The Astrophysics Spectator
The Astrophysics Spectator
The compact binary star continues as the theme. Two new pages are added to the “Stars” topical path that describe in more detail the process of how a star can overflow onto a companion star and how x-ray binary stars evolve from a pair of fusion-powered stars to a system of a fusion-powered star and either a neutron star or a black hole candidate.
We see compact binary stars because gas flows from one star to the other star, releasing gravitational potential energy. If the gas falls onto a degenerate (white) dwarf, the gas emits visible and ultraviolet light along with some x-rays, causing the binary to appear as a cataclysmic variable. If the gas falls onto a neutron star or a black hole candidate, the gas emits x-rays and gamma-rays, causing the binary to appear as an x-ray binary. Why a star loses mass to a companion star has two answers: either the star is pushing a strong wind off of its photosphere, or the star's atmosphere extends close enough to the companion to be captured. The first mechanism is seen in massive x-ray binaries, and the second mechanism is seen in cataclysmic variables and low-mass x-ray binaries.
A star is physically altered from the state it would have as a solitary star when it overflows onto its companion. Its shape is changed so that it looks like a raindrop, and its structure is changed so that it is physically larger than it would otherwise be. The overflow also alters the binary orbit, changing over time the distance between the stars and period of the orbit. If the overflowing star is less massive than its companion, these changes cause a very steady and long-term flow of gas from one star to the other, but if the overflowing star is more massive, the flow turns into a flood, and the whole binary system is soon enveloped by the gas. Such a system is effectively a single star that contains two cores.
The transfer of gas through overflow plays a critical role in the evolution of x-ray binary stars. These compact binaries consist of a neutron star or black hole candidate in orbit with a fusion-powered star. The transforming event for these systems is the creation of the compact object, whether neutron star or black hole. In particular, when the core of a massive star collapses to a neutron star, the outer layer of the star, which constitutes the majority of the star's mass, is blown away, creating a supernova. This event disrupts a binary system if the mass lost is greater than half the total mass of the system.
The existence of the high-mass x-ray binaries, which contain a fusion-powered star of more than 10 solar masses, is easily explained under these circumstances—the companion star was larger than the supernova progenitor star—but how does one explain the existence of the low-mass x-ray binaries, systems that contain a fusion-powered star of less than 1 solar mass? With a companion so much smaller than the compact star, the amount of mass lost in the supernova must inevitably be much greater than the mass remaining in the binary system. The answer is found in the runaway that occurs when an overflowing star is more massive than its companion. Once a common envelope of gas completely enshrouds the binary star system, the orbit of the two stellar cores rapidly decays, bringing the cores close together. If one of the core then collapses to a neutron star, it can blow away the surrounding gas without disrupting the close orbit of the second core. The result would be a low-mass fusion-powered star orbiting in a nearly circular orbit a more massive neutron star.
This issue adds a commentary that continues my criticism of Professor Fish's definition of academic freedom. One conclusion I come to is that the study of astronomy is only justified if one is pursuing a broad liberal arts education rather than a narrow technical education.
Next Issue: The next issue of The Astrophysics Spectator is planned for release in three weeks on September 6.
Jim Brainerd
Astronomy and the Liberal Arts. Not every subject that a scholar finds fascinating is worth studying. Universities can approach the question of what to study from two standpoints: what does a professional need to know to perform his job, and what does a man need to know to live life. Astronomy has no commercial value; its study is justified by helping us answering some of our universal and timeless questions. In short, astronomy is part of a liberal arts education. (continue)
Overflow in Binary Stars. When a star within a binary system is close to its companion, the star is distorted in shape. As the orbit of a binary star decays, forcing the stars closer together, a point is reached when gas from the distorted star flows to its companion. This loss of gas by a star is called Roche lobe overflow. This process causes mass transfer in cataclysmic variables and in low-mass x-ray binary stars. The process is stable if the overflowing star is less massive than its companion, but the process causes a flood of gas onto its companion when the overflowing star is more massive. This last process enables a low-mass x-ray binary star to survive a supernova explosion. (continue)
The Evolution of X-Ray Binary Stars. Two starkly different binary systems constitute the x-ray binary class: high-mass x-ray binaries and low-mass x-ray binaries. In the first subclass, the star losing gas to its companion is much more massive than the companion, while in the second subclass, the star losing gas is less massive than its companion. These different classes reflect two entirely different histories. The high-mass binary is created when one of the two massive stars in the system collapses to a neutron star or black hole candidate and creates a supernova. The low-mass binary is most likely created when a compact binary system evolves through a common-envelope stage before the supernova occurs; the common envelope prevents the binary star from being disrupted by the supernova. (continue)
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