Speed of light they appeared 13 billion years ago, when the universe was less than a billion years old. The fact that more distant objects appear to be younger, due to the finite speed of light, allows astronomers to infer the evolution of stars, of galaxies, and of the universe itself. Astronomical distances are sometimes expressed in light-years, especially in popular science publications and media. A light-year is the distance light travels in one year, around 9461 billion kilometres, 5879 billion miles, or 0.3066 parsecs. In round figures, a light year is nearly 10 trillion kilometres or nearly 6 trillion miles. Proxima Centauri,
Rocheworld focusing light onto the smaller sail, slowing the craft. Using the light sail system, the spaceship "Prometheus" continued to accelerate for 20 years, traveling 2 light years' distance toward Barnard's Star before going into coast mode and traveling an additional 20 years' time at a constant speed of 0.2 "c", covering the remaining 4 light years (ca. 23 trillion miles; 38 trillion km). Flouwen (the Middle English word for 'flow') are the alien creatures in the book. They are the sole intelligent inhabitants of the planet Eau, which makes up the watery half of Rocheworld. Flouwen are blob-like, happy-go-lucky aliens