Alvan Graham Clark discovers the white dwarf star Sirius B, a companion of Sirius, through an 18.5-inch (47 cm) telescope now located at Northwestern University.

Alvan Graham Clark was an American astronomer and telescope-maker.

A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: in an Earth-sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place in a white dwarf; what light it radiates is from its residual heat. The nearest known white dwarf is Sirius B, at 8.6 light years, the smaller component of the Sirius binary star. There are currently thought to be eight white dwarfs among the one hundred star systems nearest the Sun. The unusual faintness of white dwarfs was first recognized in 1910. The name white dwarf was coined by Willem Jacob Luyten in 1922.

Sirius is the brightest star in the night sky. Its name is derived from the Greek word Σείριος (Latin script: Seirios; lit. 'glowing' or 'scorching'. The star is designated α Canis Majoris, Latinized to Alpha Canis Majoris, and abbreviated α CMa or Alpha CMa. With a visual apparent magnitude of −1.46, Sirius is almost twice as bright as Canopus, the next brightest star. Sirius is a binary star consisting of a main-sequence star of spectral type A0 or A1, termed Sirius A, and a faint white dwarf companion of spectral type DA2, termed Sirius B. The distance between the two varies between 8.2 and 31.5 astronomical units as they orbit every 50 years.

Northwestern University (NU) is a private research university in Evanston, Illinois, United States. Established in 1851 to serve the historic Northwest Territory, it is the oldest chartered university in Illinois.