I want to tell you the best story that I have ever heard, and it has the added advantage of actually being a true story. I’m an astrophysicist, which means that my profession, my passion is studying things in the Universe that are incomprehensibly large, they’re extremely far away, they’re very old; the human brain doesn’t even comprehend the numbers.
I think sometimes people don’t realize that scientists actually do respond to this with some emotion. People often ask me, “What’s it like to be an astrophysicist? When you learn all these things, does it affect the way you view the rest of your life?” And the answer is yes. It’s changed the way I view absolutely everything. I want to tell you that story because I never responded to science just as the mathematics, just as the technical aspects I responded to the story and to the drama of it all.
And this morning, I want to talk to you about, as I said, my favorite story: is where we all come from. In order to start that story, we need to go to some very large scales indeed. This is a galaxy. Every image I’m going to show you today is a real picture taken by a NASA mission; this is from the Hubble Space Telescope. A lot of people know the word galaxy – that’s OK – but I don’t think people understand what monsters these really are.
Galaxies are incredibly huge. This is a galaxy that is a family of about 500 billion stars, about half a trillion stars. It’s about 100,000 light-years from end to end. Let me talk about that, because light-year is one of those famously confusing words in astronomy. When you hear the word “year”, you think it’s a unit of time. But instead it’s a unit of distance, because light travels through space at 186,000 miles per second. If you go at that speed for one year, you cover about 6 trillion miles. And that’s one light-year. This galaxy is so huge, it’s 100,000 light-years from end to end; 100,000 times 6 trillion miles.
We live in a galaxy very much like this, called the Milky Way. In fact, the Sun is spinning around the center of the Milky Way right now. All the stars in the Galaxy orbit around a common center of mass. Right now, this room, and this planet, and this whole Solar System is moving at about half a million miles an hour around the center of our Galaxy. But the Galaxy is so huge, it’s going to take us about a quarter billion years to make one transit.
I actually like the term light-year because there is a time element in it as well. This is a picture of the nearest large galaxy to us, the Andromeda Galaxy. Andromeda is 2 million light years away. You can actually see Andromeda in the night sky tonight. It’s actually up in the winter. It’s kind of a little bit of a dim smudge. If you’re in the dark sky area, you can definitely see it. The light that’s hitting your eyes tonight left Andromeda 2 million years ago. It being the nearest galaxy, Hubble has made some beautiful scans of all of the millions of stars that we can see in that galaxy. That is a 2-million-year old image as it arrives at our telescopes at this moment.
If you were on the Andromeda Galaxy, and you took a picture of Earth, you’d be looking back 2 million years ago, to the very dawn of humanity. So, there is this time aspect, too. This becomes very profound when we look farther and farther out into the Universe, because, when you look really far-out, say billions of light years away – and we can do this – the Universe is then billions of years old; you’re looking back in time. The Universe doesn’t look the same as it does today. The farthest out we can look right now with our most sensitive specialized telescopes is 137 billion years into the past. This is not something that’s conjectural; this is not a theory. This is actually a picture you can take. We have telescopes that are that powerful. When you look at what the Universe looked like 137 billion years ago, it’s very different indeed. There are no galaxies, there are no stars, there are no planets. We can look back to a time when there was nothing but hydrogen gas. Hydrogen and a little bit of helium.
When you look around you, you know, I’m a being made of carbon, and oxygen, and nitrogen, and phosphorus. The Universe is somehow very different. So what happened? What happened to change the Universe this way? Here is where we all come from. This is a star. This is the Sun, our nearest star, and stars are giant balls of hydrogen gas. In the very center of the star, it’s hot enough; it’s millions of degrees hot, that a nuclear fusion reaction is going on.
Little tiny atoms of hydrogen – hydrogen is the smallest, simplest atom – they ram together in a nuclear fusion reaction, and they make bigger and bigger atoms. The star gets energy out of that, and that’s what makes the star shine. The incredible thing is that this is where every atom in the Universe comes from, besides the original hydrogen and helium. Stars are amazing things. There’s a little tiny dot off to the side of the Sun there that I put, and that’s about the relative size of Earth.
You can barely see it. You could fit about 110 Earths to cross the diameter of the Sun, and volume-wise, you could fit a million Earths inside the Sun. But if you think the Sun is big -we talked about the scale of the galaxy – if the Sun were the size of a dot of an “I” on a page, so if you can picture a dot of an “I” would contain one million Earths, then our galaxy, the Milky Way, would be about the size of the Earth. That’s the scale of a star to a galaxy.