Short-Period Binary Stars Tend to Merge Into Single Star


Short-period binary stars have a tendency to merge into a single star over time.

A computer simulation suggests that there are many single stars that were born as two separate suns, which eventually merged into a single star during the first million years of their existence.

The scientists published their findings in the journal Astronomy and Astrophysics. Double stars, like Sirius, are quite common. Studies of star-forming regions suggest that binary stars start off with a uniform distribution of orbital periods – the number of binaries younger than a few million years that orbit each other every 1–10 years, 10–100 years, 100–1,000 years, and so on, is about the same, – but older binaries show a different pattern. They have an average orbital period of roughly 200 years, and both shorter, as well as longer periods, are rarer.


This scarcity with orbital periods is easy to explain. In order to have a long orbital period, the stars themselves must be further apart. At such great distances, the stars don’t experience much of each other’s gravity, so the gravitational tug of a passing star can disrupt the binary system.

In order to explain where the shorter period binaries went, German astronomers simulated a star cluster resembling one in the Orion Nebula. The stars interact with the gas within the cluster and modify the stars’ orbits, causing them to spiral toward each other until they merge into a single star.

This result is plausible but hard to prove. The same gas and dust that causes the closer binaries to merge also prevents them from being visible from Earth. Astronomers must use IR radiation, which penetrates the dust, but often requires space-based observatories.

Reference: “Towards the field binary population: influence of orbital decay on close binaries” by C. Korntreff, T. Kaczmarek and S. Pfalzner, 10 July 2012, Astronomy and Astrophysics.
DOI: 10.1051/0004-6361/201118019

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