'Mysterious gas-like objects spotted in the centre of Milky Way'

Astronomers have spotted strange gas-like objects close to a monstrous black hole at the centre of our galaxy, a finding that may lead to deeper understanding of the forces creating stars and other cosmic entities in the universe.

"These objects look like gas and behave like stars," said study co-author Andrea Ghez from the University of California, Los Angeles (UCLA) in the US.

According to the study, published on Wednesday in the journal Nature, the strange new objects look compact most of the time, and stretch out when their orbits bring them closest to the supermassive black hole.

The study said these objects take from about 100 to 1,000 years to complete a revolution in their orbits around the black hole named Sagittarius A*.

In previous research, the astronomers identified two unusual objects at the centre of Milky Way, which they named G1 and G2, and collectively called G objects.

They believe that G2 may have been two stars that had been orbiting the black hole in tandem, and merged into an extremely large star, cloaked in thick gas and dust.

As the scientists continued to monitor G2, they found that it had a strange signature.

"We had seen it before, but it didn't look too peculiar until it got close to the black hole and became elongated, and much of its gas was torn apart," 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's getting more compact again," she added.

One of the features of the G objects, the scientists noted, was that some of their stuff which got pulled off by the black hole, inevitably had to fall into the extremely dense region.

"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," said study co-author Mark Morris from UCLA.

In the current study, the astronomers have reported the existence of four more objects -- G3, G4, G5 and G6 -- and have determined each of their orbits.

Based on their observation of these objects, the scientists believe they were once binary stars -- a system of two stars orbiting each other -- which later merged due to the strong gravitational force of the supermassive black hole.

According to the astronomers, such merging of two stars may take more than a million years to complete.

"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," Ghez said.

She said the findings may help understand 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," Ghez added.

While the gas from G2's outer shell got stretched dramatically, the dust inside its gas did not get stretched much, the astronomers noted.

"Something must have kept it compact and enabled it to survive its encounter with the black hole. This is evidence for a stellar object inside G2," said study lead author Anna Ciurlo from UCLA.

However, the scientists do not know what this stellar object could be, and continue to probe the region for more insights.

They have identified a few other cosmic entities that may be part of this new class of objects, and are continuing to monitor them.

According to Ghez and her team, the centre of the Milky Way galaxy is an extreme environment, unlike our less hectic corner of the universe.

"The Earth is in the suburbs compared to the centre of the galaxy, which is some 26,000 light-years away. The centre of our galaxy has a density of stars 1 billion times higher than our part of the galaxy," Ghez said.

"The gravitational pull is so much stronger. The magnetic fields are more extreme. The centre of the galaxy is where extreme astrophysics occurs -- the X-sports of astrophysics," she added.