Light on a dark place

Abstract

The sharpest images ever taken of matter around the probable black hole at the centre of our Galaxy bring us within grasp of a crucial test of general relativity — a picture of the black hole's ‘point of no return’. Most galaxies are thought to have a supermassive black hole at their centres, but proving it is very difficult. The centre of our own Galaxy, occupied by the compact radio source Sagittarius A* (Sgr A*) is a good place to start looking as it is so close to us, just 26,000 light-years from the Sun. A new short-wavelength radio image of Sgr A* has made it possible establish the intrinsic size of Sgr A* for the first time. It is 1 astronomical unit (the Sun-Earth distance) across. This suggests that its mass density is more than 10 orders of magnitude greater than any other known cosmic object, well into supermassive black hole territory.