Two black holes with a combined mass of 15 billion suns have been tracked as they slowly orbit each other at the centre of a galaxy 750 million light-years away. The lumbering pair were watched for 12 years by astronomers led by Greg Taylor at the University of New Mexico in the US – who used a string of radio telescopes stretching from the Caribbean to Alaska.

The objects are separated by about 23 light-years, which is the smallest known orbit of two supermassive black holes. Writing in the Astrophysical Journal, the team describes how it tracked the motion of the black holes at a glacial 1 micro-arcsecond per year – which it says is the slowest motion ever tracked in the sky. Based on its observations, the team reckons that the black holes circle each other once every 30,000 years.

Never to meet

While the black holes are believed to be spiralling towards each other, Taylor and colleagues believe that they are moving so slowly that they may never actually combine to form one huge black hole. The binary system was probably formed when two galaxies – each containing a black hole at its core – merged.