Transcript for Q & BA Episode 2: Journey to the Center of the Sun

Hi. Phil Plait from BadAstronomy.com here. Welcome to episode 2 of Q & BA, where I answer your questions about astronomy.

Now, reader Chris Piccula wrote in and asked, "What is the density at the center of the sun? Can it be compared to an every day substance on Earth, like solid metal or liquid?"

Sure.

The center of the sun has a density of about 150 grams per cubic centimeter. Now, 150 grams for you American listeners out there, is about 5 oz, a third of a pound. For the rest of the world, it's 150 grams ::eye roll::

Now, a cubic centimeter is about this big [holding up a mini marshmallow]. This is a mini marshmallow, and it really is about a cubic centimeter. So at the center of the sun, this would weigh about 150 grams. Now, if I were to fill this with water, water has a density of 1 gram/cm³, so this would weigh about 1 gram, and the sun would have about 150 times the density of that.

Well, water's not terribly dense. You can probably think of something denser, like iron, right? Well, I happen to have something like that here. This is a meteorite [holding up a roughly fist sized meteorite]. Really is. It fell in Argentina about 6,000 years ago (I got it on Ebay). It weighs about 1,200 grams, about 3 pounds and it's made of almost solid iron. Now, it has a density of about 8 grams/cm³. It's hefty. It's dense. But the sun is about twenty times denser than this in its core. So if I were to take this and convert it into the stuff that's in the center of the sun, it would weigh about 50 pounds. I would barely be able to lift it, let alone hold it like this. So the sun is really, incredibly dense.

Now how do we know this? Well, we know that the sun is a gas. It's a hot gas and the laws that govern the behavior of gasses, they've been known for a long time. In fact, this was all done over 100 years ago. You can calculate an object, a ball of gas the size of the sun, how dense it would be in the center. And it's interesting because, these laws, they tell you a lot about the sun. For example, we know that the sun has enough energy inside of it, to heat it up to a surface temperature of about 6,000 degrees Kelvin.

Well, by knowing how much stuff is in it and the surface temperature, you can say to yourself, "If I take a ball of gas, that size and that hot, and just let it sit there, it will radiate away its heat. How long will that take for it to just cool off?"

The answer is about a million years, very roughly. And that interesting because even then, even a hundred years ago and more, they knew the earth was older than a million years because we knew about evolution and that that took much longer than a million years [caption on screen reads, "Yes, we KNOW about evolution. Deal with it."]. And a lot of geology was indicating that the Earth was much older than a million years.

Well, you'd expect the sun to be at least as old as the Earth, so if you figure the sun is just sitting there and it would die in a million years, that can't be right. There must be some energy source inside the sun, which is replenishing the heat, the energy that's lost. But back then they didn't know what it was.

Well, a few years later, this guy comes along by the name of Einstein (you might have heard of him) and he came up with this equation, E = mc², and what this means is that you can take mass and convert it to energy. And the c² is the speed of light squared which is a huge number. And what that means is that a little bit of mass can make a lot of energy. And it turns out, we know how that works.

You can convert mass to energy by fusing the nuclei of atoms together. So let's go back to my mini-marshmallow and say that it is a hydrogen nucleus [Note: not to scale]. And if I take four of these guys... Whoops. I had five there. I don't want to make five, I want to make four. If I take four of these guys and squish them together, I can make a helium nucleus. Now really, it's a little more complicated than that, but in the end, that's what happens; Four hydrogen atoms become one helium atom, one helium nucleus that is. And that releases a little bit of energy. A little bit of the mass is converted to energy.

Well, we know how much energy fusion releases. We know how much energy the sun gives off every second. And that means we can calculate how much hydrogen is converted into helium in the core of the sun, and we know that's how the sun is generating energy.

When you do these calculations, you find out that the sun is converting 700,000,000 tons of hydrogen into 695,000,000 tons of helium, every single second. Five million tons of mass is being converted into energy every second of every day in the center of the sun.

Now, five million tons is roughly the weight of about seven supertankers, filled with oil. Imagine seven supertankers. Now the oil companies would have you think that burning oil is an efficient way to heat a city and I'm here to tell you that if you convert them into energy, you can power a star. Go fusion! Little more efficient.

Well, that's pretty interesting, but those are huge numbers. It's pretty hard to understand these numbers, right? Let's bring it a little more down to Earth. Let's go back to my mini-marshmallow. If I were to take one cubic centimeter of water, one gram of water, convert it into energy using E = mc², how much energy would it produce? The answer is about 20,000 tons of TNT exploding. That's how much energy this would make and it turns out that's about the same as the yield of the atomic bombs that were dropped on Nagasaki and Hiroshima back in World War 2, pretty substantial amount of energy.

If I were to take a regulation size recycling bin [holds up a recycling bin] this is actually my recycling bin, you can see I recycle. If I were to fill this with water and convert this to energy, it would yield about 540 million tons of TNT worth of energy. Five hundred and forty megatons. Now, it turns out that all of the nuclear weapons ever exploded on the Earth, have yielded about five hundred and ten megatons. So converting this much water into energy would be the total of all the nuclear bombs ever detonated on Earth.

Trash bin. Trash bin. Converted into energy. All the nuclear weapons. Bang!

That's an incredible amount of energy and yet it is dwarfed, dwarfed, by what the sun is doing every second of every day. You know, there are a lot of people who say the sun is an "average star." You've probably heard this, "It's an average star." And I'm thinking: Five million tons of mass, converted into energy, every single second of every single day. For billions of years. Billions of years!

The sun is a lot of things, but average, it ain't.

For BadAstronomy.com and Q & BA, I'm Phil Plait.