Asked by: Brian Baker, Hook
Although stars cannot form in the voids between galaxies (since the density of matter is far too low), there are in fact large numbers of ‘intergalactic stars’. It has been estimated, for example, that 10 per cent of the mass of the Virgo galaxy cluster is in the form of these stellar interlopers.
How they got there is still a matter of debate, but there are two possible processes, both resulting from gravitational interactions. First, stars can be expelled from their parent galaxy if it collides, merges or passes close to another galaxy. Second, if a star has a close encounter with a supermassive black hole (usually residing at the galactic centre), it can be accelerated to extremely high velocities, eventually leaving its parent galaxy altogether.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.