The brightest star's hidden partner was deduced 18 years before anyone saw it
In 1844 Friedrich Bessel noticed that Sirius wobbled as it drifted across the sky, as if tugged by something invisible. He concluded it had an unseen companion. On 31 January 1862 Alvan Graham Clark, testing a new 18.5-inch lens, spotted a faint point beside the glare: Sirius B, a white dwarf.
Clark's lens was built for Dearborn Observatory and was the largest telescope in the United States at the time. What he found is the burnt-out core of what was once the more massive of two bluish stars. It exhausted its fuel first, swelled into a red giant, shed its outer layers and collapsed about 120 million years ago. The pair now circle each other every 50 years, their separation swinging between 8.2 and 31.5 times the Earth-Sun distance.
Sirius A owes its dazzle to being close and fairly powerful: roughly twice the Sun's mass and 25 times as luminous, just 8.6 light-years away. At magnitude minus 1.46 it outshines Canopus, the runner-up, nearly twice over. It is edging nearer and will brighten slightly for about 60,000 years. Around the year 66,270 it will sit within 1.6 degrees of the south celestial pole, serving as a southern pole star, and it should stay the brightest star in our sky for roughly 210,000 years before Vega takes over.
It has been a scientific first more than once. In 1717 Edmond Halley, comparing his measurements with Ptolemy's, found that Sirius had shifted by about a Moon's width, showing that the so-called fixed stars move. In 1868 William Huggins made it the first star to have its velocity measured, although his estimate was far too high and had it receding when it is actually approaching.
Cultures everywhere tracked it. Its dawn return heralded the Nile flood in Egypt, where it was worshipped as the goddess Sopdet, and the scorching dog days in Greece. For the Maori its appearance signalled winter instead, and because its path crosses directly over Fiji, Polynesian navigators used it as a latitude marker.
Source: Sirius
