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Nestled in the heart of a dwarf galaxy 54 million light years from Earth is a black hole so big it makes up 15% of the star cluster's total mass.

The supermassive black hole, discovered at the centre of galaxy M60-UCD1, is said to have a mass equivalent to 21 million suns.

By comparison, the galaxy is 500 times smaller than the Milky Way, which itself has a black hole at its heart with a mass of just four million suns.

The supermassive black hole, (pictured) discovered at the centre of galaxy M60-UCD1, is said to have a mass equivalent to 21 million suns. Scientists believe M60-UCD1 may be the remnant of a larger galaxy that had its outer regions torn away after approaching too close to another monster-sized galaxy, which it now orbits

The finding suggests that huge black holes may be more common than previously thought, and other ultracompact dwarf galaxies are likely to also contain similar supermassive black holes.

A black hole is a region where matter has become so densely squeezed that not even light can escape its gravitational pull.

Scientists believe M60-UCD1 may be the remnant of a larger galaxy that had its outer regions torn away after approaching too close to the monster-sized M60 galaxy, which it now orbits.

A similar story could be behind the formation of other dwarf galaxies, which may also harbour super-massive black holes.

'We don't know of any other way you could make a black hole so big in an object this small,' said lead scientist Dr Anil Seth, from the University of Utah.

'There are a lot of similar ultra-compact dwarf galaxies, and together they may contain as many super-massive black holes as there are at the centres of normal galaxies.'

Astronomers used the Gemini North eight-metre telescope on Mauna Kea, Hawaii, to investigate M60-UCD1. 

The dwarf galaxy orbits M60, a colossal galaxy containing a 4.5 billion solar mass black hole. 

M60-UCD1 is about 54 million light years from Earth, but just 22,000 light years from the centre of M60.

Ultra-compact dwarf galaxies are less than a few hundred light years across compared with the Milky Way's 100,000 light years.

'[The finding] is pretty amazing, given that the Milky Way is 500 times larger and more than 1,000 times heavier than the dwarf galaxy M60-UCD1,' Dr Seth said. 

'We believe this once was a very big galaxy with maybe 10 billion stars in it, but then it passed very close to the center of an even larger galaxy, M60, and in that process all the stars and dark matter in the outer part of the galaxy got torn away and became part of M60.'

Dr Seth added, however, that the dwarf galaxy M60-UCD1 may be doomed, although he cannot say when because its orbit around M60 isn't known.

M60 is among the largest galaxies in what astronomers refer to as 'the local universe.'

'Eventually, [M60-UCD1] may merge with the centre of M60, which has a monster black hole in it, with 4.5 billion solar masses - more than 1,000 times bigger than the supermassive black hole in our galaxy. 

'When that happens, the black hole we found in M60-UCD1 will merge with that monster black hole.' 

The astronomers estimated the mass of the dwarf galaxy's supermassive black hole by using the Gemini North telescope (pictured) to measure the speed and motion of stars in orbit around it. They found that the galaxy contains more mass than would be expected by the amount of starlight it emits

The study – conducted by Dr Seth and 13 other astronomers – was funded by the National Science Foundation in the U.S., the German Research Foundation and the Gemini Observatory partnership, which includes the NSF and scientific agencies in Canada, Chile, Australia, Brazil and Argentina.

Ultracompact dwarf galaxies are among the densest star systems in the universe. 

M60-UCD1 is the most massive of these systems now known, with a total of 140 million solar masses. 

Astronomers have debated whether these dwarf galaxies are the stripped centers or nuclei of larger galaxies that were ripped away during collisions with other galaxies, or whether they formed like globular clusters - groups of perhaps 100,000 stars, all born together. 

There are about 200 globular clusters in our Milky Way, and some galaxies have thousands, Dr Seth continued.

The astronomers estimated the mass of the dwarf galaxy's supermassive black hole by measuring the speed and motion of stars in orbit around it.

They found that the galaxy contains more mass than would be expected by the amount of starlight it emits. 

The stars at the centre of M60-UCD1 move at about 230,000 mph – faster than stars would be expected to move without the black hole.

An alternate theory is that M60-UCD1 doesn't have a supermassive black hole, but instead is populated by a lot of massive, dim stars.

But Seth said the research team's observations with the Gemini North telescope, and analysis of archival photos by the Hubble Space Telescope, revealed that mass was concentrated in the galaxy's centre, indicating the presence of a supermassive black hole.   

The astronomers studied M60-UCD1 because they had published a paper last year showing the galaxy was an X-ray source and was extremely dense. 

The X-ray emissions suggest gas is being sucked into the black hole at a rate typical of supermassive black holes in much larger galaxies.

The new research is published in the journal Nature.