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Whether flowers or chocolates or whatever, when your valentine gives you a gift, here's what you should not do. You should not ask where did this gift come from, especially if your sweetheart is our science correspondent, Robert Krulwich, because if you ask he may give you the long answer.

ROBERT KRULWICH: So let's say you buy your sweetheart something nice and Valentine-y made of gold. And she says -

Unidentified Woman: Oh, it's so lovely. Where did it come from?

KRULWICH: And then you say well, do you want the short answer or do you want the long answer?

Unidentified Woman: Oh, I think I'd like the long answer.

KRULWICH: Okay. Since you asked, the gold in this necklace comes from a star.

Unidentified Woman: Oh, Robert.

KRULWICH: No. But I mean that the gold I'm giving you today was made in the center of a star.

Unidentified Woman: No.

KRULWICH: Yes.

Dr. NEAL DEGRASSE TYSON (Hayden Planetarium): That's what stars do.

Unidentified Woman: Who's that?

Dr. TYSON: They are in the business of cosmic alchemy.

KRULWICH: That is Neal DeGrasse Tyson, director of the Hayden Planetarium in New York City. And since you were interested, I asked him.

Unidentified Woman: You did?

KRULWICH: Yeah. To tell me how gold gets made like from the very beginning. Like what do you need?

Dr. TYSON: Okay. First, you need a universe, because that's starting -

KRULWICH: That's good. That's good.

Dr. TYSON: We're on the right track.

Unidentified Woman: Oh, no.

KRULWICH: Shh. When the universe began, he says, there were just two kinds of atoms - very simple - hydrogen, which has one proton in its nucleus, and helium with two protons.

Dr. TYSON: And that's pretty much all she wrote at the beginning of the universe.

KRULWICH: And the thing is you can't smoosh these different atoms together to make a new kind of atom with three protons or with four protons or five protons, because unless you apply a lot of heat, protons don't like each other. They will not combine.

Dr. TYSON: Usually, they'll resist each other because of the same charge. Okay?

KRULWICH: So how hot do you have to make it to get two protons who don't like each other to sit together?

Dr. TYSON: About 10 million degrees.

KRULWICH: Which just happens to be the temperature you find inside a star. So in our sun right now, it is so hot that two protons that would normally say I really, really don't like you are being slammed together so hard, finally each one says okay.

Dr. TYSON: Now I really, really like you.

KRULWICH: This is called fusion. The sun is a furnace that slams protons into heavier and heavier atoms.

Dr. TYSON: And then it keeps going. It gets carbon and oxygen and nitrogen and silicon. And it goes - it just plows its way up the period table of elements.

KRULWICH: So carbon has six protons, nitrogen, seven protons, oxygen, eight protons. A hot sun can cook all the way up until a 26-proton atom - that's iron. But when it gets to iron, says Neal Tyson, that is about as far as it can go.

Dr. TYSON: It's dead matter. You can't fuse it. You can't fission it.

KRULWICH: So when a star, when you hit iron, you start to run out of fuel. You can't cook and that's trouble.

Dr. TYSON: That's a bad day for the star. And at that moment, the entire star collapses, and in that collapse, the star reaches stratospheric temperatures and blows its guts to smithereens.

(Soundbite of explosion)

KRULWICH: This is what's called a supernova. When a big star, bigger than our sun, explodes - now you wouldn't actually hear this in outer space because you can't hear things out there - but it's so cataclysmic because the atoms inside are slamming so furiously into each other creating heat that is so intense -

Dr. TYSON: Oh, oh. Hundreds of millions of degrees.

KRULWICH: That only in a supernova is it possible to create atoms with 30 protons or 40 protons or 50 protons or 60 protons. And while nature prefers even numbers for stability, every so often, the star will forge also an odd-numbered atom like gold.

Dr. TYSON: Gold is number 79. It's odd.

KRULWICH: So there aren't a lot of gold atoms. When you compare different sorts of atoms -

Dr. TYSON: There is about a million iron atoms for every gold atom in the universe.

KRULWICH: And then after this explosion, those lonely gold atoms are cast deep into the universe and they hang around for a long, long time, until a few of them might join in the cloud and may become a planet. And once inside the planet, they may migrate near the surface where we can dig them up.

But every atom of gold in Neal Tyson's wedding ring, which is a lot of atoms, right?

Dr. TYSON: It's been a while since I've counted, but I agree. Yes, it's got scads of atoms.

KRULWICH: Every one of them was forged in a collapsing star traveled across the universe to get to his finger.

And my very last question is, how many years are represented by your gold ring? How many miles did the gold have to travel to get to you?

Dr. TYSON: Oh, bigillions.

KRULWICH: Well, I thought we can get a slightly harder number than that. So I asked Dr. Tyson to calculate the distance a gold atom will travel from its birth in a supernova through space to his wedding ring.

Dr. TYSON: So what do we get? We get three million light year journey.

KRULWICH: To you finger.

Dr. TYSON: To end up as the expression of someone's love.

KRULWICH: All that distance just for you.

Unidentified Woman: For me?

KRULWICH: Yeah. Yeah. For you.

Robert Krulwich, NPR News in New York.

SIEGEL: And there are more ideas for expressing your love, ideas that cost less than a gold ring at NPR.org.

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