Star Turbulence: What Happens when Two Stars Approach One Another

When we look at the night sky, we see stars as tiny points of light eking out a solitary existence at immense distances from Earth. However, more than half of these "single" stars are actually binary stars with a companion. Now, scientists have taken a closer look at what happens when two stars approach one other.

In this latest study, the researchers used new methods to model what occurs when two stars that are involved go through a phase in which they are surrounded by a common envelope consisting of hydrogen and helium. This phase is extremely short, which is why models are so important for studying it.

More than half of the stars we know of have evolved in binary star systems. The energy for their luminosity comes from the nuclear fusion of hydrogen at the core of the stars. As soon as the hydrogen fueling the nuclear fusion is exhausted in the heavier of the two stars, the star core shrinks. At the same time, a highly extended stellar envelope evolves, consisting of hydrogen and helium. The star, effectively, becomes a red giant.

While the envelope of the red giant expands, the companion star draws the envelope to itself via gravity, and part of the envelope flows towards it. During this time, the stars come closer to one another. Finally, the companion star may fall into the envelope of the red giant and both stars are then surrounded by a common envelope.

"Depending on what the system of the common envelope looks like initially, very different phenomena may ensue in the aftermath, such as thermonuclear supernovae," said Sebastian Ohlmann, one of the researchers, in a news release. Currently, researchers are looking a bit more closely at this stage.

The findings are published in the journal The Astrophysical Journal Letters.

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