The Universe is lavish beyond imagining

Carl Sagan, Shadows of Forgotten Ancestors, page 13.

Stars formed in batches from huge clouds of gas and dust. A dense clump of material attracts adjacent gas and dust, grows larger and more massive, more efficiently draws matter into it, and is off on its way to stardom. When the temperatures and pressures in its interior become high enough, hydrogen atoms - the most abundant material in the Universe by far - are jammed together and thermonuclear reactions are initiated. When it happens on a large enough scale, the star turns on and the nearby darkness is dispelled. Matter is turned into light.

The collapsing cloud spins up, squashes down into a disk, and lumps of matter aggregate together - successively the size of smoke particles, sand grains, rocks, boulders, mountains and worldlets. Then the cloud tidies itself up through the simple expedient of the largest objects gravitationally consuming the debris. The dust-free lanes are the feeding zones of young planets. As the central star begins to shine, it also sends forth great gales of hydrogen that blow grains back into the void. Perhaps some other system of worlds, fated to arise billions of years later in some distant province of the Milky Way, will put these rejected building blocks to good use.

In the disks of gas and dust that surround many nearby stars, we think we see the nurseries in which worlds, far-off and exotic, are accumulating and coalescing. All over our galaxy, vast, irregular, lumpy, pitch-black, interstellar clouds are collapsing under their own gravity, and spawning stars and planets. It happens about once a month. In the observable Universe - containing as many as a hundred billion galaxies - perhaps a hundred solar systems are forming every second. In that multitude of worlds, many will be barren and desolate. Others may be lush and fertile, on which beings exquisitely adapted to their circumstances are growing up, coming of age, and attempting to piece together their beginnings. The Universe is lavish beyond imagining.