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About 80 years ago, American astronomer Edwin Hubble discovered that the universe is expanding.

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Other stars and galaxies are moving away from us, and the further away they are, the faster they are moving.

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This is called Hubble’s Law.

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The fact that the universe is expanding means that it must have started from somewhere.

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Astronomers now believe that it was created in the “Big Bang” about 15,000 million years ago and has been expanding ever since.

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The Big Bang was an enormous explosion that created all the matter and energy in the universe.

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About 75 percent of this matter was hydrogen and about 25 percent was helium, the two lightest elements.

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Before the Big Bang, there was nothing.

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The Big Bang theory was first proposed by Russian‒ American physicist George Gamow.

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A theory like this is impossible to prove.

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But it was accepted by most astronomers after American physicists Arno Penzias and Robert Wilson discovered what is called “background radiation.”

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This is radiation left over from the Big Bang.

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Penzias and Wilson received the 1978 Nobel Prize in Physics for their discovery.

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The Nobel Prize is never awarded to people who have died, so Gamow, who died in 1968, did not share the award.

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About 300 million years after the Big Bang, stars and galaxies started to form.

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Stars, such as our sun, are enormous balls of hydrogen and helium brought together by the force of gravity.

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Gravity forces them to contract and causes very high pressures and temperatures.

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The pressure at the center of our sun is over one billion atmospheres and the temperature is over 10 million degrees Celsius.

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The high temperatures cause hydrogen atoms to combine and form helium atoms in what is called a fusion reaction.

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This reaction releases heat and light energy, which is radiated out into space.

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Galaxies are groups of stars.

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Most galaxies contain hundreds of millions of stars.

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Our galaxy is called the Milky Way.