Stars reproduce

Close encounters with massive double stars produce frenzied loners

Star outliers

Kagoshima (Japan) / Leiden (Netherlands) - Around every fifth massive star in our Milky Way galaxy moves through space at an unusually high speed. These young "outlier stars" mainly come from dense star clusters. There they are accelerated by close encounters with a double star and catapulted out of the star cluster. This is now shown by computer simulations by a research duo from Japan and the Netherlands. The results of the simulations successfully reproduce the speed and mass distribution of the observed outliers, write the two astronomers in the journal "Science".

While most of the stars in the Milky Way move at comparatively low speeds of up to 30 kilometers per second, outliers can move up to 300 kilometers per second. So far there have been two explanatory models for the high speed: Either the outliers come from double star systems in which one of the stars has exploded and the force of the explosion has accelerated the remaining star. Or close encounters between single and double stars catapult one of the three celestial bodies onto a new, rapid path.

The computer simulations by Michiko Fujii from the University of Kagoshima in Japan and Simon Portegies Zwart from the Leiden Observatory in the Netherlands now show that the second model is apparently the right one. "The model replicates the main features of the outliers in the Milky Way and also provides an explanation for the massive stars around young star clusters," said the two scientists. In their computer simulations, a massive double star is always formed in the center of a young star cluster, which then ejects an average of 23 stars from the cluster at high speed - a number that, when added up over the entire Milky Way, provides a sufficient number of outliers.

Fujii and Zwart also specifically compare their simulation with the young cluster R136 in the Large Magellanic Cloud, a satellite galaxy of the Milky Way. Observations show six stars with more than eight times the mass of the suns flying away from R136 at high speed - the simulation provides an average of five such outliers. Additional outliers from star explosions are therefore not necessary - and would not even be possible with R136: Some of the stars were ejected from the cluster, which was only around two million years old, before a star explosion could even occur there.