Discovering Sirius B

The proper motion of Sirius and its companion. The dark sinuous curve is the path of Sirius while the dotted curve is that of Sirius B. The straight line is the path of the center of mass of the system.

In 1844 the German astronomer Friedrich Bessel noticed small odd irregularities in the proper motions of the two bright stars, Sirius and Procyon. They did not move uniformly in a straight line, as expected relative to other stars in the sky, but rather seemed to move with irregular motions which seemed to repeat every fifty years. Bessel attributed these motions to the presence of unseen massive companions whose orbital motion produced the irregularities. Binary stars were well known at the time but until Bessel’s discovery it was always possible to identify both stars in a system.

The dark star associated with Sirius was accidentally discovered on the evening of January 31, 1862 in Cambridgeport Massachusetts. Alvan Graham Clark and his father Alvan Clark, two members of the famous family of nineteenth century telescope makers, were testing the lens for an 18 ½-inch telescope, the largest refracting telescope in the world at the time. The Clarks were not aware of the significance of their discovery at the time but did report it to George Phillips Bond, Director of the Harvard College Observatory in Cambridge Massachusetts. Using the Harvard’s 15-inch “Great Refractor” Bond succeeded in observing the small companion exactly one week later. Bond immediately recognized the significance of the discovery and promptly published the news in scientific papers in the US and in Europe.

The previously unknown part of the discovery of companion, Sirius B, involves Europe’s most famous astronomer at the time, Urbain J. J. Le Verrier. In 1846 Le Verrier had predicted the position of Neptune, and the new planet promptly discovered in Berlin later that same year. Le Verrier was also aware of the significance of Sirius’ dark star and conducted his own search for it in January 1862. The story also involves the development of the first reflecting telescope to a use metal-on-glass primary mirror. This revolutionary telescope, an 80-cm reflector, had been developed by Léon Foucault and went into service at the Paris Observatory in January 1862.

The new companion of Sirius turned out to be a remarkably faint yet massive star. It would require more than five decades before astronomers appreciated that the small companion represented an entirely new type of star, a white dwarf.

More about the fascinating events surrounding the discovery of Sirius B can be found below.