Radius of Proxima Centauri

Proxima is startlingly small: 0.154 of the Sun's radius makes it only about 1.5 Jupiters across, though 130 times Jupiter's mass — matter at the bottom of the main sequence is crushed to enormous density, and Proxima's mean density is some forty times the Sun's. Feed this radius and its 3000 K effective temperature into the Stefan–Boltzmann law and the luminosity comes out near 0.0016 L⊙: if Proxima replaced the Sun, Earth would receive less light than a heat lamp across a room, and its habitable zone huddles twenty times closer than Mercury.

The radius is not imaged but inferred — from the measured luminosity and temperature, cross-checked by interferometry at the VLT that just resolves a disc about one milliarcsecond across, the angular size of a coin at four thousand kilometres. That such a measurement is possible at all for so small a star is itself a fact worth knowing about modern optics.