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Inner Lagrangian Point and Roche Lobes, from Accreting Binaries, University of Tennessee |
7. EVOLUTION OF BINARY STARS
More than half of the stars in the sky belong to binary systems, and most of these are close binaries, in which the two stars are separated by only a few stellar radii and have orbital periods of days. We have seen how single stars evolve. Now we need to discuss the evolution of binary stars. As you will see, the possibility -- indeed, the certainty -- that these stars will interact with each other when they approach the end of their lifetimes opens a rich variety of new possibilities for the final fates of these stars and for the phenomena that we can observe.
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Inner Lagrangian Point and Roche Lobes, from Accreting Binaries, University of Tennessee |
Mass Transfer in Binary Systems: In all orbiting systems, there is a location called the Inner Lagrange Point (also called the L1 point), which is the place where the gravitational pull of the two objects is exactly equal, as illustrated above. For the Earth-Sun system, for example, the L1 point is located at a point about 0.01 AU (4 times the distance of the Moon) away from Earth toward the Sun. The SOHO satellites are located there. The L1 point is on a mathematical surface called the Roche Lobe. As a star swells up due to its evolution, its outer atmosphere will finally reach this Roche Lobe. Then, gas will start to spill across the L1 point onto the other star. An example is shown below
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A supercomputer simulation of mass transfer in a binary star system. From Interacting Binary Stars, by John Blondin, North Carolina State University |
When both stars in a binary system are smaller than their respective Roche lobes, no mass transfer will occur. Such systems are called detached binary systems. But when one star fills its Roche Lobe and begins to pour matter onto the other star, the system is called a semi-detached binary system. Finally, it is possible that the second star will also swell up so that it also fills its Roche Lobe. In that case, the stars merge into a common-envelope binary system, which has a single dumbell-shaped outer surface and two cores. In such systems, the stars may eventually merge to form a single star. But in some cases the system may expel its common envelope and turn back into a detached or semi-detached binary.
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Illustration of the Algol binary system. From Interacting Binary Stars, by John Blondin, North Carolina State University. Note that the red star is larger, but the blue star is more massive (it moves less). |
The "Algol Paradox": Algol (Beta Persei) was known by the ancient Arabs as the "Demon Star" because it was seen to dim periodically. We now understand that it is an eclipsing binary system. The red star is bigger (has a larger radius) than the blue star. Evidently, it is nearing the end of its lifetime and becoming a red giant. But the red star is less massive (0.8 MSun) than the blue star (3.7 MSun), which appears to be a main sequence star.
Here's the "paradox": we believe that both stars in a binary system are formed at the same time. We also believe that more massive stars evolve faster than less massive stars. So, how can it be that the lighter red star is more evolved than the heavy blue star?
The answer is that the red star in Algol was originally more massive, so it evolved sooner than the blue star. But when it ran out of hydrogen in its core and began to swell up, it passed most of its mass to the blue star. The blue star became more massive and the red star became less massive.
This process is illustrated in John Blondin's Algol page. Be sure to hit the answer. That shows a supercomputer simulation of how the gas passes from the evolved star to the less evolved star. In particular, the simulation shows how an accretion disk is formed due to the rotational motion of the gas as it is transferred in a binary system.
Another example of a mass transfer binary Phi Persei. Be sure to check the jpeg links there, which illustrate the evolution of this system.
The mass transfer that occurs in binary systems of various types leads to a variety of wonderful phenomena, as we shall describe in Lesson 7.
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Last modified February 20, 2002
Copyright by Richard McCray
