A mysterious cloud that somehow survived a close encounter with a supermassive black hole has now been unmasked.
According to a new study of the object, called G2, it's actually three baby stars, shrouded in a thick cloud of the gas and dust from which they were born. This interpretation offers a very tidy solution to the questions that remained unanswered after G2 skimmed past Sgr A* – the supermassive black hole at the heart of the Milky Way – back in 2014.
"We propose that the monitored dust-enshrouded objects are remnants of a dissolved young stellar cluster whose formation was initiated in the circumnuclear disk," the researchers wrote in their paper.
G2 was discovered in 2011 (described in a study published in 2012). At that time, it was hurtling towards an event known as perinigricon – the point in its orbit in which it is closest to the black hole.
Astronomers fully expected that the close encounter would result in G2 getting torn apart and slurped up by by Sgr A*, producing some supermassive black hole accretion fireworks.
The fact that nothing happened was later referred to as a "cosmic fizzle". G2 stretched out and elongated as it drew close to the black hole; then, after perinigricon, it returned to a more compact shape.
Another vexing characteristic of G2 is that it's very hot, far hotter than a cloud of dust should be. It's possible that Sgr A*, or other stars, could have heated the object, but it remained the same temperature no matter where it was. This suggested that whatever was heating G2 was coming from within the cloud itself, not external influences.
Both these behaviors, astronomers found, are more consistent with the behavior of a star. A team of researchers last year suggested that the G2 cloud could harbor a hidden star within – the product of a collision between two stars that produced a huge cloud of gas and dust around them.
But the same study also revealed the discovery of four more similar objects in the galactic center, bringing the total number of G objects to six. That's a lot of merged binary stars.
Now, a team of researchers led by astrophysicist Florian Peißker from the University of Cologne in Germany has come up with an alternative explanation, after conducting a detailed review of 14 years' worth of observations taken with the Very Large Telescope's SINFONI instrument.
According to their analysis, G2 should be concealing three stars, at around 1 million years old. That's very young, for stars; by contrast, the Sun is 4.6 billion years old. The G2 stars are so young that they would still be surrounded by material from the cloud in which they formed.
"That G2 actually consists of three evolving young stars is sensational," Peißker says, noting that the discovery makes the three stars the youngest stars ever observed around SgrA*.
The stars could have originated in the same stellar nursery, forming a cluster, which has since dissolved, with individual stars breaking away and generating new orbits around Sgr A*.
Even if not associated with the S-cluster, the G2 stars were likely part of a larger cluster of stars at some point. Other dusty objects orbiting Sgr A* could also have been members of this cluster, which would have been disrupted by gravity after moving towards the supermassive black hole from a greater distance.
Because the environment around Sgr A* is not considered conducive to star formation, more work will be needed to discover where G2 and the other G objects might have originated. Astronomers may also be able to use the new findings to understand more about black holes.
"The new results provide unique insights into how black holes work," Peißker says.
"We can use the environment of SgrA* as a blueprint to learn more about the evolution and processes of other galaxies in completely different corners of our Universe."
The research has been published in The Astrophysical Journal.