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Dutch astronomers believe they have found a super-massive black hole that's recoiling out of its galaxy at high speed.

Utrecht University student Marianne Heida discovered the bizarre star by comparing hundreds of thousands of X-ray sources, picked up by chance, with the positions of millions of galaxies.

Normally, each galaxy contains a super-massive black hole at its centre that sometimes lights up under X-rays. But this star was clearly not located in the centre of the system.

Recent calculations have indicated that a black hole created when two smaller black holes merge could be flung out of its galaxy at high speed. The calculations reveal that the speed of the hole depends mainly on the direction and speed with which the two black holes rotate around their axes before merging.

Heida says her research result is probably the tip of the iceberg.

"We have found even more of this strange class of X-ray sources," she says. "However, for these objects we first of all need accurate measurements from NASA's Chandra satellite to pinpoint them more precisely."

Finding more recoiling black holes will provide a better understanding of the characteristics of black holes before they merge. In future, it might even be possible to observe this process with the planned LISA satellite.

Astronomers hope to use this satellite to measure the gravity waves that the two merging black holes emit. Ultimately this information will reveal whether super-massive black holes in the cores of galaxies are the result of many lighter black holes merging.