Sirius is Alpha Canis Majoris, apparent magnitude -1.46, the brightest star in the night sky by a comfortable margin. Its brilliance is largely a matter of proximity: at 8.6 light years it is one of the nearest stars, and while it is genuinely more luminous than the Sun, about twenty-five times so, it is not an exceptional star in absolute terms. Several stars in this collection are thousands of times more luminous and appear far fainter.
It is a binary. Sirius A is a main-sequence star of type A0, and Sirius B is a white dwarf, the first ever identified. Its existence was inferred by Friedrich Bessel in 1844 from a wobble in the primary's proper motion, and it was seen by Alvan Graham Clark in 1862. Sirius B has roughly the mass of the Sun packed into a volume comparable to the Earth. Its discovery and the subsequent realisation of what it was forced the development of degenerate matter physics and is one of the foundational episodes of twentieth-century astrophysics.
Sirius is low enough in the sky from mid-northern latitudes that atmospheric refraction breaks its light into flashes of colour, and it is the star most often reported to authorities as an unidentified flying object.
There is a genuine and unresolved historical puzzle: several classical sources, including Ptolemy, describe Sirius as red. It is white. Explanations offered include translation and terminology problems, atmospheric effects on a low-altitude star, and, far less plausibly, on stellar evolution timescales, actual change. No explanation is agreed.
Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with Almagest (Mathematike Syntaxis)Claudius Ptolemy