How did the universe begin?
Why is the universe the way it is?
How will it end?
"All my life, I have been fascinated by the big questions that face us, and have tried to find scientific answers to them. If, like me, you have looked at the stars, and tried to make sense of what you see, you too have started to wonder what makes the universe exist. The questions are clear, and deceptively simple. But the answers have always seemed well beyond our reach. Until now.
"The ideas which had grown over two thousand years of observation have had to be radically revised. In less than a hundred years, we have found a new way to think of ourselves. From sitting at the center of the universe, we now find ourselves orbiting an average-sized sun, which is just one of millions of stars in our own Milky Way galaxy. And our galaxy itself is just one of billions of galaxies, in a universe that is infinite and expanding. But this is far from the end of a long history of inquiry. Huge questions remain to be answered, before we can hope to have a complete picture of the universe we live in.
"I want you to share my excitement at the discoveries, past and present, which have revolutionized the way we think. From the Big Bang to black holes, from dark matter to a possible Big Crunch, our image of the universe today is full of strange sounding ideas, and remarkable truths. The story of how we arrived at this picture is the story of learning to understand what we see."
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A light-year is the distance light travels in one year. It is 9.5 trillion (9,500,000,000,000) kilometers. The size of a galaxy may be as little as a thousand light-years across or as much as a million light-years across.
A galaxy contains stars, gas, and dust which are held together as a group by gravity. There may be millions, or even billions, of stars in one galaxy. There are billions of galaxies in the Universe.
Une galaxie contient les étoiles, le gaz, et la poussière qui sont liés en tant que groupe par gravitation. Il peut y avoir des millions, ou même des milliards, d'étoiles dans une galaxie. Il y a des milliards de galaxies dans l'univers.