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Compact Cosmos

48 HABITABLEZONES life, the universe, and everything This wonder we call life could well be a rare and precious thing. For earthly evolution we have needed a planet big enough to hold an atmosphere,close enough to the sun for water to be liquid,and yet far enough away to avoid an inferno. The width of the comfortable circumstellar habitable zone has shrunk over time top left as the sun has become brighter. Alas,in a billion years or so the Earth itself will be too hot to sustain life as the sun continues its expansion into a red giant. Our position in the galactic disk top right is a crucial balancing act. The planet and all its living bodies contain and require heavier elements synthesized in supernovae,yet the explosions of these dying suns sterilize enormous areas of space. Life struggles to develop near these fearsome events,making the stillactive galactic center a nogo zone. Conversely, the outer edge of the disk has too few heavy elements,making life there also unlikely. Should a species survive long enough to form an intelligent society, there are several stages to look forward to main opposite. Type 0civilizations are divided by squabbles and inefficient use of resources. Rising to Type I status requires planetary consensus,global communication,and renewable energy sources. Colonizing its solar system allows a Type II society to spread to neighboring planets and moons, so outliving its mother planet. Interstellar travel grants the next giant leap to a galactic Type III civilization,while eventually,with resources on a cosmic scale,Type IV beings can manipulate the very essence of spacetime. 48 49
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