Binary Stars
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Basic Information

Binary Stars are groupings of two stars that orbit each other.1 Depending on the arrangement, they may orbit each other as quickly as every few hours, or as slowly as hundreds of years.

Binaries are a form of multiple star system consisting of exactly 2 stars. Modern naming convention typically gives both stars the same name, but adds an "A" or "B" at the end to differentiate them. So, nearby Alpha Centauri is made up of Alpha Centauri A and Alpha Centauri B, and they are sometimes collectively called Alpha Centauri AB. Some binary systems were originally trinary systems. Three-star systems tend to be unstable and may eventually eject one of the three stars as a runaway star.

If the two stars orbit particularly close to each other, or if one of them is a compact high-density object (such as a white dwarf, neutron star or black hole), matter and mass may be exchanged between them, causing the larger of the two stars to slowly grow. This mass transfer affects stellar evolution and may cause the creation of an accretion disk around the larger star. Eventually, the growth may destabilize the system, and cause a nova or cause the stars to drift apart (with one or both stars becoming runaway stars).

By contrast, so-called optical binaries are pairs of stars that are light years apart and do not orbit each other, but merely look close together from the specific vantage point of our observatories here on earth.

The Goldilocks Zone of a binary system is further out from the stars than it would be in a single-star system. So habitable worlds are likely to have a longer year and longer seasons than we have here on earth. This also means that life may be possible on worlds orbiting two stars or star-like objects (brown dwarf, etc) that are not individually hot or energetic enough to sustain life. Low-mass stars of this sort are far more common than stars that have features similar to our sun, so if it turns out that life commonly evolves near such stellar pairs, that would suggest the galaxy may be more heavily populated than we'd otherwise expect. (This would have an effect on the Drake Equation.) The best binary systems for life are ones where the two stars are close together and orbit each other every 10 days or less. If the binaries are far apart, the planets orbiting them will have unstable orbits that are perturbed by the stars individually. These variations may make it harder for life to adapt and flourish. The "sweet spot" seems to be for two stars a little smaller than our own, orbiting close together.


2. space dot com - talks about which binaries are good for supporting life

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