There’s something really strange about the center of the Milky Way.
The vicinity of a supermassive black hole is a pretty strange place to start, but in 2020 astronomers discovered six objects unlike anything in the galaxy orbiting Sagittarius A*. They are so unusual that astronomers have assigned them to a new class of objects called G objects.
The original two objects, G1 and G2, first caught the attention of astronomers nearly 20 years ago, and their orbits and bizarre properties gradually combined in the years that followed. They looked like gigantic clouds of gas, 100 astronomical units in length, stretching out the gas and dust emission spectrum as they got closer to the black hole.
However, G1 and G2 did not behave like gas clouds.
“This object looks like gas, but behaves like a star.” Andrea Ghez, a physicist and astronomer at the University of California, Los Angeles, explained in 2020.
Ghez and her colleagues have been studying the Galactic Center for over 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 on very different orbits than G1 and G2 (see figure above). Overall, G objects have orbital periods ranging from 170 to 1,600 years.
Exactly what that is is unclear, but Ghez believes the big clue is that G2 emerged intact from perigee in 2014, i.e. from its closest orbit to a black hole.
“When we got close, the G2 got really weird signals,” she said.
“I’ve seen it before, but it didn’t seem all that strange until it got close to the black hole, lengthened, and ripped away most of the gas. Moving away from the black hole, it came off a pretty innocuous object. The black hole really stretches and distorts when it gets closest, breaking its outer shell. I lost it and now it’s getting more compact again.”
Previously, it was thought that G2 was a cloud of hydrogen gas being torn apart and sucked in by Sgr A*, creating supermassive black hole accretion sparks. The fact that nothing happened was later referred to as a “cosmic fizzle”.
Astronomers believe the answer lies in giant binary stars. For the most part, these twin stars locked in mutual orbits spend their time acting as friend stars. But sometimes, like colliding binary black holes, they can collide with each other to form one large star.
When this happens, it creates a huge cloud of dust and gas that surrounds the new star for about a million years after the collision.
“Something must have been [G2] It’s compact and was able to survive an encounter with a black hole,” Ciurlo added. “This is proof that there is a star inside G2.”
So what about the other 5? Well, they might as well be binary star mergers. Most stars at the center of galaxies are very massive and most are binary. And the extreme gravitational forces acting around Sgr A* could be enough to destabilize binary orbits at relative frequencies.
“Stellar mergers can happen in space more often than we thought, and they’re probably very common,” Ghez said.
“Black holes may be driving binary stars to merge. It is possible that many stars we do not watch and understand are now the end products of serene mergers. We are learning how galaxies and black holes evolve. How binary stars interact with each other.” The way they interact and interact with black holes is very different from the way single stars interact with other single stars and black holes.”
The G objects seem to have a lot in common with whatever they are, and expanding the data set can only provide more information to solve the puzzle. However, there is still much to be learned. Discovered like a mysterious spark burning in Sgr A*.
Was it a delayed response at G2’s perihelion? Wasn’t the turmoil of the universe not so raucous after all? We may have to keep watching the corner of this strange little supermassive black hole to see what happens next.
This study nature.
An earlier version of this article was published in December 2020.
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