Are Orbits of Close Binary Stars Truly Circular?

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Discussion Overview

The discussion revolves around the nature of orbits in close binary star systems, specifically questioning whether these orbits are truly circular or if the circular assumption is primarily for mathematical convenience. It also explores the implications of mass transfer between stars and the behavior of Lagrangian points in these systems.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants suggest that the assumption of circular orbits in close binary stars is made for mathematical simplicity, as it allows for easier solutions to equations.
  • Others argue that tidal forces in close binaries tend to circularize the orbits over time, leading to a physical basis for the near-circularity of these orbits.
  • Questions are raised about the behavior of Lagrangian points during mass transfer events, with some noting that these points change as the mass ratio between the stars varies.
  • It is proposed that the roles of donor and receiving stars may reverse during mass transfer, affecting the dynamics of the system.
  • A participant inquires about the relationship between Lagrangian points and asteroids, questioning how these points would be affected by mass transfer and whether asteroids would move toward new Lagrangian points.

Areas of Agreement / Disagreement

Participants express differing views on the nature of orbits in close binary stars, with some supporting the idea of near-circular orbits due to tidal forces, while others emphasize the mathematical convenience of the circular assumption. The discussion on Lagrangian points and mass transfer also reveals multiple perspectives without clear consensus.

Contextual Notes

The discussion highlights the complexity of binary star dynamics, including the assumptions made regarding orbits and the effects of mass transfer, which may not be fully resolved within the conversation.

shirin
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in analysis of close binary stars, it is assumed that their orbits is circular and then by a rotating coordinate system, potential is obtained and then lagrangian points and so on...
Is the orbit of CLOSE binary stars nearly circular in reality or is it assumed circular because otherwise it is hard to solve ?
 
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Yes the equations are easier to solve in a circular orbit, as many things become time independent (separation between the two components, shape of the equipotential surfaces and so on), but I think a more physical reason is:
Close binaries interact through tidal forces, which circularize the obit. So after a bit of evolution and interaction the orbits will be very close to circular.
 
two more questions;
Does the location of lagrangian points differ when mass trasfer is happening?
For a binary star, is the mass transfer always from one star to another or may it change?
Thanks in advance
 
When mass transfer is happening the Lagrangian points indeed change (as the mass ratio between the object changes) and it will even change the orbits of the stars. The donor star and receiving star may reverse their roles. It usually is a big star that is the donor. Say you have a red giant and a normal sun-like star. The red giant can transfer mass to the other. When it evolves and becomes a white dwarf it may later receive mass when the sun-like star becomes a red giant.
 
I've heard something about locating asteroids in lagrangian points. Which binary system do these lagrangian points belong to? Earth and what?
There is no mass transfer between Earth and its companion. But it were so, what would happen to these asteroids? I mean, as these lagrangian points differ due to mass trasfer, do the astroid move toward the new lagrangian point or whatelse happans?
 

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