The center of our Milky Way galaxy harbors a large, dark secret: There is a graveyard of stars surrounding the supermassive black hole Sagittarius A*. We couldn’t see this stellar cemetery before now because the extreme and chaotic environment surrounding the black hole blocked it from view. But now, scientists have delved through the mayhem by looking at Sagittarius A*’s black hole using NASA’s NuSTAR X-ray telescope. The study has been published in Nature.

X-ray image of the supermassive black hole at the center of the Milky Way, Sagittarius A* via Perez et al./Nature

The image above shows the brightest X-ray regions in white. Yellow to red demonstrates a decrease in the amount of X-rays being emitted, leading to black, which is the dark, icy vacuum of space. The two regions enclosed by the white dotted areas were the sections the scientist's examined and are located around 10 parsecs (33 light-years) away from the central black hole. The X-ray emissions coming from these regions corresponded to those of stars nearing the end of their life, otherwise known as white dwarfs. When stars reach the end of their life, they blow up and spew their outer layers of stellar matter into the universe. Only the hot, inner core of the star remains; this is a white dwarf. A white dwarf cools down over time until it simply runs out of energy.

A newly formed white dwarf has a lot of energy left over from the supernova explosion that gave rise to it. The infant white dwarf is burning so hot that the gas on the surface has enough energy to emit X-rays. The researchers believe the X-ray emissions surrounding Sagittarius A* are from white dwarfs, despite not being able to see them, because they have compared the X-ray spectrum to those of known white dwarfs elsewhere in the universe.

There are some other possible candidates for X-ray emitters surrounding the supermassive black hole, such as rapidly spinning neutron stars, for example. However, when comparing emission spectrums, the results seem to point to white dwarfs. Kerstin Perez, the lead author of the paper, commented on this revelation in a NASA Jet Propulsion Laboratory news release: "Almost anything that can emit X-rays is in the galactic center."

The question that naturally leads from the discovery of this stellar 'mass grave' is "why is it there?" At the moment, it is another cosmological mystery. “Regardless of origin, these observations reveal novel processes present within the inner parsecs of the Galaxy,” said Perez.