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 way (up to congruence) to place the vertices at distinct locations such that adjacent vertices are a unit distance apart, and this placement also puts the two endpoints of the missing spoke at unit distance . posed the problem of estimating how many pairs of points in a set of "n" points could be at unit distance from each other. In graph theoretic terms, how dense can a unit distance graph be? The hypercube graph provides a lower bound on the number of unit distances proportional to formula_1 By considering points in a square grid with carefully chosen spacing, Erdős