Bizarre objects near Milky Way black hole

Orbits of G objects at the centre of our galaxy, with the supermassive black hole marked with a white cross.

ASTRONOMERS from the University of California at Los Angeles (UCLA) and W.M. Keck Observatory have discovered a new class of bizarre objects at the centre of our galaxy, not far from the supermassive black hole called Sagittarius A*. The study, which is part of UCLA’s Galactic Centre Orbits Initiative, consists of 13 years of data taken from Keck Observatory on Mauna Kea in Hawaii. The results have been published in a recent issue of Nature.

“These objects look like gas but behave like stars,” said co-author Andrea Ghez, director of the UCLA Galactic Centre Group. These new objects, four in all and which are called G objects, look compact most of the time and stretch out when their orbits bring them closest to the black hole. Their orbits range from about 100 to 1,000 years, said lead author Anna Ciurlo, a UCLA postdoctoral researcher.

Andrea Ghez’s research group identified an unusual object at the centre of our galaxy in 2005, which was later named G1. In 2012, astronomers in Germany made a puzzling discovery of a bizarre object named G2 in the centre of the Milky Way that made a close approach to the supermassive black hole in 2014. Andrea Ghez and her research team believe that G2 is most likely two stars that had been orbiting the black hole in tandem and merged into an extremely large star, cloaked in unusually thick gas and dust. The four new objects have been labelled G3, G4, G5 and G6. “At the time of closest approach, G2 had a really strange signature,” Andrea Ghez said. “It went from being a pretty innocuous object when it was far from the black hole to one that was really stretched out and distorted at its closest approach and lost its outer shell, and now it is getting more compact again.”

While G1 and G2 have similar orbits, G3, G4, G5, and G6 all have very different orbits. Andrea Ghez believes all six objects were binary stars—a system of two stars orbiting each other—that merged because of the strong gravitational force of the black hole. The merging of two stars takes more than 1 million years to complete, Andrea Ghez said. ”We are learning how galaxies and black holes evolve. The way binary stars interact with each other and with the black hole is very different from how single stars interact with other single stars and with the black hole,” she added.

“The unique dataset that Professor Ghez’s group has gathered during more than 20 years is what allowed us to make this discovery,” Anna Ciurlo said. “We now have a population of ‘G’ objects, so it is not a matter of explaining a ‘one-time event’ like G2.”