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 pairs might also be at unit distance apart. For instance, the Möbius-Kantor graph has a drawing of this type. According to this looser definition of a unit distance graph, all generalized Petersen graphs are unit distance graphs . In order to distinguish the two definitions, the graphs in which non-edges are required to be a non-unit distance apart may be called strict unit distance graphs . The graph formed by removing one of the spokes from the wheel graph "W" is a subgraph of a unit distance graph, but is not a strict unit distance graph: there is only one