New Analysis Reveals All Stars are Born in Pairs

A new analysis indicates that the sun almost certain had a twin, though not an identical one, when it was born 4.5 billion years ago.

The analysis by a theoretical physicist from the University of California, Berkeley, and a radio astronomer from the Smithsonian Astrophysical Observatory at Harvard University provides new evidence that all stars are born in pairs.

The new assertion is based on a radio survey of a giant molecular cloud filled with recently formed stars in the constellation Perseus, and a mathematical model that can explain the Perseus observations only if all sunlike stars are born with a companion.

Many stars have companions, including our nearest neighbor, Alpha Centauri, a triplet system. Astronomers have long sought an explanation.

Astonomers have searched for a companion to our sun, a star dubbed Nemesis because it was supposed to have kicked an asteroid into Earth’s orbit that collided with our planet and exterminated the dinosaurs.

It has never been found.

“We are saying, yes, there probably was a Nemesis, a long time ago.” noted Steven Stahler, a University of California, Berkeley, research astronomer and co-author of a paper already accepted for publication in the Monthly Notices of the Royal Astronomical Society.

“We ran a series of statistical models to see if we could account for the relative populations of young single stars and binaries of all separations in the Perseus molecular cloud, and the only model that could reproduce the data was one in which all stars form initially as wide binaries. These systems then either shrink or break apart within a million years,” said Stahler.

In this study, “wide” means that the two stars are separated by more than 500 astronomical units, or AU, where one astronomical unit is the average distance between the sun and Earth, namely 93 million miles, or 150 million kilometers. A wide binary companion to our sun would have been 17 times farther from the sun than its most distant planet today, Neptune.

Based on this model, the sun’s sibling most likely escaped and mixed with all the other stars in our region of the Milky Way galaxy, never to be seen again.

“The idea that many stars form with a companion has been suggested before, but the question is: how many?” first author Sarah Sadavoy, a NASA Hubble fellow at the Smithsonian Astrophysical Observatory, was quoted as saying in a news release from UC Berkeley. “Based on our simple model, we say that nearly all stars form with a companion. The Perseus cloud is generally considered a typical low-mass star-forming region, but our model needs to be checked in other clouds.”

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 four 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 one 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.” (Xinhua)