Our Universe, Billions and billions

Carl Sagan in his book, Billions and Billions writes:

Our Universe is composed of some hundred billion galaxies, one of which is the Milky Way or as we like to call it, “Our Galaxy,” although we certainly do not have possession of it.

It is composed of gas and dust and about 400 billion suns. One of these, lodged in an obscure spiral arm of the Galaxy, is our Sun or local star, — drab, humdrum, ordinary.

Accompanying our Sun in its 250 million year journey around the center of the Milky Way is a retinue of small worlds. Some are planets, some are moons, some asteroids, some comets.

We humans are one of the 50 billion species that have grown up and evolved on a small planet, Earth, third from the Sun. From the Earth we have sent spacecraft to examine some 70 other worlds in our system and so far have reached the surfaces of four of them—the Moon, Venus, Mars, and Jupiter, engaging ourselves thereby in a truly mythic endeavor.

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