Re: What are Roche lobes?

Hi Elsy,

A star only has a Roche Lobe if that star is part of a binary system, in which two stars are orbiting around each other. This is because the Roche Lobe of a star is defined as the domain surrounding the star in which material released in co-rotation with the binary falls back on the star. Outside the Roche Lobe, material is either flung out of the system by "centrifugal forces," or falls on the companion star. There are also 5 points called the Lagrange point (L1 through L5) at which matter can hover indefinitely. L1 lies in between the two stars and is approximately the point where the gravity of the two stars pull in equal and opposite magnitude (approximately because rotation throws in a fudge factor). Check out:

http://www.astro.keele.ac.uk/~zac/rochelob.html

As you can imagine, then, if one of the stars in a binary expands beyond its Roche Lobe, the gas which is outside of the Roche Lobe is stripped off. Usually what happens is that only one star expands beyond its Roche Lobe at a time, when it reaches the end of its life and puffs up as a red giant. In that case, the mass usually follows a stream from the expanding star onto the smaller companion, as in this image . This particular image is of a "cataclysmic variable," in which the fairly normal star in the foreground is orbiting a white dwarf star (a white dwarf is a burned-out remnant of a star like the Sun that has used up all of its nuclear fuel). The normal star is so close to the white dwarf that it is larger than its Roche Lobe, and is losing gas to the white dwarf, which forms a disk of gas orbiting the white dwarf.

The same sort of thing can happen to a red giant, if it is orbiting a smaller companion star. In some extreme cases, the red giant may even lose most of its mass to the companion. The companion may have started out being smaller than the red giant, but, afterwards, it may have more mass than what is left of the red giant!