There is something really strange at the centre of the Milky Way. The vicinity of a supermassive black hole is a pretty strange place to start, but in 2020 astronomers found six objects orbiting Sagittarius A* that are unlike anything else in the galaxy. They are so strange that they were assigned to a whole new class, which astronomers have dubbed G objects.

The original two objects, dubbed G1 and G2, first came to the attention of astronomers about two decades ago, and their orbits and strange nature were gradually pieced together over the following years. They appeared to be giant gas clouds 100 astronomical units wide, with emission spectra of gas and dust, which became more elongated as they approached the black hole.

However, G1 and G2 did not behave like gas clouds.

"These objects look like gas but behave like stars," physicist and astronomer Andrea Ghez of the University of California, Los Angeles, said in 2020.

Ghez and her colleagues have been studying the galactic centre for more than 20 years. Based on this data, a team of astronomers led by UCLA astronomer Anna Ciurlo detected four more of these objects: G3, G4, G5 and G6.

They are on quite different orbits from G1 and G2 (pictured above); together, the G bodies have orbital periods ranging from 170 years to 1,600 years.

Exactly what they are is unclear, but G2's solid exit from the periapsis, the point in its orbit closest to the black hole, in 2014 was a big clue, according to Ghez.

"At the moment of closest approach, G2 had a really strange signature," he said.

"We've seen it before, but it didn't look very peculiar until it got close to the black hole and elongated and most of its gas was broken up. While it was a fairly innocuous object when it was away from the black hole, at its closest approach it turned into an object that was really stretched and distorted and lost its outer shell, and now it's becoming more compact again."

It was previously thought that G2 was a cloud of hydrogen gas that would be torn apart and swallowed by Sgr A*, producing a supermassive black hole accretion firework. The fact that nothing happened was later labelled a "cosmic fiasco".

Astronomers believe the answer lies in massive double stars. Most of the time these twin stars hang out locked in a mutual orbit and do their friendly star thing. But sometimes - just like colliding binary black holes - they can collide into each other to form a massive star.

When this happens, they produce a vast cloud of dust and gas that surrounds the new star for about a million years after the collision.

"Something must have kept [G2] compact and allowed it to survive its encounter with the black hole," Ciurlo added. "This is evidence that there is a stellar object inside G2."

But what about the other five? They too could be binary star mergers. Most of the stars in the galactic centre are very massive, and many are binaries. And the extreme gravitational forces around Sgr A* may be enough to destabilise their binary orbits with relative frequency.

"Stellar mergers in the Universe may be happening more often than we thought, and are probably quite common," Ghez said.

"Black holes may be pushing binary stars to merge. It is possible that many stars we are watching and don't understand are the end product of mergers that are currently quiet. 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 the way single stars interact with other single stars and with the black hole."

It seems that whatever G objects are, they have a lot in common, and expanding the data set can only provide more information to solve the puzzle. However, there is still much to be solved. Like some of the mysterious fireworks from Sgr A* that have been detected before.

 

Source: https://www.sciencealert.com/