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I'm currently building a habitable tatooine-like binary system with the following stars:

  • Mass: 1.09 / 0.85 M⊙
  • Luminosity: 1.41 / 0.52 L⊙
  • Radius: 1.07 / 0.88 R⊙
  • Temperature: 6,087 / 5,230 K
  • Spectral Type: G0V / K0V
  • Age: 6 ± 1 Gyr
  • Average separation: 0.20 AU
  • Eccentricity: 0.5
  • Periapsis / Apoapsis: 0.10 / 0.30 AU (15 - 45 Million km)
  • Unstable Zone: ≤ 0.90 AU (135 Million km)
  • Habitable Zone: 1.32 - 1.96 AU (198 - 294 Million km)
  • Frost Line: 6.74 AU (1 Billion km)

And with the following orbits:

  • 0.97 AU: Venus-style steamball planet
  • 1.36 AU: Earth-style habitable planet
  • 1.85 AU: Earth-style habitable planet
  • 2.49 AU: Europa/Ganymede-style ice planet
  • 3.32 - 4.48 AU: Asteroid belt
  • 7.33 AU: Jupiter-style gas giant
  • 12.67 AU: Jupiter-style gas giant
  • 20.65 AU: Neptune-style ice giant
  • 33.33 AU: Neptune-style ice giant
  • 53.33 AU: Neptune style ice giant

To the question: What exactly happens to the secondary when the primary becomes a red giant in about 1 or 2 Billion years from now? I know that the primary might engulf the secondary and there could be some sort of mass transfer to the secondary, but could this delay the red giant phase or prevent the primary from becoming super huge (I'm thinking 1 to 1.3 AU in radius)?

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  • $\begingroup$ Forgive me if I'm wrong, but wasn't this same question asked a few days ago? $\endgroup$
    – Joachim
    Sep 24, 2023 at 10:07

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To the question: What exactly happens to the secondary when the primary becomes a red giant in about 1 or 2 Billion years from now?

Exactly? That'll require some expensive numerical simulation, but in broad strokes: the larger star will expand until it exceeds its Roche lobe, which will transfer mass to the smaller star until it equilibrates as either a modified non-contact binary with somewhat more equal masses, or as an over-contact binary--essentially, a single peanut-shaped star--depending on how much drag there is in the process and exactlyhow much the smaller star grows.

but could this delay the red giant phase or prevent the primary from becoming super huge (I'm thinking 1 to 1.3 AU in radius)?

Yes, it would, at least for a while. The mass transfer will both directly reduce the size of the larger star and reduce its luminosity and slow its evolution by reducing pressure in the core. Meanwhile, it will accelerate the evolution of the initially-smaller star, and if the first round of expansion does not result in an over-contact binary, the second star can eventually expand and transfer mass back, in a cycle that repeats until each star has evolved enough to expand into an over-contact binary.

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  • $\begingroup$ And once the two stars have evolved into an overcontact binary, they slowly merge into a single star within a few million years and that single, larger star then evolves into a red giant that swallows the innermost planets and burns the entire system to a crisp, right? $\endgroup$ Sep 25, 2023 at 13:58
  • $\begingroup$ @SussusAmogus Essentially, yes, but it's more a matter of merging as the two lobes expand, rather than merging and then expanding. $\endgroup$ Sep 25, 2023 at 14:24

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