Alvan Graham Clark (July 10, 1832 – June 9, 1897) was an American astronomer and telescope-maker.

[1] On January 31, 1862, while testing a new 18.5-inch (470 mm) aperture great refractor telescope in Cambridgeport, Massachusetts, Clark made the first ever observation of a white dwarf star.

This discovery of Sirius B, or affectionately "the Pup", proved an earlier hypotheses (Friedrich Wilhelm Bessel in 1844) that Sirius, the brightest star in the night sky with an apparent magnitude of −1.46, had an unseen companion disturbing its motion.

Clark's 18.5 inch refracting telescope was then delivered to his customer, the landmark Dearborn Observatory of Northwestern University in Evanston, Illinois, where it is still being used today.

[2] Alvan Graham Clark died in Cambridge, Massachusetts on June 9, 1897.