via Astronomy Now.
For the past three decades, the binary star system DI Herculis, 2,000 light years from us, has presented a bit of a problem – the two stars don’t appear to be following the normal rules of gravitational attraction when in orbit of one another. They seem to lag a little, the orbit taking four times as long as it should for two objects of their mass in the orbit they’re in. Now however, a surprising observation has solved the problem.
Astronomers from the Massachusetts Institute of Technology (MIT) used a high-resolution spectrograph called Sophie on the 1.93-metre telescope at the Observatoire de haute-provence in France to carefully observe the light from the stars. What they found was the stars were tipped over relative to one another. One is seventy degrees off where it was assumed the rotational pole was, the other eighty degrees.
As stars are huge balls of fast rotating gas, they aren’t perfect spheres, they bulge a bit round their equators. The result of this is that the gravitational attraction of the star changes if you are in an orbit that doesn’t follow a line of longitude. The effect of that is tidal effects (the difference between near and far side gravitational attraction) change during orbits. This in turn leads to the lag in the orbits.
Now that’s all fine and well, mystery solved, but it leaves one minor problem – how did the stars end up tip over like this? They are separated by 30 million kilometres (a fifth of the Earth-Sun distance) and are of similar ages and masses, suggesting they came from the same gas cloud. One gas cloud normally means one plane of rotation and so everything pops out fairly well aligned, like other binary star systems. Does this mean that stars really start out tipped over and pull each other into alignment? Could the stars have been disrupted, perhaps by the close passage of another star? Exoplanets, and even Uranus in our own solar system, have displayed similar misalignments, often attributed to interactions with other planets. One possibility is that the stars started out as individual stars and simply came together, but the similarities in age and mass seem to make this rather improbable.