Astronomers have finally figured out what a mysterious object at the heart of the galaxy really is: a pair of binary stars merged together orbiting a black hole.

Astronomers were previously puzzled by this object, known as G2, and once thought that it was a big cloud of hydrogen gas being sucked into the Milky Way's supermassive black hole. However, that explanation made no sense, because what we know about black holes tells us that a black hole would tear this gas cloud apart and that the state of the black hole itself would change.

"G2 survived and continued happily on its orbit; a simple gas cloud would not have done that," says Andrea Ghez, UCLA professor of physics and astronomy. "G2 was basically unaffected by the black hole. There were no fireworks."

So what exactly is G2?

Now we know, thanks to Ghez and a team of astronomers using Hawaii's W.M. Keck Observatory, which has the world's largest optical and infrared telescopes. G2 is actually a pair of binary stars pulled together into one star by the black hole.

Black holes are regions of spacetime with collapsed matter that have such strong gravitational pulls that nothing, not even light, can escape from them. When they pull on nearby stars, the effects are evident to telescopes like those at Keck.

The UCLA team praised the technology of the Keck telescopes, stating that they wouldn't have made this discovery without it. Adaptive optics allows the telescopes to correct those distortions caused by Earth's atmosphere better than older technology. This makes details of the black hole at the center of the galaxy more visible.

G2 is one of many stars that astronomers now believe were once binary stars, especially considering that in the Milky Way, most stars come in pairs. This suggests that there are other stars in our galaxy that were also the same: two stars instead of one.

G2 is currently in what astronomers call its "inflation" stage. This happens after two stars merge and the new combined star expands, a process that takes about 1 million years. It is also being spaghettified, a process that happens when a black hole pulls on an object and makes it longer.

"We are seeing phenomena about black holes that you can't watch anywhere else in the universe," says Ghez. "We are starting to understand the physics of black holes in a way that has never been possible before."

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