A triple-star system has been captured in the middle of its birth, giving astronomers one of the first direct glimpses into how multiple-star systems come to be. The image of the star trio, taken by the ALMA telescope in Chile, shows two young stars forming in a spiral around an older star in the system’s center. And this positioning confirms a major theory that researchers have had about how multiple stars wind up next to each other.

One of the first direct glimpses into how multiple-star systems come to be

That theory says that extra stars can pop up within another star’s protoplanetary disk — the dense saucer of rotating gas that surrounds a newly formed sun. These disks are the byproducts of the large clouds of dust and gas that form stars. Sometimes a massive cloud will collapse in on itself and pull in more materials, causing the core of the cloud to grow so dense and so hot that it begins the nuclear reaction that births a star. As this happens, the cloud starts rotating around the star, forming the disk.

Many researchers believe that the spinning disk can start pulling in even more materials, growing so gravitationally unstable that parts of the disk collapse again into new stars. And the new image of the three-star system, called L1448 IRS3B, is the first direct evidence proving that this process can indeed happen. The system, detailed today in the journal Nature, definitively shows the two younger stars forming within the spirals of the older star’s disk. The study team also looked at how the mass was distributed throughout the disk, finding that the gas cloud was gravitationally unstable.

"This is the first smoking gun that indicates that process can happen," says Stella Offner, an assistant professor of astronomy at the University of Massachusetts, who was not involved in the study.

Researchers have been extra curious about how binary and multiple stars form together, since these types of systems are abundant throughout the galaxy. About half of all the stars like our Sun are in binary systems — two stars that rotate around another star. "If you pick a random star in the sky, there’s a 50 percent chance it’s binary," study author John Tobin, an astrophysics professor at the University of Oklahoma, tells The Verge. "They don’t tend to form alone."

It’s thought there are two main ways that multiple stars end up together. One is the protoplanetary disk idea. The other is that multiple stars can emerge within the giant gas cloud that is forming a new sun. As the new star begins to form, the gas cloud becomes super turbulent, creating multiple centers of gravity that can collapse in on each other. This type of process tends to create systems in which the stars are super spread out. Stars forming within a protoplanetary disc are much closer together — on the scale of the planets in our Solar System.

An image of the proto-triple-star system L1448 IRS3B from the VLA telescope. (John Tobin from data obtained with the VLA telescope)

Both processes are equally valid ways to create multi-star systems, Tobin says. "It doesn’t necessarily have to be one or the other," he says. "Both can be happening at the same time. This route just may be forming the ones that are closer together."

The new triple system also tells us a lot about our own Solar System, too. Since we only have one Sun, it’s likely that the star’s protoplanetary disc wasn’t massive enough to form an additional star. Tobin and his team hope to study more newborn star systems to better understand why our Solar System only has one lonely star. "That can then tell us what conditions are favorable for a single source star," says Tobin.

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