Astronomers have speculated about the origins of binary and mulitple star systems for hundreds of years, and in recent years have created computer simulations of collapsing masses of gas to understand how they condense under gravity into stars, and of the interaction of many young stars recently freed from their gas clouds. Several years ago, one such computer simulation by Pavel Kroupa of the University of Bonn led him to conclude that all stars are born as binaries.
Yet direct evidence has been scarce. As astronomers look at younger and younger stars, they find a greater proportion of binaries, but why is still a mystery.
According to Stahler, astronomers have known for several decades that stars are born inside egg-shaped cocoons called dense cores, which are sprinkled throughout immense clouds of cold, molecular hydrogen that are the nurseries for young stars.
The Perseus molecular cloud is one such stellar nursery, about 600 light-years from Earth and about 50 light-years long. Last year, a team of astronomers completed a survey that used the Very Large Array, a collection of radio dishes in New Mexico, to look at star formation inside the cloud. Called VANDAM, it was the first complete survey of all young stars in a molecular cloud, that is, stars less than about 4 million years old, including both single and mulitple stars down to separations of about 15 astronomical units.
The VANDAM survey produced a census of all Class 0 stars, those less than about 500,000 years old; and Class I stars, those between about 500,000 and 1 million years old. Both types of stars are so young that they are not yet burning hydrogen to produce energy.
Sadavoy, a member of the VANDAM team, took the results from VANDAM and combined them with additional observations that reveal the egg-shaped cocoons around the young stars. By combining these two data sets, Sadavoy was able to produce a robust census of the binary and single-star populations in Perseus, turning up 55 young stars in 24 multiple-star systems, all but five of them binary, and 45 single-star systems.
Using these data, Sadavoy and Stahler discovered that all of the widely separated binary systems, those with stars separated by more than 500 AU, were very young systems, containing two Class 0 stars. These systems also tended to be aligned with the long axis of the egg-shaped dense core. The slightly older Class I binary stars were closer together, many separated by about 200 AU, and showed no tendency to align along the egg's axis.
The two researchers mathematically modeled various scenarios to explain this distribution of stars, assuming typical formation, breakup and orbital shrinking times. They concluded that the only way to explain the observations is to assume that all stars of masses around that of the sun start off as wide Class 0 binaries in egg-shaped dense cores, after which some 60 percent split up over time. The rest shrink to form tight binaries.
"As the egg contracts, the densest part of the egg will be toward the middle, and that forms two concentrations of density along the middle axis," Stahler said. "These centers of higher density at some point collapse in on themselves because of their self-gravity to form Class 0 stars."
"Within our picture, single low-mass, sunlike stars are not primordial," he added. "They are the result of the breakup of binaries."
