The overall orderliness of the main sequence suggests that the properties of stars are not random. In fact, a star’s exact position on the main sequence and its evolution are functions of only two properties: composition and mass.
Composition can be evaluated if we have a spectrum of the star, its fingerprint. But how can we determine the mass of a star?
Fortunately, most stars don’t travel solo, but in pairs known as binaries. (Our sun is an exception to this rule.) Binary stars orbit one another.
Some binaries are clearly visible from the earth and are called visual binaries, while others are so distant that, even with powerful telescopes, they cannot be resolved into two distinct visual objects. Nevertheless, these can be observed by noting the Doppler shifts in their spectral lines as they orbit one another. These binary systems are called spectroscopic binaries. Rarely, we are positioned so that the orbit of one star in the binary system periodically brings it in front of its partner. From these eclipsing binaries we can monitor the variations of light emitted from the system, thereby gathering information about orbital motion, mass, and even stellar radii.
However we observe the orbital behavior of binaries, the key pieces of information sought are orbital period (how long it takes one star to orbit the other) and the size of the orbit. Once these are known, Kepler’s third law can be used to calculate the combined mass of the binary system.
Why is mass so important? Mass determines the fate of the star. It sets the star’s place along the main sequence and it also dictates its life span.
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