Peering into the heart of the Milky Way, astronomers have spotted the 'howls' of dead stars as they feed on stellar companions. These zombie stars are thought to be leaving a 'mass grave' of white dwarf stars near the centre of the Milky Way. But why there are so many of these stars here - which are left behind when a larger star runs out of fuel - remains a mystery. Scroll down for video . The Nuclear Spectroscopic Telescope Array, or NuStar, has captured a new high-energy X-ray view (magenta) of the bustling center of our Milky Way galaxy. The smaller circle shows the center of our galaxy where the NuStar image was taken . The finding, published in Nature, was made by scientists at Haverford College in Pennsylvania, US using Nasa's NuStar telescope. 'We can see a completely new component of the centre of our galaxy with Nustar's images,' said Kerstin Perez of Columbia University in New York. 'We can't definitively explain the X-ray signal yet - it's a mystery. More work needs to be done.' When a star runs out of fuel it usually collapses in on itself. Stars more than eight times the mass of our sun will explode in a supernova, leaving behind a neutron star or black hole. Smaller stars, though, will shed their outer layers and leave behind a hot dense core – known as a white dwarf. A white dwarf contains a similar amount of mass to its previous star but will be much smaller – about the size of Earth – making gravity at its surface much more intense. They are thought to be able to burn for tens or hundreds of billions of years - at which point they  become black dwarfs – but as the universe is not this old, no black dwarfs exist in the universe yet. When they peered at the Galactic Centre, they found an emission of X-rays that corresponded to thousands of dead stars being present. The emission could also be due to millisecond pulsars - rapidly rotating neutron stars - or dense magnetic fields. 'However, all these interpretations pose significant challenges to our understanding of stellar evolution, binary formation and cosmic-ray production in the Galactic Centre,' the researchers wrote in their paper. The emissions were found to be coming from a region of the Galactic Centre 13 by 26 light-years in size around the central supermassive black hole, called Sagittarius A*. On theory is that when stars die, they don't always go quietly into the night. Unlike stars like our sun, collapsed dead stars that belong to stellar pairs, or binaries, can siphon matter from their companions. This zombie-like 'feeding' process differs depending on the nature of the normal star, but the result may be an eruption of X-rays. But it is the potential for these to be white dwarf stars that is the most intriguing theory. Scientists at Haverford College in Pennsylvania say they have spotted X-ray emissions consistent with thousands of white dwarfs (one illustrated centre) at the galaxy's centre. These are stars left behind after larger ones use up their fuel. But why thousands of these stars are here remains a mystery . There are already known to be many young, massive stars orbiting our galaxy's central supermassive black hole - but why there would be thousands of white dwarf stars as well is not clear. White dwarfs are what are left when medium-mass stars - including our sun - shed their outer layers at the end of their life. Behind is left the hot, dense core which burns white. If the white dwarf gathers too much material from a companion, its mass will reach a critical point and the star will explode. The researchers say the latest finding needs further study to get to the bottom of the mystery. 'This new result just reminds us that the galactic center is a bizarre place,' said co-author Chuck Hailey of Columbia University. 'In the same way people behave differently walking on the street instead of jammed on a crowded rush hour subway, stellar objects exhibit weird behavior when crammed in close quarters near the supermassive black hole.' Shown is the region around the supermassive black hole at the Milky Way's Galactic Centre - Sagittarius A* - where the X-ray emissions consistent with thousands of white dwarfs were found. Other possible explanations include millisecond pulsars, which are rapidly rotating neutron stars . Nasa's NuStar telescope (illustrated) is a space-based X-ray telescope that focuses high-energy X-rays from distant astrophysical sources. Its primary goal is to conduct a survey for black holes, but it has also been used for other purposes such as this latest research .
Scientists at Haverford College in Pennsylvania made the finding . They spotted X-ray emissions consistent with thousands of white dwarfs . These are stars left behind after larger ones use up their fuel . But why thousands of these stars are there remains a mystery .