Chapter 14: The Milky Way

Accretion Disk Binary System

Mass transfer is the idea that material can move from one star to another. Many binaries orbit slowly with a large distance between the two stars. There is no interaction between stars that are at "arm's length" like this. However, gravity holds some stars in a tighter embrace. As we would expect from Kepler's laws, close binaries orbit more rapidly. This tight configuration allows material to be transferred from one star to another, with interesting consequences.

A series of snapshots in the life of a binary star before mass transfer and during its common envelope evolution. The binary has a mass ratio M1/M2=3. The black line is the Roche equipotential surface. The CoM is the centre of mass of the binary system.

How do astronomers describe mass transfer? We can picture a system of two closely orbiting stars as containing an imaginary Roche surface, or lobe. A Roche surface is the gravitational boundary of the gas in a binary system. The concept was proposed by Edward Roche, a French astronomer from the mid-19th Century. Roche was also known for his theory that the rings of Saturn were caused by the disruption of a moon that came too close to the giant planet. The cross-section of the system is a figure eight, with one lobe around each star. Slow-moving material inside either lobe orbits around the star in that lobe. Material that moves beyond either lobe is not gravitationally bound to either star. Mass can move from one star to the other through the point of contact between the Roche surfaces. When either star evolves into a red giant state, it expands until it fills its lobe. Its outer layers then assume the teardrop shape of the lobe, and the Roche surface becomes the new surface of the star.

A Hertzsprung-Russell diagram showing color and size of stars.

Astronomers can predict what will happen when two stars of different masses start life together as a binary system. Evolution can take a single binary through three stages. In the first stage, neither star fills its Roche lobe. In the second stage, the more massive star leaves the main sequence and swells into a giant as it evolves. The giant may fill its Roche lobe. Any further tendency to expand causes matter to be shed, mostly through the point common to the two lobes. If the giant were a single star, it would be spherical, and what little mass it did lose would stream off in all directions. But pressure within the giant and the companion's gravity force the bloated giant to lose gas through its distorted, pointed tip. Like sand in an hourglass, this gas enters the lobe of the second star. Most of the gas spirals around the small star and crashes onto it, making it gain mass as the large star loses mass.

Eclipsing binary star of type beta Lyrae animation

Mass drives stellar evolution, so the evolution of both stars in a binary system will be altered, compared to their destinies had they been single. Later, the second star evolves to the giant state, partly through its normal evolution, but partly because of the added mass. In the third stage, both stars fill their lobes, producing what is called a contact binary. At the junction of the teardrop regions of space, the two stars actually touch as they orbit each other as a single unit! If two stars start very far apart, however, the expansion as a giant may not fill either Roche lobe, and the binary may never evolve beyond the first stage.