This week an international team of astronomers reports the first multiple-star system to be observed during the earliest stage of formation. This finding supports model predictions about how two- and three-star systems form by University of Massachusetts Amherst astrophysicist Stella Offner.

Writing in the current issue of Nature, first author Jaime Pineda of the Institute for Astronomy at ETH Zurich, with others in England and the United States, say understanding why and how multiple star systems form is essential for grasping phenomena such as star and planet formation, planet frequency and habitability.

They say the number of stars in a system is determined during the earliest stage of star formation but critical processes occurring then are usually hidden by dense clouds of dust and gas.

Offner says the new observations also help to explain why some pre-stellar gas condensations go on to form a system with only a single star like ours, while others form binary (two stars) or multi-star systems.

Up to half of all stars reside in systems with two or more stars, including the sun's nearest stellar neighbor, Alpha Centauri. However, astronomers do not know exactly what determines how many stars will form together or what initial conditions determine the type of star system to develop. Results they report this week advance understanding of these conditions in situations where multiple stars are very widely separated.

Offner, who performs simulations on stars' natal environments, had predicted that stars in many forming multi-star systems will be widely separated by a distance of several thousand times the distance between Earth and our sun. Conditions in this initial configuration in a "stellar nursery" are governed by gas velocities and gravity, she notes.

"It seems like a simple question," she says. "Why is our sun a single star while the nearest star to us, Alpha Centauri, happens to be a triple system? There are competing models for how multiple star systems are born, but now we know a little more than we did before."

In their new paper, Pineda and colleagues report discovering the star system in the act of forming within the "stellar nursery" region of the constellation Perseus by following up on intriguing observations made by the Very Large Array (VLA), an astronomical radio observatory in Socorro, N.M., and the Green Bank Telescope (GBT), the world's largest fully steerable radio telescope, in West Virginia.

The new, high-resolution observations show three gas condensations, which are fragments of a dense gas filament, and one very young star that is still gaining mass. They estimate the condensations will each form a star in about 40,000 years, a relatively short timescale in astronomical terms. The filament fragmentation "provides a new pathway to create stellar systems," Pineda and colleagues note.

"This is the first time that we have been able to study these young systems in formation, and it is thanks to the combination of both GBT and VLA that we can do it," Pineda adds.

Although the distances between the gas condensations are now several times the size of our solar system, the authors estimate the gravitational attraction between them is sufficiently strong that the new stars will comprise a quadruple star system. However, they say it is likely that interactions between the stars will cause one of the four to be expelled in less than a million years, leaving a triple system.

Unfortunately, the knowledge gained from witnessing this system as it is forming does not include signatures or characteristics to help astronomers locate more like it.

"We would like to know how common this configuration is," Offner notes. "Unfortunately, we couldn't predict what was there from the initial GBT survey, so we don't know what to look for in other places. It will take more survey work and more numerical modeling to be able to identify other very young systems like this one."

"In terms of what this means for the formation of our sun," she adds, "it suggests that its early conditions did not look like this forming system. Instead, the sun likely formed from something that was more spherical than filamentary. The distribution of the planets in our solar system also suggests that our sun was never part of a multiple system like this one."