The proximity 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 in the galaxy. They are so unique that they have been given a new class, what astronomers call G objects.
The two original objects, dubbed G1 and G2, first came to the attention of astronomers nearly two decades ago, and their orbits and bizarre natures gradually coalesced in subsequent years. They looked like giant clouds of gas 100 astronomical units in diameter stretching further as they got closer to the black hole, with emission spectra of gas and dust.
But G1 and G2 did not behave like gas clouds.
“These objects look like gas but act like stars,” physicist and astronomer Andrea Ghez of the University of California, Los Angeles explained in 2020.
Ghez and her colleagues have been studying the galactic center for more than 20 years. Based on this data, a team of astronomers led by UCLA astronomer Anna Ciurlo identified four more objects: G3, G4, G5, and G6.
(Anna Ciurlo/Tuan Do/UCLA Galactic Center Group)
They are in very different orbits from G1 and G2 (pictured above); Collectively, the G objects have orbital periods ranging from 170 to 1,600 years.
It’s unclear exactly what they are, but G2 is intact. The emergence of periapsis in 2014 – that is, the closest point in its orbit to the black hole – was, Ghez believes, a big clue.
“At the time of closest approach, G2 had a really weird signature,” she said.
“We’ve seen this before, but it didn’t seem too weird until it got close to the black hole and stretched out, and a lot of its gas was torn apart. It went from being a fairly harmless object when I was away from him.” black hole to one that was really stretched and distorted at its closest point and lost its outer shell, and now it’s getting more compact again.”
Previously, G2 was thought to be a cloud of hydrogen gas, which would be torn apart and sucked into Sgr A*, producing some supermassive black hole accretion fireworks. The fact that nothing happened was later called “cosmic crackling”.
Astronomers believe the answer lies in massive binary stars. Most of the time, these twin stars, trapped in each other’s orbit, just hang out doing their friendly star thing. But sometimes, like colliding binary black holes, they can merge with each other, forming a huge star.
When that happens, they produce a large cloud of dust and gas that surrounds the new star for about a million years after the collision.
“Something must have been preserved [G2] compact and allowed it to survive its encounter with the black hole,” Ciurlo added. “This is evidence of a stellar object within G2.”
And the other five? Well, they could also be binary star mergers. Most of the stars in the galactic center are very massive and most are binary. And the extreme gravitational forces acting around Sgr A* could be enough to destabilize their binary orbits with relative frequency.
“Star mergers may occur in the Universe more frequently than we thought, and they are probably quite common.” ghez said.
Black holes could cause binary stars to merge. It is possible that many of the stars that we have observed and do not understand are the end product of mergers that are now quiet. We are learning how galaxies and black holes evolve. stars interact with each other and with the black hole is very different from how individual stars interact with other individual stars and with the black hole.”
It seems that the G objects have a lot in common, whatever they are, and expanding the data set can only provide more information to solve the puzzle. However, there is still much to discover. As some mysterious fireworks seen going off from Sgr A*, which was also sighted.