Why Do Stars in Binary Systems Assume a Teardrop Shape?

  • #1
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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  • #2
Joe Prendergast said:
it is said
By whom? Where?
 
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  • #3
In almost any astrophysics textbook, for example, Modern Astrophysics by Carrol and Ostlie
 
  • #4
<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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  • #5
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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  • #6
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.
 
  • #7
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.
 
  • #8
Vanadium 50 said:
By whom? Where?
Most depictions in pop media of such binary stars employ teardrop-shapes.

1705596307535.png

1705596341269.png
 
  • #9
"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.
 
  • #10
Well, this is what Wiki has under Roche Lobe:
1705627823346.png
 
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  • #11
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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1. Why do stars in binary systems sometimes assume a teardrop shape?

Stars in binary systems can assume a teardrop shape due to the intense gravitational pull exerted by a close companion star. This gravitational force can distort the shape of one or both stars, especially when they are in close proximity. The side of a star facing its companion experiences stronger gravitational pull, causing it to stretch towards the other star, resulting in a teardrop-like shape.

2. How does the distance between stars in a binary system affect their shape?

The distance between stars in a binary system plays a crucial role in determining their shape. In closely orbiting binary systems, the gravitational forces are strong enough to cause significant tidal distortion, leading to the teardrop shape. Conversely, in binary systems where stars are farther apart, each star is more likely to maintain a spherical shape due to the weaker gravitational interaction.

3. Are all stars in binary systems affected equally by this phenomenon?

No, not all stars in binary systems are affected equally by this phenomenon. Factors such as the mass of the stars, their size, and the distance between them influence the extent of the gravitational interaction and the resulting shape distortion. Generally, less massive stars are more susceptible to deformation than their more massive companions.

4. Does the teardrop shape of stars in binary systems affect their evolution?

Yes, the teardrop shape of stars in binary systems can impact their evolutionary processes. The distortion can lead to increased tidal heating and changes in the stars' rotation rates. Additionally, mass transfer can occur if one star's material is pulled towards its companion, potentially leading to changes in mass, composition, and subsequent stages of stellar evolution.

5. Can we observe the teardrop shape of stars in binary systems from Earth?

Directly observing the teardrop shape of stars in binary systems from Earth can be challenging due to the vast distances involved and the small size of the stars relative to these distances. However, astronomers can infer the presence of such shapes through indirect methods such as studying the light curves, spectral lines, and timing variations in eclipsing binaries, or by using high-resolution imaging techniques like interferometry when conditions allow.

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