The brightest star in the night sky, Sirius, is actually a binary system—two stars orbiting each other.

Sirius lies approximately 8.6 light-years from Earth in the constellation Canis Major. The system consists of Sirius A, a main-sequence star, and Sirius B, a faint white dwarf companion. Astronomer Friedrich Bessel first suspected the companion's existence in 1844 after detecting irregularities in Sirius A's motion. In 1862, Alvan Graham Clark finally observed the faint star while testing a new telescope lens.Sirius B is a stellar remnant—the collapsed core of a dead star that has exhausted its nuclear fuel. It contains about 98% of the Sun's mass but is compressed into a sphere just 12,000 kilometers in diameter. This extreme compression creates a density of roughly 2.1 million grams per cubic centimeter. The gravitational pull at its surface is nearly 350,000 times stronger than Earth's gravity.The two stars orbit their common center of mass every 50.1 years in an elliptical path. Because Sirius A is about 10,000 times more luminous than Sirius B, the smaller star is often drowned out by its much brighter partner, making "The Pup" a challenging target for amateur astronomers. Studying this system has provided NASA and the European Space Agency with crucial insights into stellar evolution and the physics of degenerate matter.