Hubble image of Messier 60 and M60-UCD1
This NASA/ESA Hubble Space Telescope image shows the massive elliptical galaxy Messier 60 (also called M60, or NGC 4649). M60 is 120 million light-years across and contains an estimated 400 billion stars. Highlighted in the inset is the dwarf galaxy M60-UDC1 which orbits the giant elliptical.
Lying about 50 million light-years away, M60-UCD1 is a tiny galaxy with a diameter of 300 light-years – just 1/500th of the diameter of the Milky Way! Despite its size it is pretty crowded, containing some 140 million stars.
The dwarf galaxy may actually be the stripped remnant of a larger galaxy that was torn apart during a close encounter with Messier 60. Circumstantial evidence for this comes from the recent discovery of a monster black hole, which is not visible in this image, at the centre of the dwarf. The black hole makes up 15 percent of the mass of the entire galaxy, making it much too big to have formed inside a dwarf galaxy.