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Binary and Variable Stars


For a casual observer, binary star systems can often be confused with variable stars, and vice versa. The reason for this is that they both seem to vary their output of light. However, these two phenomenon are in no way related, for binary stars are two stars that are locked in each others' gravitational pull and are destined to go through the universe spinning around each other. A variable star, on the other hand, is a star that, for one reason of another, changes its luminosity in either a predictable pattern or at random.

Binary Star Systems

Estimates are that at least 60% of stars are members of a binary (or more) system. These common systems can form in two different ways. Most frequently they are born the same way all other stars do - in a nebula. The only difference is where they form: In the parent nebula, the two constituent stars of the future binary system form closely to one another. After they are "born" (initiation of hydrogen fusion), their solar winds blow away all of the surrounding nebular material, and because of their proximity, the two stars cannot escape each other's gravitational pull. Therefore, they are locked together. (Please note that this is also applicable to more than two stars; our closest neighbor, Proxima Centauri, is part of a triple star system, examples of four star systems abound, as are five, six, and more.)Binary System Schematic

The other way for a binary system to form is if a free star happens to fall into another star's gravitational pull, and become locked in orbit because it does not have enough velocity to escape. (This is the same way some planets get their moons: An asteroid is captured by a planet's gravitational pull.)

Historical Note: An older theory of binary formation proposed that the two stars were once one star, and they later split into two.

A binary star system works very simply. In the example (right), the system consists of a high-mass neutron star and a blue supergiant (not to scale). Because the neutron star has a higher mass than its companion, the center of gravity of the system will lie closer to it than the blue supergiant. Its orbit around the center of gravity will be smaller than the other star's. The time of orbit can last anywhere from less than half an hour (the record is five minutes in a white dwarf binary system) to millions of years, depending on how close the stars get.

The reason that binary systems are sometimes mistaken for variable stars is because some eclipse periodically, as seen from Earth. This means that at certain points in their orbits, one of the stars will pass in front of the other one, and thus block out part of its light.

Sometimes, if one of the stars is a neutron star or black hole, it can pull material from its companion star. In the case of a neutron star, the material will build up on the surface, and then it will periodically erupt in a massive explosion. This is called a cataclysmic variable (see below).

Variable Stars

A variable star gets its name because it can change in size, which we on Earth observe as a change in brightness. When the star is as its smallest, it is brightest, and when it is at its largest, it is at its dimmest.

The reason for this is somewhat counter-intuitive. Throughout the cycle, the star produces the same amount of energy. When it is large, that energy has to fill a greater volume; therefore the energy is more spread out within the star, so it is dimmer. The converse is then also true.

There are many different types of variable stars. Mira Variables are named after the most famous Mira variable, Mira, in the constellation the Cetus, the Whale. They are old, red giant stars. Over time, they have grown unstable and pulsate. Their pulsation cycle can last for anywhere between a few hundred days to several years, and can vary between eight different magnitudes - a measure of brightness. Since they are so unstable, their cycles are not necessarily constant from one to the next.

The Cephied Variables are named after the first one to be discovered: Delta Cephied. Cephieds have cycles that last a few days, and they vary by as much as two magnitudes. Unlike Mira variables, the Cephied variable stars have extremely regular, and their cycles are directly linked to their absolute magnitude - the true measure of a star's brightness. This makes Cephied variables one of the most important types of stars, for they can act as a "cosmic yardstick." By measuring the star's pulsation period, astronomers are able to judge its absolute magnitude. Then, by measuring its apparent magnitude, astronomers can tell how far away the star is. Henrietta Leavitt was the first to prove this in 1900, by showing that out of any two Cephieds with the same period of variation, the one with the brighter average magnitude is closer to us.

RR Lyrae stars are a sub-classification of Cephied variables. They have a much shorter oscillation period - usually less than a day - and are all about 90 times brighter than the Sun.

Stellar AccretionCataclysmic variables always have a binary companion, and do not pulsate via the same mechanism as the other variable stars so-far discussed. They draw matter from their companion star, as shown in the diagram above. In this diagram, a very massive neutron star is drawing material from its binary blue supergiant *. After they draw a sufficient amount of material, they erupt in an explosion that can raise their luminosity by as much as 10 magnitudes. However, the stars are not destroyed, and can repeat their cycles again and again, each cycle lasting what is believed to be hundreds of thousands of years.

Variable stars are not variable for their entire lifetimes. Mira variables, for example, become so only when they are old and unstable. Another example is Polaris, the North Star; its fluctuations seem to have been dying off recently, indicating that it is coming out of its unstable phase.

*Diagram is not to scale.

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