Mass of Sirius A

Sirius A's mass is known the only way stellar masses are ever truly known: from an orbit. Bessel announced in 1844 that Sirius's proper motion wobbles, and deduced an unseen companion — the first object in astronomy inferred purely from its gravity, a method that would later find Neptune, exoplanets and dark matter. Alvan Graham Clark spotted the faint companion in 1862 while testing a new telescope lens, and the pair's 50.13-year mutual orbit, now tracked by Hubble across more than three revolutions, yields both masses to about one per cent through Kepler's third law.

Twice the Sun's mass buys a short, bright life: Sirius A burns 25 times the Sun's luminosity through an A-type surface at 9940 K, and will exhaust its core hydrogen within a billion years of its birth — a fate its companion has already met. The two-solar-mass benchmark also marks a structural boundary: stars above it fuse hydrogen through the CNO cycle in convective cores, unlike the Sun.