Why Do Stars in Binary Systems Assume a Teardrop Shape?

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Discussion Overview

The discussion revolves around the shape of stars in binary systems, specifically addressing the claim that one star can take on a teardrop shape due to the gravitational influence of its companion. Participants explore the implications of this shape, comparing it to tidal effects seen on Earth and questioning the accuracy of representations in popular media versus scientific literature.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant states that binary stars can assume a teardrop shape due to gravitational pull, but questions the contradiction with tidal bulges on Earth.
  • Another participant challenges the initial claim by asking for sources and references supporting the assertion about teardrop shapes.
  • A reference to "Modern Astrophysics" by Carrol and Ostlie is provided as a source for the teardrop shape claim.
  • Some participants argue that most binary systems do not exhibit teardrop shapes and criticize the reliance on popular media representations.
  • Discussion includes the concept of the Roche lobe and its relevance to the shape of stars in binary systems, with one participant suggesting that the revolution of the system must be considered.
  • Participants express skepticism about the accuracy of artistic depictions of binary stars and emphasize the need for credible references in discussions.
  • One participant elaborates on the conditions under which a binary star may appear teardrop-shaped, particularly when one star fills its Roche lobe and transfers mass to the other star.
  • References to astronomical phenomena related to mass transfer in binary systems, such as Algol systems and X-ray binaries, are mentioned.
  • Another participant discusses the nature of stars and their photospheres, suggesting that close binary stars would likely share a common envelope.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the existence or prevalence of teardrop shapes in binary stars, with multiple competing views presented regarding the accuracy of representations and the underlying astrophysical principles.

Contextual Notes

Some claims rely on specific definitions and assumptions about binary star systems and Roche lobes, which may not be universally accepted or understood among participants.

Joe Prendergast
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In binary star systems, it is said that one of the stars can assume a teardrop shape resulting from the gravitational pull of the companion star. This shape is typically portrayed as asymmetric (i.e., there is no teardrop shape on side of the star away from the companion star). Yet the gravitational force of the moon on the earth causes tidal bulges on both the sides of the earth: the side of the earth facing the moon as well as the side of the earth facing away from the moon. Does anyone know the reason for this apparent contradiction?
 
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Joe Prendergast said:
it is said
By whom? Where?
 
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In almost any astrophysics textbook, for example, Modern Astrophysics by Carrol and Ostlie
 
<sigh>

More teeth pulling. Most binaries are not teardrop shaped. So what exactly are you talking about? "It's in lots of books" is not helpful.
 
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Probably thinking about the Roche lobe. Wiki has an article on this. Key point being that there's the revolution of the system to take into account when drawing the equipotential (Earth-Moon tidal deformations can neglect revolution).
 
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Vanadium 50 said:
<sigh>

More teeth pulling. Most binaries are not teardrop shaped. So what exactly are you talking about? "It's in lots of books" is not helpful.
See the answer below. This is what I was looking for.
 
Bandersnatch said:
Probably thinking about the Roche lobe. Wiki has an article on this. Key point being that there's the revolution of the system to take into account when drawing the equipotential (Earth-Moon tidal deformations can neglect revolution).
Thank you, just what I was looking for.
 
Vanadium 50 said:
By whom? Where?
Most depictions in pop media of such binary stars employ teardrop-shapes.

1705596307535.png

1705596341269.png
 
"Pop media" is unreliable. I don't know what those artists' conceptions are supposed to be of, but I doubt very much that there are many contact binaries that look anything at all like that. The top one is less unrealistic than the bottom.
 
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Well, this is what Wiki has under Roche Lobe:
1705627823346.png
 
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Vanadium 50 said:
"Pop media" is unreliable. I don't know what those artists' conceptions are supposed to be of, but I doubt very much that there are many contact binaries that look anything at all like that. The top one is less unrealistic than the bottom.
Educate yourself with a college level textbook. Start with Roche lobes.
 
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  • #12
Joe Prendergast said:
Educate yourself with a college level textbook. Start with Roche lobes.
That is not acceptable. On PF you are required to provide actual references. Do you have any or not?
 
  • #13
russ_watters said:
That is not acceptable. On PF you are required to provide actual references. Do you have any or not?
I did provide a reference above, but I will repeat it for you: Modern Astrophysics by Carrol and Ostlie. Additionally, Teardrop stars (associated with Roche Lobes) are covered on page 662.
 
  • #14
Vanadium 50 said:
I doubt very much that there are many contact binaries that look anything at all like that.
Do you have any references? What you "doubt" is not helpful.
Props to Bandersnatch for answering the question.

A binary will look like that when one star is large enough to fill its Roche lobe, and starts transferring mass to the other star.
"Mass transfer due to Roche-lobe overflow is responsible for a number of astronomical phenomena, including Algol systems, recurring novae (binary stars consisting of a red giant and a white dwarf that are sufficiently close that material from the red giant dribbles down onto the white dwarf), X-ray binaries and millisecond pulsars."
https://en.wikipedia.org/wiki/Roche_lobe
 
  • #15
Since you like Wkipedia, you can see a more accurate picture at https://en.wikipedia.org/wiki/W_Ursae_Majoris_variable

Neither picture Dave posted share an envelope and are not even the same color. Actual stars that close to each other surely would. Remember, a star doesn't really have a surface - it has a photosphere, which is a tenuous region where the cattering finally becomes low enough to allow the light to escape so we can see it.

These stars tend to have a common envelope, lots of mass transfer, and it is not completely wrong to describe them as one outer envelope heated by two cores.
 
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