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,
Unit distance graph find a unit distance graph "G" in which some pair of vertices are at distance "A" in all unit distance representations of "G" . This result implies a finite version of the Beckman–Quarles theorem: for any two points "p" and "q" at distance "A", there exists a finite rigid unit distance graph containing "p" and "q" such that any transformation of the plane that preserves the unit distances in this graph preserves the distance between "p" and "q" . The full Beckman–Quarles theorem states that any transformation of the Euclidean plane (or a higher-dimensional space) that preserves unit distances must