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| About
4.6 billion years ago an interstellar cloud of gas and dust
began to collapse under the force of its own gravity. |
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| As the
cloud collapsed, it began to rotate and flatten into a spinning
disk. The large clump of gas at the center became the Sun. |
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| Gas and
dust around the newborn Sun began to clump into larger objects.
The surfaces of early planets were bombarded by comets and
asteroids left over from the original disk. |
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| The bombardment
gradually declined during the first half-billion years, but
the comets and asteroids remaining still occasionally collide
with planets — sometimes with dramatic consequences.
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