Binary stars are stars bound in orbit about one another by their gravitational attraction. Most stars seem to form in binary systems or in systems containing more than two stars. This is not really surprising: stars form from condensations in giant clouds of gas, so where one star forms, others are likely, and they may form close enough to each other to be bound together forever. We saw this in the numerical simulation reproduced in Figure 12.2 on page 138.

We have already studied special cases of binaries: planets in motion around the Sun, and the Moon around the Earth. These orbits allow us to measure masses in the Solar System. We learn the Sun's mass once we know the radius and period of the Earth's orbit. Similarly, we measure the mass of the Earth by studying the motion of the Moon (and of artificial Earth satellites). In the same way, binary star orbits are used to measure the stars' masses. Binaries are often our best, indeed our only, way of measuring the masses of stars.