A pair of researchers from the Harvard’s Smithsonian Astrophysical Observatory and the University of California, Berkeley, used radio telescope observations to study the distribution and orientation of stars in Perseus, a massive, very cold gas-filled cloud located about 600 light-years from Earth. Perseus is home to dozens of sun-like stars less than a million years old, the baby versions of our 4.5 billion-year-old yellow orb in the sky. Using statistical models, the researchers turned back the clock and investigated how the star population within the cloud unfolded over time. They found that in order to explain what they were seeing, they had to assume that the stars inside Perseus first formed and lived in binaries, before other forces may have started pushing them around.
So, yes, the sun may have had a twin, in the very beginning—but not for long.
To understand why, let’s look at Perseus. Perseus, like other similar clouds, is home to the birthplace of stars: egg-shaped cocoons of molecular gas, known as dense cores. The researchers observed several kinds of systems across Perseus. There are wide binaries, which contain two young stars orbiting each other, separated by more than 500 astronomical units, or AU. (One AU is the average distance between the sun and Earth, about 93 million miles). These young stars appear to be aligned with the long axis of a dense core, which suggests they may have formed together. There are tight binaries, containing slightly older stars orbiting less than 500 astronomical units apart and showing no alignment to dense cores. And there are single-star systems, like our sun.
The researchers, based on their simulations, determined that all the stars started off as wide binaries, aligned with their egg-shaped cocoons. About 60 percent of the pairs split up a few million years later, flying apart. The rest spiraled closer together, forming tight binaries.
If our sun had a twin, it would have orbited at least 17 times farther from the sun than Neptune. After a few million years together, it drifted away for good, into interstellar space, mingling with the other stars of the Milky Way.
“We don’t know exactly how we lost it,” said Steven Stahler, a theoretical physicist at the University of California, Berkeley, and one of the researchers. Stahler said friction between the young stars and surrounding gas can cause them to shrink their orbits, moving closer and closer together. Mergers are rare, since young stars emit winds powerful enough to drive off clouds of gas and reduce friction. But the reasons for breaking apart remain more of a mystery. One star may be ejected from its dense core, or the core itself could splinter, leaving two halves with two stars to float apart.
Younger stars are trickier to observe than their older counterparts, thanks to their dusty, dense cores, which block out their light, said Sarah Sadavoy, the lead author and an astronomer at the Smithsonian Astrophysical Observatory at Harvard. But clusters of young stars are the best places to investigate the mysteries of star formation, she said.“There’s been less time for things to get disrupted,” she said.