Bessel and Sirius B
Friedrich Wilhelm Bessel was director of Konigsberg Observatory,
where he was carrying out careful measurements
of the positions and motions of stars. In 1834 he
realized that Sirius seemed to "wobble" in its motion
over the years, which could be explained if it had a faint, unseen companion. This proved
to be the white dwarf, Sirius B, which was not seen directly until 1862.
Bessel also predicted a companion for the nearby star
Procyon (Procyon B was later found to be another white dwarf), and he carried out the
first measurement of the distance to a star when he succeeded in measuring
the parallax of 61 Cygni in 1838.