The nearest stars to Earth are part of the stellar system known as the Alpha Centauri system. Alpha Centauri is actually a triple star system consisting of three stars: Alpha Centauri A, Alpha Centauri B, and Proxima Centauri.
Proxima Centauri, the closest star to Earth, is located about 4.24 light-years away. Alpha Centauri A and Alpha Centauri B are a bit farther, with a distance of about 4.37 light-years.
The concept of a light-year is a unit of distance, not time. It represents the distance that light travels in one year, which is approximately 9.46 trillion kilometers or 5.88 trillion miles. We use light-years to express astronomical distances because it provides a more convenient and understandable scale for the vast distances in space.
The distances to stars are determined using a technique called stellar parallax. It relies on the fact that as the Earth orbits the Sun, the position of a nearby star appears to shift slightly against the more distant background stars due to the change in viewing angle. By measuring this apparent shift in position over a period of six months, astronomers can calculate the distance to the star.
The accuracy of the measurements has improved over time, and modern telescopes and techniques can determine stellar distances with great precision. This allows us to estimate the distances to nearby stars and express them in units of light-years.