A deeply twisted magnetic field within a stellar nursery started astronomers on a hunt for the cause. They think they've found it in a star almost invisible from Earth because it sits just behind another from our field of view. The relationship between the two stars could not only account for the twisted field, but teach us something important about how stars come to exist in pairs.
The majority of stars in the galaxy travel in pairs (binaries), so understanding the process of their formation is essential to a full knowledge of stellar lifecycles. We've seen snapshots of the process where two stars form from the same patch of gas, just as a star and a planet might, but not everyone believes that is the whole story.
It's been theorized that stars sometimes form independently, or in very distant orbits around each other but subsequently join up. A paper in The Astrophysical Journal next week (preprint on Arxiv.org) describes what appears to be an example of this process in action, opening a window on how many pairs may form.
Although most of the material within a star-forming gas cloud ends up becoming stellar matter, some is shot outwards at high speed as a by-product of star creation. Magnetic fields form in parallel to these outflow jets. However, when Dr Erin Cox of Northwestern University took a close look at the Lynds 483 dark nebula, around 700 light-years away in Serpens, she found the field lines were twisted at 45 degrees compared to the outflows. Field line direction is identified by studying polarization of light from the source, which in turn is caused by dust that aligns itself along the field.
Cox suspected a second star might be present, distorting the direction of the field lines, and won time on the Atacama Large Millimeter/submillimeter Array (ALMA) to search for it. Cox and co-authors found a second star cocooned in the same stellar envelope as the first.
“These stars are still young and still forming,” Cox said in a statement. “The stellar envelope is what supplies the material to form the stars. It’s similar to rolling a snowball in snow to make it bigger and bigger. The young stars are ‘rolling’ in material to build up mass.”
The two stars are about as far from each other as the Sun and Neptune. Each could eventually develop its own family of planets, for whom the other star would occupy an intermediate place between Sun and ordinary star, rather than being like Tatooine's twin suns.
However, Cox doesn't think they started at this distance. “There is newer work that suggests it’s possible to have two stars form faraway from each other, and then one star moves in closer to form a binary,” Cox said. “We think that’s what is happening here.”
That would explain the twisted field, but it also raises a bigger question – what caused these stars to move towards each other in the first place? So far, that hasn't been answered.
“Learning how binary stars form is exciting because planet and star formation take place at the same time, and binary stars dynamically interact with each other,” Cox said. “In our census of exoplanets, we know planets exist around these double stars, but we don't know much about how these planets differ from the ones that live around isolated stars.” It's something she thinks a new generation of astronomical instruments may fix.