Radius of Sirius A

Sirius A is wide enough to measure directly, which almost no star is. In 2003 Pierre Kervella's team combined the light of two telescopes at ESO's Very Large Telescope Interferometer in Chile and found its disc 6.039 thousandths of an arcsecond across: a quarter seen from about 800 km away. With the Hipparcos parallax putting the star 8.6 light-years off, that angle becomes a radius of 1.711 Suns, give or take 0.013, or about 1.19 million km.

Set beside the mass of Sirius A, the radius gives everything a formula wants from the surface: gravity of about 193 m/s², some 70 per cent of the Sun's because the extra mass is spread over a wider ball, and an escape velocity near 680 km/s. The same measurement, run through stellar models, puts the star's age at roughly 200 to 250 million years. Its companion, Sirius B, is about 211 times narrower yet holds half as much mass: the gap between a star still burning hydrogen and one that has finished.