Yo ho! It's hot, the sun is not a place where we could live.

But here on earth there'd be no life without the light it gives.

We need its light. We need its heat. We need its energy.

Without the sun without a doubt there'd be no you and me. -They Might Be Giants

Ahh, the Sun. Beautiful and blinding to the naked eye, it's still the source of energy that gave rise to all of the life on Earth that we know.

Image credit: GOES satellite, in the X-ray.

The Sun emits energy all over the light spectrum, from long-wavelength radio waves (many meters long) to visible light to X-rays (just a small fraction of a nanometer). The surface of the Sun is an astounding 5,800 Kelvin, hot enough to melt (or sublime) absolutely everything we know, including tungsten, diamonds, and all known chemical compounds.

But not to worry, we're a good 93 million miles (150 million km) away from it, where the temperatures are much cooler.

Image credit: the International Space Station crew.

I was recently asked a question that sounds simple, but is actually very deep:

How long would it take for us to notice if the Sun stopped shining?

Well, there are two ways to interpret this, and so I'll give you the answer to both interpretations.

1.) The Sun simply goes out. If you heat anything up to a temperature of 5,800 Kelvin, it's going to emit light, much like your standard household bulb does. What if, for example, you suddenly cooled the Sun down to a much lower temperature so that it didn't emit light anymore? How long would it take us, here on Earth, to notice that?

The only special thought you need to have is that the last of the Sun's rays would still be on their way towards us when you turned the Sun off. And light doesn't travel infinitely fast; it travels at the speed of light! So, depending where in its orbit the Earth is, it would take just under eight-and-a-half minutes (anywhere from 8:11 to 8:27, according to my calculations) for the lights to go out. And in this scenario, the Sun would go from its current colossal brightness to pretty much zero, instantaneously.

But what if I wanted a more physically realistic interpretation?

2.) The Sun's source of power -- nuclear fusion in the core -- suddenly stopped. This is a lot more interesting, and a lot more difficult to answer. Let's take a look at the science of what goes on inside the Sun.

The core of the Sun -- the innermost quarter of it -- isn't at 5,800 Kelvin. It's more like 14 million Kelvin. Inside, it converts about 620 million tonnes of Hydrogen into Helium every second, releasing the energy equivalent of 1.8 billion of the most powerful nuclear weapon ever detonated. Every second.

But that energy that gets made in the core? Yeah, you wish it looked like sunlight. Instead, it's ultra-high-energy gamma radiation, which will either give you green skin and huge muscles when you get angry,

or will cause cancer and certain death, depending on whether you listen to Stan Lee or medical science.

But, fortunately, these gamma rays have hundreds of thousands of kilometers of Sun to go through before they head towards us. And this, perhaps unsurprisingly, is hard.

Each gamma-ray photon, once made, travels a maximum of a few millimeters before it gets absorbed and re-radiated. But when this happens, it gets re-radiated in a random direction and at a lower energy. It's still a photon, mind you, so it still moves at the speed of light, but it has to bounce around an incredible number of times, losing all that energy time and time again, until it finally reaches the surface of the Sun. Some take longer and reach lower energies, others get out more quickly while they still have higher energies, producing a nice, smooth spectrum.

How long does this process take? Between 10,000 and 170,000 years. That's not an uncertainty, mind you. The ones that "get lucky" and get out quickly take about 10,000 years to do it, and are responsible for the highest energy (high frequency and short-wavelength) emissions of light, such as X-ray and ultraviolet light. The ones that take longer have more collisions and lose more energy, as they spend more time bouncing around in the Sun.

So what does this mean, overall? If the Sun suddenly stopped fusing hydrogen into heavier elements, here's how things would change (neglecting the collapse of the interior of the star that would ensue):

For the first 10,000 years, everything would appear the same as always; there would be no drop in any measurable quantity coming from the Sun. After 10,000 years, the X-rays, UV-rays, and bluer forms of light would start to disappear from the Sun's spectrum. To us, on Earth, it would appear that the Sun was gradually cooling. After 170,000 years of gradual cooling, the Sun would have changed colors from yellow to orange to red to deep, dark red, and would have gotten progressively dimmer. Over these 160,000 years, the Sun would drop from its current brightness to about one-thousandth of that brightness. In other words, the Sun would look as bright from Earth as it appears from Pluto today. Beyond 170,000 years, the Sun will fade out of the visible spectrum and will only emit infrared light; at this point, there's nothing left to prevent the Sun from collapsing under its own gravity.

So the answer, surprisingly, is that if the Sun "went out," we'd have about 10,000 years before we noticed anything, and even then, it would just be a gradual dimming and cooling for over 100,000 years while we figured out what to do about it!

Luckily, with the laws of physics in place, this isn't something we have to worry about, but that doesn't mean I won't cash in on the film rights...

What do you say, Hollywood?

Update: I have been informed that this movie was already made: Sunshine. And it looks like I nailed the movie poster, too. (Thanks, SteveM and Nemo.) What can I say, Hollywood? There's still a chance to get it right...