Binary Stars (Part I)

And now, we bring you a topic that hopefully will have you seeing double — in terms of stars, that is. Some astronomers say that a majority of stars are in binary or multiple star systems, but this is rather controversial, so suffice it to say that a significant number of stars are binaries or multiples. They are so important to astronomy that we can’t cover everything in one post, so we’ll split our discussion of binaries into a trilogy, with Part I (this post) covering general properties and evolution, Part II the types, and Part III the related math.

Before we really get into binaries, we should make it clear that a binary is not a “double star”, or optical double. An optical double is simply a pair of stars that, by chance, appear in nearly the same position in the sky but do not interact with each other in any way — perhaps the most famous example is Mizar and Alcor in Ursa Major. A true binary system has its stars gravitationally bound to each other, with both orbiting around the center of mass (also called the barycenter).

A schematic of a very basic binary star system (Credit: University of Oregon)

The formation of binary systems is still shrouded in mystery, with many competing theories all seeking to explain this stellar phenomenon. The old explanation for binary formation was that a rapidly rotating star could deform so much that it distorted into a “barbell” shape and eventually split into two stars that would then orbit around each other. However, this theory has been discredited in recent years due to simulations that show that stars tend to form accretion disks when spinning rapidly, rather than turning into barbells.

Binaries may also form from the fragmentation of molecular gas clouds as they collapse into protostars. However, the original cloud may not be able to immediately fragment into multiple clumps, so it may have to first collapse, then stop collapsing before it can divide into smaller chunks that then give birth to a gravitationally bound multiple star system. Alternatively, an accretion disk around a protostar may continue to…accrete…more mass from the molecular cloud around it. If this disk grows more massive than the star it orbits, it becomes unstable, and may clump together under its own gravity to form a second star and therefore produce a binary.

Stars that have formed separately may interact with each other to form a binary system, but this requires very high densities of stars, such as in globular clusters. Gravitational capture of an object requires a loss of energy from the system (referring to the two stars that will eventually become the binary), because of the principle of conservation of energy. In tidal capture, the excess energy goes into distorting the interior of the two stars as they pass each other at close quarters. However, this method of binary formation requires the two stars to interact at a very precise distance — too great a separation and the interaction won’t drain enough energy from the system to form a binary, but too small a separation and the two stars will just smash into each other to form a single, larger star. In three-body gravitational capture, excess kinetic energy is transferred to a hapless third star, which is then flung away at high speed while the other two stars become a binary system.

Castor sextuple star system, made up of three pairs of binary stars… because if you’re going to do it, you might as well overdo it. (Credit: Jodrell Bank Center for Astrophysics, University of Manchester)

The evolution of binary systems depends heavily on the degree to which the two stars in the system transfer mass. Each star has a Roche lobe, which is basically the space where a star has gravitational influence. If a star expands outside its Roche lobe, then material can flow to the companion star and lead to odd stellar evolution such as the Algol paradox, named for a binary system composed of a K0 subgiant and a B8 main sequence star. The theories of stellar evolution predict that the more massive B8 star should have evolved off the main sequence to the giant phase before the K0 star, but this is not the case — thus a paradox. However, astronomers have resolved the paradox by positing that the Algol system started out as a pair of main sequence stars, with one much more massive than the other. As the more massive star entered its red giant phase, it overfilled its Roche lobe and transferred away so much mass that it ended up as a subgiant while its companion became a massive blue main sequence star.

Gas flow simulations in the Algol system (Credit: M. Ratliff and M. Richards, PSU)

In a detached binary (wide binary), the two stars are both within their Roche lobes, so stellar evolution proceeds just as it would if the two stars evolved separately.

A semi-detached binary occurs when one star fills its Roche lobe and transfers mass to the other. Semi-detached binaries can produce interesting objects such as novae or x-ray binaries. Novae form from binary systems of a white dwarf and a main sequence or giant star, where mass streams onto the white dwarf and eventually ignites a nova outburst. An x-ray binary, on the other hand, forms from a system of two massive stars, where one has gone supernova — without disrupting the binary system — and left behind a neutron star or black hole. When the second star becomes a red giant, it streams mass onto an accretion disk surrounding the NS/BH, which emits strongly in x-rays. The x-ray radiation may even be powerful enough to vaporize the companion star that powered it in the first place.

Contact binaries are the strangest of the lot. The two stars share much of their mass (both are overfilling their Roche lobes) and orbit within a common envelope. The components may spiral in towards each other, due to loss of orbital energy to friction of orbiting within an atmosphere, and eventually merge into a single rapidly-rotating star. For more examples of what may happen to interacting (semi-detached and contact) binaries, check out the links below, especially this paper by P. Podsiadlowski of Oxford University.

Types of binary systems (Credit: David Darling)

Once both stars in a binary system have reached their end stages of evolution, end results vary wildly. One binary system made of two low-mass stars may end up as a pair of orbiting white  dwarfs (remember RX J0806.3, 2012 Astronomy folks?). Meanwhile, another binary system composed of a neutron star and a supergiant might turn into two runaway stellar remnants heading in opposite directions at high speeds, if the system is blown apart when the supergiant eventually goes supernova.

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TL; DR — Binary systems are pairs of stars that are gravitationally bound together. They may form due to fragmentation of molecular clouds or protostellar disks, or more rarely, from gravitational capture. Stars within binary system may transfer mass to each other if they expand outside their Roche lobes, and mass transfer leads to fascinating examples of stellar evolution in semi-detached and contact binary systems.  Even more than that the amount of mass in the system or for each component can also change the properties, leading to many variations of the system.

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Sources and links for further reading:

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