Several of the stars of the month are double stars, but double stars where one of the stars is of specific interest in itself. The star for February 2020 is such a case. The German astronomer Friedrich Bessel—best known for his measurement of the parallax of 61 Cygni announced in 1844—realised from his study of the motion of the bright star Sirius (which needs no introduction) it must have an unseen companion. In effect the mass of the unseen star makes Sirius “wobble”. But there was a big puzzle: as the star could not be seen it had to be faint and yet to make Sirius wobble, it also had to be massive. This unseen star, Sirius B, was first seen by accident when Alvan Graham Clark was testing the 18.5 inch refractor destined for the Dearborn observatory near Chicago. His father Alvan Clark had established a famous telescope factory at Cambridgeport, Massachusetts, and on 31 January 1862, his son was testing the new telescope. As he was waiting for Sirius to appear from behind a nearby building, he realised he could see Sirius’s hitherto invisible companion as his eye was shielded from the bright light of the main star. Of course we now know that Sirius B is a white dwarf, a star which is indeed small, dim and massive, which is basically the carbon-rich ember from a supernova explosion.
Sirius A and B are both about 230 million years old, but as a massive star of five solar masses (Sirius A is only twice as massive as our sun), Sirius B exploded after only 100 million years. As it has a relatively small orbit round Sirius A (20 AU), the separation varies considerably over time. In the 1990s and 2000s, Sirius B was close to the main star and could not be seen easily. Observing an 8 magnitude star next to a very bright star is difficult anyway. Even seeing the companion of Rigel although it is almost as bright as the main star (1.3 against 0.0) is difficult. But currently we have a short window during which the separation is relatively large (at present the separation is 11 seconds) and the maximum will be 11.3 seconds in 2023. However, you will need excellent seeing (difficult when Sirius is usually low in the sky), a fairly large telescope (at least eight inches, and twelve inches or more is better) and high magnification (300 times if possible). With a reflector, Sirius B is liable to be hidden by a diffraction spike.