Galactic mystery: Strange objects that look like gas but behave like stars lurk at the heart of Milky Way

These G objects were once binary stars — a system of two stars orbiting each other — that merged because of our supermassive black hole, Sagittarius A*


                            Galactic mystery: Strange objects that look like gas but behave like stars lurk at the heart of Milky Way
The center of our galaxy (Getty Images)

A class of strange objects has been lurking at the center of the galaxy, orbiting the supermassive black hole, Sagittarius A*, shows research.

These objects, called G objects, look like gas but behave like stars. "They look like compact, gas-emitting objects. In practice, we believe they hide a star inside, surrounded by dust and gas," Dr. Anna Ciurlo, a UCLA postdoctoral researcher and lead author, tells MEA WorldWide (MEAWW).

The discovery of the population of G objects can help scientists unveil the mystery behind the supermassive black hole's extreme environment. "The gravitational pull is so much stronger and the magnetic fields are more extreme," Dr. Ciurlo adds.

The team identified the first of these objects in 2005 and named it G1. After seven years, another group discovered G2. Over the years, the team has identified four other companions of G1 and G2, which are now named G3, G4, G5 and G6. These objects can take about 100 to 1,000 years to orbit Sagittarius A*. 

Orbits of the G objects at the center of our galaxy, with the supermassive black hole indicated with a white cross. Stars, gas and dust are in the background. (Anna Ciurlo, Tuan Do/UCLA Galactic Center Group)

Shortly after G2's discovery, scientists thought these objects were gas clouds. But G2's close encounter with the supermassive black hole changed that line of thought.

"At the time of closest approach, G2 had a really strange signature. We had seen it before, but it did not look too peculiar until it got close to the black hole and became elongated or stretched," explains co-author Dr. Andrea Ghez, director of the UCLA Galactic Center Group.

During this time, the objects lost some of its material to the black hole, says Ciurlo. This meeting changed how G2 appeared: it went from being a pretty innocuous, compact object to one that was really stretched out and distorted at its closest approach. 

A gas cloud could not have survived this encounter. This led the team to believe that these objects were once binary stars — a system of two stars orbiting each other — that merged because of the supermassive black hole.

Its gravity alters the orbits of the two stars until the duo crash into each other. Also, such mergers release materials that can feed the black hole, says the study.

The stretching and the loss of materials or gas were restricted to only the G2's outer shell. The core containing the dust did not stretch much and remained intact. Something helped G2 survive this encounter with the black hole and evidence points to the presence of a stellar object inside G2, adds Dr. Ciurlo.

Ghez and the team speculate that such events may be quite common. "Black holes may be driving binary stars to merge. It is possible that many of the stars we have 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," explains Dr. Ghez.

These observations stem from data collected over 20 years to make these observations, thanks to the WM Keck Observatory in Hawaii. After G2, other objects are expected to get close to the supermassive black hole — but that could take decades or even longer.

For now, however, the team is keeping their eyes glued to the galactic center to find out if there are more G objects. In fact, they have already identified a few likely candidates and are continuing to analyze them.

The study has been published in Nature.

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