Why Do Stars in Binary Systems Assume a Teardrop Shape?

[Joe Prendergast]

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?

 

[Vanadium 50]

it is said

By whom? Where?

 

[Joe Prendergast]

In almost any astrophysics textbook, for example, Modern Astrophysics by Carrol and Ostlie

 

[Vanadium 50]

<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.

 

[Bandersnatch]

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).

 

[Joe Prendergast]

<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.

 

[Joe Prendergast]

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.

 

[DaveC426913]

By whom? Where?

Most depictions in pop media of such binary stars employ teardrop-shapes.

 

[Vanadium 50]

"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.

 

[DaveC426913]

Well, this is what Wiki has under Roche Lobe:

 

[Joe Prendergast]

"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.

 

[russ_watters]

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?

 

[Joe Prendergast]

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.

 

[James Demers]

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

 

[Vanadium 50]

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.