Measuring the masses of stars would be a hopeless task if it were not for the fact that there are many binary or double stars in the sky. In fact, about half of the stars you see in the sky are not single stars like the Sun, but consist of two stars in orbit about each other just like the Earth and Moon revolve about their common centre. Figure 4 shows the results of careful observations taken over many years, and now the masses of the two stars can be calculated. It is the exact opposite of the problem of sending astronauts to Moon: in the Apollo missions, we know the masses of the Earth and Moon and want to calculate the orbit. For a binary star, we see the orbit and want to calculate the masses.
In many binary stars, the two stars are so close together that even the most powerful telescope cannot resolve them. We are sure that many stars are binaries because the brightness varies in a periodic fashion as shown in fig 4. The obvious interpretation of this light-curve is that the star is really a double and that the two stars alternately pass in front of each other. This is called an eclipsing binary.
The result of almost two centuries of observing binary stars is that astronomers have a good idea about the masses of stars. The masses are usually expressed in terms of the mass of the Sun; this is called the solar mass. Obviously, the mass of the Sun is one solar mass (actually 2 x 1030 kg ), and the masses of the other stars lie in the range of one tenth up to about 50 solar masses. High mass stars are extremely rare and most stars contain one solar mass or less.
Figure 2 The Double Star Kruger 60
Careful observations over many years often reveal that two stars located near each other in the sky are actually in orbit about each other. Indeed, about half of the stars in the sky are double stars. Kruger 60, in the constellation of Cepheus, is an excellent example. (Yerkes Observatory)
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