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BINARY STARS

BINARY STARS - solar system we can deduce the mass of the...

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BINARY STARS The first binary, or double, star was discovered in 1650 by Jean Baptiste Riccioli, who observed two stars separated by about 1/1000 of a degree. He did not observe motion, but in 1805 William Herschel observed a famous binary pair, one star bright and the other faint, and the faint one moved about 1/7000 of a degree in one year. After a few years he saw that the motion was not a straight line. Given Newton's laws of motion, he had to conclude that the faint star was feeling a force. And the only reasonable assumption was that this was the gravitational force exerted by the bright star. Eventually it was seen that the dim star orbitted the bright star. The bright star would also feel gravitational force due to the dim star, but it is more massive (usually brightness correlates with mass) and so its motion would have been less -- too little to be detectable. Following the method we have used in connection with the
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Unformatted text preview: solar system , we can deduce the mass of the bright star from the orbital parameters of the dim star. This is in fact the first determination of the mass of a star, and it is the main technique we have today for determining stellar masses. From observation of many binary stars that we have gotten a good idea of the range of star masses. Among other things we find that the sun's mass falls in this range, which adds to our conviction that the sun is a star. Figure 3 below is a logarithmic scale which shows the range of star masses, and where the solar system planets fit in. A logarithmic scale is one in which a fixed spacing on the line corresponds to multiplying by a constant factor (the factor here is 10). Each mass is given relative to the sun. Note the large "empty space" between the smallest stars and Jupiter, the largest planet....
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