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Troubling binary stars
are tipped over

Posted: September 18, 2009

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Tipped over stars are to blame for a peculiar double star system in which the stars are revolving around one another four times slower than they should, according to new research published in the 17 September issue of Nature.

The puzzling binary star system in question is DI Herculis, 2,000 light years away, and it has been confounding astronomers for over thirty years. To finally solve the riddle, 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. By examining the starlight carefully, they discovered that the two stars of DI Herculis are not rotating in the same plane – one of them is tipped over by 70 degrees, and the other tipped in the other direction by 80 degrees, relative to one another. As most stars rotate quickly, it causes them to bulge at their equators. The bulges on these two tipped over stars creates unusual tidal interactions between the pair that affects the rate at which they precess around one another.

An illustration (not to scale) of the orbits of the two stars of DI Herculis, and how their rotational axes are tipped over relative to one another. Image: Simon Albrecht.

This is highly unusual, and there is the possibility that it is a fluke, or maybe it is telling us that there is more to learn about binary star systems in general.

“Maybe most binary stars are formed misaligned,” muses Professor Joshua Winn, of MIT. Over time, their gravitational interactions would then straighten them up. If this is the case, this is very surprising because binary stars that are as close as these two are (30 million kilometres apart, or 0.2 astronomical units) should have formed from the same protostellar cloud of gas and dust. This cloud will have been rotating in one direction only, and the stars that formed from it should be rotating in the same direction. Indeed, all other binary star systems that we see appear to be aligned upright with one another.

In that case, perhaps the two stars of DI Herculis did not form from the same gas cloud; perhaps one was captured by the gravity of the other after a close passage. Winn’s co-researcher, Simon Albrecht, thinks this is unlikely. “These stars are so similar in mass and lifetime that we wouldn’t expect that,” he says.

Parallels can be drawn to exoplanets that have been discovered orbiting around their stars backward, or on steeply inclined orbits (see a related story here). In these cases, it is thought the planets had a close encounter with another planet that disrupted their orbit. Perhaps in the case of DI Herculis, there was another star that passed close by, upsetting the twin stars and knocking them on their sides. Of course, in our own Solar System Uranus is knocked on its side, presumably from a giant collision with another body that it suffered early in its history. Even Earth has a tilt of 23.4 degrees, which brings us our seasons.

At one stage prior to these findings it looked as though DI Herculis’ strange behaviour even flouted the Theory of General Relativity, so scientists are relieved to find that relativity is still intact. However, it has only thrown up a host of other mysteries; solve one puzzle, and it only opens the door to many more.

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