At the heart of the Milky Way, there's a monster black hole with a mass 4 million times that of the sun, Sagittarius A* (Sgr A*). In the last decade, scientists looking in the black hole's cosmic neighborhood saw two peculiar objects, which seemed to be orbiting the black hole. They were dubbed G1 and G2. 

The nature of these so-called "G sources" is controversial. Some astronomers believe they're gas clouds, others contend they look more like strange stars shrouded in dust. In a new study, astronomers reveal they have detected four more of these mysterious objects which look very similar to G1 and G2 -- and suggest they are members of a new class of cosmic phenomena.

"These objects look like gas and behave like stars," said Andrea Ghez, an astronomer at UCLA and co-author of the new study, published in the journal Nature on Wednesday.

Using near-infrared data collected over the last 13 years by the Osiris imager, installed at the W.M. Keck Observatory in Hawaii, the team were able to study the mysterious objects in greater detail. And they had a good template to work from -- both G1 and G2 have been studied fairly intensely and G2, in particular, has enraptured astronomers in the past.

In 2014, astronomers observing G2 watched on as it barreled directly towards Sgr A*. Astronomers predicted G2 was a gas cloud and so it would offer the massive black hole a snack -- as it approached it would be ripped apart and gas would fall into the black hole. But... that's not what happened. G2 got perilously close to Sgr A* and survived, prompting a rethink about what it could be. 

"G2 survived and continues happily on its orbit; a gas cloud would not do that," said Ghez, back in 2014. G1 also survived its own close encounter, relatively unchanged.

Ghez postulated what astronomers were seeing was not a gas cloud, but the product of merged binary stars, after two stars orbiting each other collided and formed a single, massive star. 

A second team of researchers, from the Max Planck Institute, countered that G2 could certainly be a gas cloud, suggesting it was part of a larger stream of gas that also incorporated G1. Modelling showed this seemed to fit the data well.

But those hypotheses were generated with just two weird objects. Now, with four more to study, the mystery deepens.

"I think the gas streamer hypothesis worked well when we just had G1 and G2, but with 6 objects, orbiting at very different inclinations, this hypothesis is harder to apply," said Anna Ciurlo, an astronomer at UCLA and first author on the new study.

Ciurlo and the team suggest the binary merger hypothesis is a "compelling explanation". Astronomers do see single stars living and orbiting Sgr A* in a similar part of the galactic neighborhood -- and they're weird, too -- but they don't seem to exhibit quite the same properties. Conditions around monstrous black holes are pretty extreme, so it may not be all that unusual to see a series of big, merged binary stars, considering where we're looking.

Stefan Gillessen, an astronomer at the Max Plack Institute and proponent of the streamer hypothesis, suggests its unlikely all of the objects were formed from a binary merger, and postulates the G sources may be a handful of different, weird cosmic phenomena like young stellar objects or stellar wind knots. 

The newly discovered G sources will drive by the supermassive black hole in years to come, which should provide astronomers with more information about their nature -- however, they'll be waiting for some time. The orbital periods of the new objects range between 170 years and 1,600 years. To improve their odds of understanding what the strange not-gas-maybe-stars are, the team will need to study the region in more detail -- but also look further out into the cosmos.

"We need to look for more objects in this region," says Ciurlo. "But we also need to look for similar objects elsewhere, to understand if this process is unique to the environment of the black hole or not."

Originally published 11 a.m. PT
Updated 1:07 p.m PT: Added comments from Ciurlo and Gillessen