Strange things lurking near Sagittarius A*

Astronomers have discovered four more “bizarre” objects at the centre of the Milky Way not far from the supermassive black hole Sagittarius A*.

Part of a new class called G objects, they look compact most of the time and stretch out when their orbits bring them closest to the black hole. Those orbits range from about 100 to 1000 years.

“These objects look like gas and behave like stars,” says Andrea Ghez, director of the Galactic Centre Group at the University of California, Los Angeles (UCLA) and co-author of a paper in the journal Nature.

The new discoveries are known simply as G3 to G6. G1 was discovered by Ghez’s research group back in 2005, and G2 by astronomers in Germany in 2012.

“The fact that there are now several of these objects observed near the black hole means that they are, most likely, part of a common population,” says co-author Randy Campbell, from the Keck Observatory in Hawaii. 

The new study is part of UCLA’s Galactic Centre Orbits Initiative, which draws on 13 years of data taken from the observatory. 

The research team, led by UCLA’s Anna Ciurlo, has determined that while G1 and G2 have similar orbits, the new four all have very different orbits. 

Ghez believes all six were binary stars – a system of two stars orbiting each other – that merged because of the strong gravitational force of the supermassive black hole. The merging of two stars takes more than a million years to complete. 

“Mergers of stars may be happening in the universe more often than we thought, and likely are quite common,” she says. 

“Black holes may be driving binary stars to merge. It’s possible that many of the stars we’ve been watching and not understanding may be the end product of mergers that are calm now. 

“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.”

Co-author Mark Morris, also from UCLA, says one of the things that gets people excited about G objects is “that the stuff that gets pulled off of them by tidal forces as they sweep by the central black hole must inevitably fall into the black hole”.

“When that happens, it might be able to produce an impressive fireworks show since the material eaten by the black hole will heat up and emit copious radiation before it disappears across the event horizon,” he says.

The researchers made their observations using powerful technology called adaptive optics (AO), which Ghez helped develop at the Keck Observatory. 

This corrects the distorting effects of the Earth’s atmosphere in real time and when combined with the observatory’s OH-Suppressing Infrared Imaging Spectrograph (OSIRIS), allows spectroscopic measurements to be made of the galactic centre’s gas dynamics.

An animation of the orbits of the G objects, together with the orbits of stars near the supermassive black hole. Credit: Advanced Visualisation Lab, National Centre for Supercomputing Applications, University of Illinois.