What can stars of a binary system do?

Answer 1

Planet's revolve around stars because of gravity. However, gravity is not restricted to only act between large and small bodies, stars can revolve around stars as well. In fact, 85% of the stars in the Milky Way galaxy are not single stars, like the Sun, but multiple star systems, binaries or triplets. If two stars orbit each other at large separations, they evolve independently and are called a wide pair. If the two stars are close enough to transfer matter by tidal forces, then they are called a close or contact pair. Binary stars obey Kepler's Laws of Planetary Motion, of which there are three. 1st law (law of elliptic orbits): Each star or planet moves in an elliptical orbit with the center of mass at one focus. http://abyss.uoregon.edu/~js/images/ellipse.gif Ellipses that are highly flattened are called highly eccentric. Ellipses that are close to a circle have low eccentricity. 2nd law (law of equal areas): a line between one star and the other (called the radius vector) sweeps out equal areas in equal times. http://abyss.uoregon.edu/~js/images/equal_areas.gif This law means that objects travel fastest at the low point of their orbits, and travel slowest at the high point of their orbits. 3rd law (law of harmonics): The square of a star or planet's orbital period is proportional to its mean distance from the center of mass cubed It is this last law that allows us to determine the mass of the binary star system (note only the sum of the two masses, see previous lecture). http://abyss.uoregon.edu/~js/images/binary_star_orbit.gif