Strange spinning binary star explains 30-year enigma

An international team led by Leiden astronomers has discovered why the two stars of binary star DI Herculis rotate so strangely around one another, which once even proved problematic for Einstein's theory of relativity. Their findings were published in Nature.

The two stars, that orbit each other every ten days, lie both on their side with their rotation axis almost 90 degrees tipped over to one side, which was previously thought to be impossible. This explains their strange orbits that have been observed for the past 30 years.


Incompatible with the theory of relativity

Binary star, DI Herculis, is at a distance of 2000 light years in the constellation of Hercules. As early as the 1980s, observations were made of the irregular orbits by these stars that seemed to be in conflict with Einstein's theory of relativity. This theory predicts that the orientation of stellar and planetary orbits changes over the course of time, an effect that was first explained by Albert Einstein in 1915 for Mercury's orbit, which at the time meant an enormous. However, the calculations did not seem to work for DI Herculis, which suggested that Einstein's theories may not apply under the extreme conditions of this binary star.

Flattened off

The researchers have now discovered the cause of this discrepancy, thus resolving a 30-year-old enigma. 'It is caused by the fact that both stars are so far tilted, that with their rotational axis they are at almost at a right angle with respect to the orbital plane,' says Simon Albrecht, former Leiden PhD researcher who was recently awarded a prestigious NWO Rubicon fellowship, and now works at the Massachusetts Institute of Technology (MIT).  'As a result of the rapid rotation of the stars, they are flattened off in a direction almost perpendicular to the orbit. This strange configuration has the effect of counteracting the effect of relativity,' Albrecht explains.  

New problem

‘But this does not mean that the old puzzle has been completely resolved,' says fellow researcher Ignas Snellen. ‘What causes these stars to spin so strangely? We have a reasonable insight into the origin of binary star systems, but we expect that the rotational axes of both stars and the orbital axis are both set in the same direction.' It may be that there is a third star that has not yet been discovered, that has disturbed the orbits of the two other stars.  New research will hopefully provide the answer.

(16 September 2009)

Last Modified: 29-09-2009