Beta Lyrae variable
Beta Lyrae variables are a class of close binary stars. Their total brightness is variable because the two component stars orbit each other, and in this orbit one component periodically passes in front of the other one, thereby blocking its light. The two component stars of Beta Lyrae systems are quite heavy (several solar masses each) and extended (giants or supergiants). They are so close, that their shapes are heavily distorted by mutual gravitation forces: the stars have ellipsoidal shapes, and there are extensive mass flows from one component to the other.
These mass flows occur because one of the stars, in the course of its evolution, has become a giant or supergiant. Such extended stars easily lose mass, just because they are so large: gravitation at their surface is weak, so gas easily escapes (the so-called stellar wind). In close binary systems such as beta Lyrae systems, a second effect reinforces this mass loss: when a giant star swells, it may reach its Roche limit, that is, a mathematical surface surrounding the two components of a binary star where matter may freely flow from one component to the other.
In binary stars the heaviest star generally is the first to evolve into a giant or supergiant. Calculations show that its mass loss then will become so large that in a comparatively very short time (less than half a million years) this star, that was once the heaviest, now becomes the lighter of the two components. Part of its mass is transferred to the companion star, the rest is lost in space.
The light curves of beta Lyrae variables are quite smooth: eclipses start and end so gradually that the exact moments are impossible to tell. This is because the flow of mass between the components is so large that it envelopes the whole system in a common atmosphere. The amplitude of the brightness variations is in most cases less than one magnitude; the largest amplitude known is 2.3 magnitudes (V480 Lyrae).
The period of the brightness variations is very regular. It is determined by the revolution period of the binary, the time it takes for the two components to once orbit around each other. These periods are short, typically one or a few days. The shortest known period is 0.29 days (QY Hydrae); the longest is 198.5 days (W Crucis). In beta Lyrae systems with periods longer than 100 days one of the components generally is a supergiant.
Beta Lyrae systems are sometimes considered to be a subtype of the Algol variables; however, their light curves are different (the eclipses of Algol variables are much more sharply defined). On the other hand, beta Lyrae variables look a bit like W Ursae Majoris variables; however, the latter are in general yet closer binaries (so-called contact binaries), and their component stars are mostly lighter than the beta Lyrae system components (about one solar mass).
Examples of β Lyrae stars
Nearly a thousand β Lyrae binaries are known: the latest edition of the General Catalogue of Variable Stars (2003) lists 835 of them (2.2% of all variable stars). Data for the ten brightest β Lyrae variables are given below. (See also the list of known variable stars.)
|star||type*||period (days)||apparent visual
|TU Mus||EB/KE||1.3||8.17-8.75||O7.5V + O9.5V||15500|
|υ Sgr||EB/GS||137.939||4.53-4.61||B8pI:+O9V ? (or F2p?)||~1700|
|*) EB = Beta Lyrae variable; for other codes see: General Catalogue of Variable Stars|