So, you might as well know how it works. The sun is really nothing more than a giant, massive nuclear explosion that just keeps on exploding, and exploding, and exploding, and exploding.

So, the sun was formed about 4.5 billion years ago, the same way that all other stars form. There was a bunch of gas in the universe, and some of the gas started clumping together because of gravity.

The more that that gas clumped together, the more gas wanted to be there, and it clumped together so hard that eventually two atoms of hydrogen fused together into Helium, producing a huge amount of energy. And that started off a chain reaction that became the power of our sun.

So, now, at its core where all the, you know, real interesting action happens, uh, the temperature there is about 27 million degrees Fahrenheit.

From there it takes about 170 thousand years for that energy to reach the surface of the sun. And by that time it's cooled to a balmy 10 thousand degrees.

Often you'll hear people say, "We're gonna make this as hot as the surface of the sun!" And it's important to know, the surface of the sun, not that hot compared to 27 million degrees.

Other examples of hydrogen fusion that you're probably familiar with and do not occur on the sun, include the Tsar Bomba, the most powerful nuclear weapon ever detonated.

It was detonated in the Soviet Union in 1961. And the way that fusion bombs work is they actually have, basically, a bunch of fission bombs going on at once to create enough pressure to make the hydrogen atoms fuse. Just the fission bomb that made the Tsar Bomba go off was equivalent to 50 megatons of TNT. The amount of stuff that was fused in the Tsar Bomba was about that big.

By contrast, the area of the sun where fusion is happening is about the size of 240 thousand Earths. So, if the sun were to blow up all at once, like the Tsar Bomba did, it would basically melt everybody's faces off from here to Alpha Centauri.

So, the sun and the Tsar Bomba are basically the same thing on different scales; the question is why doesn't the sun blow up all at once?

Lucky for us, the sun naturally controls its pressure, and its temperature, and its fusion rate, with a kind of a natural thermostat. 

Basically, when the enormous heat and explosive energy in the core make it expand, the core becomes less dense. Which makes the fusion rate slow down, and then gravity pulls the core back in. Then it starts all over again, and that's just the way the sun rolls.

That's not to say that some stars don't blow their wad all at once or, at least, a lot faster. Our sun happens to be a G-type star, which is a tasteful, middle-of-the- road kind of mass for a star. As it happens, the less mass a star has, the dimmer it is, the redder it is, and the longer-lived it is.

Dim red dwarf, for example, can take trillions of years to burn out, while one of those big, blue, O-type stars can burn through its entire allowance of hydrogen in, like, one million years. So our sun is doing pretty good. It looks like it's got another 4.5 million years or so, before it, you know, sputters out. Ah, good old sun!

Olivia: Okay, so, the sun wins. And while, hopefully, you and I will be dead before it explodes, it can still sometimes wreak havoc on our lives. Here's Hank explaining what causes our skin to burn in the sun.

Hank: It's summer here in the Northern Hemisphere, and you know what that means: sunscreen season! You've probably heard this before but it's worth repeating anyway, exposure to the sun, especially when you get tanned or burned, increases your risk of developing skin cancer.

So, stop going outside! But, you can go outside. But wear sunscreen!

But if you do get tanned or sun burned, we will assume by accident, there's a lot going on inside your skin. The sun is not actually tanning or burning you. The whole process is actually a defense mechanism, and you are tanning and burning yourself.

The trouble with sunlight is that it contains lots of radiation in the ultraviolet part of the spectrum, and some of that has enough energy to damage the cells in your skin, especially their DNA. We divide UV radiation into three types based on wavelength: UVA, which has a longer wavelength and packs the least energy; UVB in the middle; and UVC, with a shorter wavelength and the most energy. But we don't really need to worry about UVC because it's absorbed by the atmosphere, but UVB can cause enough damage to sunburn, and UVA might make you tan.

When receptors in special skin cells called melanocytes detect the UVA radiation in sunlight, they start producing extra melanin, a pigment that darkens skin color. Melanin's real job is to absorb UV radiation before it can damage too many of your skin cells, basically, when you tan, your body is realizing that more sun exposure might be coming and it tries to protect you.

But sometimes that's not enough. When UVB rays damage a cell's DNA, it'll often destroy itself in a process called apoptosis. If too many cells do this, an immune response kicks in. That immune response is otherwise known as a sunburn. Basically, your skin cells are like, "My DNA has been damaged, so I must murder myself before I give you cancer!" Blood flow increases to the area to help with healing, which is why sunburns are usually red and warm, and after a while all those dead cells can start to peel away. Get enough exposure and so much of your tissue will destroy itself that you might even end up with blisters, like in a second degree burn.

Sunscreen helps because it contains substances like benzophenone which absorb or even reflect UV radiation so your trip to the beach doesn't have to involve sacrificing skin cells or increasing your risk of cancer.

Olivia: So, wear your sunscreen. Not only will the sun cause skin damage, it also causes things to fade. Here's a video inspired by a Patreon patron on why the sun does that.

Hank: I'm going to hazard a guess here. Somewhere in your town, there's a little shop. The window displays look like they haven't been changed in a century, give or take, and you can tell, because the colors of everything in the window have faded.

Colors seem to do this a lot; leave them in the sun for long enough and they start to disappear. Even stranger, sunlight will eventually cause some kinds of plastic to crack or develop dark streaks. It's almost like it's been sunburned. Well, that's not too far off, actually, in the sense that it's being damaged by ultraviolet light.

Colors fade and plastics degrade because over time they're being ripped apart by sunlight's high energy UV light. When certain molecules absorb UV light, the light provides enough energy to break some chemical bonds, destroying, or at least rearranging, the molecules in a process called photodegradation. That's a problem for pigments and dyes, which only work in the first place because their specific chemical structures reflect certain colors of light. Destroy those structures, and they aren't going to reflect light in the same way anymore. Over time, more and more molecules absorb the extra energy and break down. Eventually, most of the color fades. For your average window display, photodegradation is more of an inconvenience than anything else. All the store has to do is switch it up every so often. It's more of a concern in the art world, where hanging a painting in the wrong spot can damage it forever.

But sunlight messing with chemical bonds does more than just fade colors. It also degrades certain kinds of plastic, like the kinds used in PVC piping and polypropylene rope. After spending enough time in the sun, pipes start breaking and ropes start snapping. And if there's one thing you don't want a rope to do, it's snap.

There are ways to make inks and plastics a little more resistant to sun damage, though. Manufacturers can include UV-resistant chemicals like benzophenone, which basically act as sunscreen, absorbing the ultraviolet light before it can do much harm. But if you've got a Picasso lying around, you might want to keep it out of the sun.

Olivia: OK, so the sun isn't always the best. But sometimes it's awesome. Here's a video on the science of those beautiful sunbeams.

Hank: You know those pictures that your parents and your friends and your parents' friends are always posting all over social media? The ones that have some sort of inspirational quote plus a picture where the sun seems like it's using beams of light to break through a wall of clouds? Those trails of light are called crepuscular rays, and they are actually pretty common. If you look at the sky on a partly cloudy day, especially if it's around dawn or dusk, odds are you will see those rays for yourself, though you probably won't see any inspirational quotes floating around in the air.

Crepuscular rays happen when sunlight passes through air that has a bunch of stuff in it. Sunlight is always bouncing off of stuff in the air. The way it bounces depends on what it is hitting. Nitrogen in the air, for example, scatters the bluer parts of the sun's light, which is why the sky is blue. Crepuscular rays happen when the sunlight hits bigger particles, like dust and water, which bounce all the light's colors pretty equally. When we see all the colors of light at once, we call that white light, which is why a lot of crepuscular rays look white.

Sometimes the rays don't look white, which just means that the light hitting them wasn't white to begin with. So crepuscular rays around sunset will be redder, since the sunlight looks pretty red by that point.

But crepuscular rays aren't always visible whenever there's a lot of dust or moisture in the air. There also needs to be something blocking some of the sunlight. Without anything in the way, we wouldn't be able to see the paths of the individual rays, no matter how much stuff was in the air. Light would be bouncing off of everywhere equally, and everything would look equally lit up. These two things, particles in the air and something to block parts of the sunlight, are why clouds are so common in those Facebook pictures. Clouds are denser pockets of water vapor and dust that are really good at blocking sunlight. And the area around clouds also tends to have some dust and water, so if some light does manage to break through a hole in the clouds, it hits a whole bunch of material that scatters it.

But you don't need clouds. Mountains and buildings can also create crepuscular rays. And if the sun is at the horizon, like at dawn or dusk, you can get crepuscular rays that stretch all the way across the sky. So intense!

Like all crepuscular rays, they look like they're radiating out from the sun, like a bunch of lines coming from a single point. But the ones right above you will look parallel and the ones going behind you, directly opposite where the sun is, all meet again, where they are called anticrepuscular rays. And they look like they're radiating out from some invisible sun on the darker side of the horizon. So what's actually happening here? Are the rays parallel or do they spread out from the sun? Well, both. The sun is so far away that when its light gets to earth, it's all pretty much going int he same direction. So all of the sunlight causing crepuscular rays is virtually parallel, which means that the rays themselves are always parallel line. They just look like they're meeting at the sun because of perspective, which makes things look smaller when they're farther away, just how train tracks look parallel when you're standing next to them, but they look like they're getting closer together the farther away they are from you.

So those beginnings of crepuscular rays far away from you are like those distant train tracks that seem like there's hardly any space between them. Then the rays directly above you are like the train tracks by your feet, so they look parallel. The anticrepuscular rays are also really far away from you -- they're just as far away as the original ones, so they look like they're coming together, just like the original crepuscular rays did. If you could see crepuscular rays from space, you would see them for the parallel lines that they really are. And this is a thing that we know because astronauts have seen crepuscular rays from space, and this is what they see. So the next time someone puts up some photos of supposedly inspirational crepuscular rays, don't forget to congratulate them on their excellent picture of dusty light.

Olivia: I've saved my favorite for last. In this early SciShow, Hank explains the science of solar storms.

Hank: I don't know about you, but that, that enormous, unstoppable, torrent charging at the earth like a tsunami of radioactivity makes me want to go hide under my bed. This footage was captured by the uninterestingly-named STEREO-A, one of two spacecraft launched in 2006 to help NASA study solar activity. These eruptions that you're seeing here happened in December 2008, so I suppose we're safe, since it happened a long time ago, and it's the first time that we've actually seen what it looks like when a solar storm engulfs our planet.

A single, normal one of these eruptions can send a billion tons of ionized gas, a billion tons of solar plasma, hurtling toward our planet. And the thing of it is, that in terms of space weather, this, that happened in December 2008, ah, not that big of a deal. Practically San Diego. When things get really intense, it's at a period called solar max, which happens on an eleven year cycle. And frankly, that's when the really crazy crap happens.

As you might expect, the sun has some pretty strong magnetic fields. And just like the earth, it has north and poles. But because the sun is not, in fact, a mass of incandescent gas — it is an enormous ball of juicy plasma — its magnetic fields are always churning around and stretching and contorting. And by the time they're done shifting, every eleven years or so, the poles have reversed. South becomes north and vice versa. And while this is happening, the magnetic fields cross and sort of poke out from the middle of the sun and they go all crazy, basically, and cause all sorts of crazy, dangerous-looking phenomena. Very much like what we just saw, which is called a coronal mass ejection. This is when the sun's corona emits these huge plumes of ionized gas, and when I say huge, I mean that this disc in the middle of this picture is the sun. That's the sun, and those giant popping bubbles are pretty much as big as the sun itself.

And sometimes magnetic fields that have been going in opposite directions connect, effectively short-circuiting themselves. This causes solar flares, which are localized blasts of radiation. Even short ones, lasting just a couple of hours, can unleash enough energy to power the entire United States for a million years. And where these magnetic fields actually emerge from the surface of the sun, then we get what's called sun spots. These spots become more frequent, and eventually peak at solar max, telling us that things are about to get real.

Now, because of their enormous energy, these storms can actually affect things that use electromagnetism here on the surface of the earth, which today, is practically everything. Transformers on power grids get thrown offline; radio and satellite communications go on the fritz. During one of the big flares ever, recorded in 1859, telegraph machines picked up so much energy that the ones that had been turned off started working. And that's lovely, because it is the exact opposite of the effect that it would have on cell phones, which now just stop working during solar flares.

And of course these radiation plumes pose health risks to astronauts, people who are living outside of the earth's electromagnetic field, but at least they will get a lovely view of these auroras from space.

Unfortunately, these storms remain massively difficult to predict, which I guess isn't that surprising, given that the sun is a pretty complicated system. For example, in 2006, scientists said that 2010 would be a huge solar flare year, almost beating out 1958, when it was said that the Northern Lights could be seen as far south as Mexico. And of course, 2010 turned out to be something of a nothing year for solar flares. 

Olivia: Thanks for learning about the sun with me. We're uploading summer-inspired videos all week. So check back later to learn more. And if you want to continue getting smarter while you're relaxing on the beach or by the pool, check out these SciShow beach towels, available at DFTBA.com/SciShow. Don't forget to be awesome, and that the sun is terrifying and awesome.

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