Ken Croswell captures the beauty of the Milky Way:

Heavy elements are ... essential for life: Witness the oxygen we breathe, the calcium in our bones, the iron in our blood. When a star explodes in a lesser galaxy, this raw material for life shoots out into space at millions of miles an hour and is lost.

But in the Milky Way, the elements encounter interstellar gas and dust and are restrained by the strength of the galaxy's immense gravitational field. These impediments slow their speed, so they can enrich star-forming gas clouds with the ingredients for new generations of stars and planets. That's what happened 4.6 billion years ago, when the sun and the Earth emerged from a now-vanished interstellar nebula.

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