so small that there are more atoms in a single glass of water than there are glasses of water in all the oceans of the Earth! So how can something so small produce all the energy we see in the Sun?

The reason is because of something that is in a very famous mathematical equation: E equals MC squared (written as E=MC2). This formula was discovered by Albert Einstein, who is probably one of the most famous and influential scientists in the history of the world. He lived from 1875 to 1955 and made some of the greatest discoveries into the nature of physics and gravity. He was well known for coming up with thought experiments like the one we used to imagine how big the Sun is and using these to put together mathematical formulas that described some of the greatest scientific insights in history.

E in this formula stands for “energy” or the amount of energy that something contains (like the light and heat of the Sun).

M in this formula stands for “mass” or the amount of matter that is in a particular object (like the trillions of tons of matter in the Sun).

C in this formula represents the figure equal to the speed of light. This is an amazingly huge number: 299,792,458 meters per second, or around 186,000 miles per second. C squared, means C times C.

So what Albert Einstein discovered is that Energy equals the amount of Mass in an object multiplied by the speed of light times the speed of light! This means that for every bit of matter (M) we find, it gets multiplied by an incredibly huge number (C times C) when it is converted into energy (E). In other words there is a ton of energy stored in every bit of matter!

So even though the Sun is being powered by tiny atoms being squeezed together by the enormous pressure of gravity at the center of the star, these tiny amounts of matter become enormous amounts of light and heat when they are released as energy through fusion.

Did you Know?

Many countries now think that we might be able to find ways to fuse atoms together here on Earth. There are big reactors that are now close to being able to keep a fusion reaction going without using up too much energy in the process. Since we don’t have trillions of tons of matter to create the pressures we find on the Sun, these reactors have to use other ways to generate the incredible heat that causes nuclear fusion. Some use high powered lasers, for example, shooting into very small and concentrated spaces to heat atoms until they stick together! Imagine if we found a way to make this happen in a controlled and safe way here on Earth. We would be able to have a miniature Sun that we could use to power electric devices and energy plants!

Magnetism and the Sun

All the bubbling, heat and energy of the Sun creates huge currents, or streams, moving through the Sun, and even big arcs that jump out of the Sun’s surface, like what we can see in this picture.


Close up of the surface of the Sun, churning with activity. 
Credit: NASA

On the Earth, we see matter in the three forms we are the most familiar with: solid, liquid, and gas. The ground beneath our feet is (usually) solid, for example. The ocean is made out of water: a liquid. And the air is a gas. If you visited the Sun, it is so hot that you would mostly find only one thing: a state of very hot matter we call plasma. Plasma is what happens when you take a gas and heat it up to extremely hot temperatures. It behaves in ways that are different than what happens in the states of matter we are the most familiar with here on Earth.

Enormous currents of plasma seem to flow around the outside and inside of the Sun. This movement of atoms creates gigantic magnetic fields. These magnetic fields are so huge and powerful that they carry enormous streams of plasma thousands of miles into the space around the Sun. We call these huge arcs of plasma solar “prominences.”


Plasma prominence on the Sun’s surface. 
Credit: NASA

Sometimes these streams of charged particles, or prominences, snap and whip off the Sun’s surface, flinging charged particles off into space at tremendous speeds. We call this process a “solar flare,” or if it’s really big, a “coronal mass ejection” or “CME.” Since these charged particles are moving so fast, with lots of energy, we need to be careful when they come towards the Earth. They can disrupt electronic devices like satellites in space, or even power lines and cell phones here on the ground. So if you’re calling your parents and your cell phone drops the call, then remember that it might be because of a gigantic solar storm the size of the entire Earth that blasted off of the Sun millions of miles away!

In addition to these solar flares, light and heat, the Sun is constantly pouring charged particles out across the solar system. We call this flow of material outward from the Sun the solar wind. In a way, we are actually living in a distant part of the Sun’s atmosphere. So what happens in the Sun itself actually can have a huge effect on us here on Earth. Because of this, NASA and other international space agencies have set up programs to track and predict weather on the Sun, just like we do here on Earth! The NASA team that handles this is the NASA Space Weather Bureau, and their full time job is to follow the weather on the Sun so that they can alert people on Earth to anything that might be coming that would affect us here.

Think About It

One of the great mysteries of the Sun has to do with its temperature. The surface of the Sun, called the “photosphere,” is actually a lot cooler than the bright outer plasma atmosphere of the Sun, called the “corona.” The corona is visible when there is a solar eclipse, or when the surface of the Sun itself is blacked out in a picture. Of course, both the surface of the Sun and its corona are very hot; but while the surface of the Sun is around 10,000 degrees Fahrenheit, the corona is around 1,800,000 to 5,400,000 degrees!


A picture of the Sun’s corona, with the Sun itself blocked out. 
Credit: ESO.

If you think about it, this is a very strange thing. It is almost like walking away from a fire or a heater, and getting a lot hotter for a while before it gets cooler. Scientists think that this might have something to do with the Sun’s magnetic field heating this outer atmosphere, or that there might even be some sort of shockwave that hits this outer atmosphere, heating it up in the process. But no one really knows. So even if the Sun is there every morning when we wake up, there is actually a lot more for us to discover about our local star.

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