Popular Astronomy

Share to Facebook Share to Twitter Stumble It More...

Guide to Beta Lyrae

An eclipsing variable in which primary and secondary eclipses can be followed visually, and which also shows smaller scale brightness changes between eclipses.

Although the brightness changes in Beta Lyrae were first recognised in the 1780s, astronomers do still not fully understand what is happening in this system.

The orbital period is just under 13 days and is slowly increasing. Hence eclipse predictions made using old values for the orbital period will be incorrect.

Indeed, it is currently many orbits "behind" where it should be if predictions are based on the orbital period quoted in the General Catalogue of Variable Stars (GCVS)

The period changes are believed to be related to the transfer of mass between the two stars.

The brightness also varies slightly outside of eclipses because the two stars have distorted each other gravitationally - they have become egg-shaped, rather than spherical, and so their profile changes as the stars move around their orbits. Midway between eclipses, we see the "eggs" side on and hence they appear slightly bigger than when we see then "end on" near eclipses. Consequently, Beta Lyrae appears brightest midway between eclipses and then has already started to fade before the eclipses actually start. 

Extreme brightness range 3.4 - 4.3
More typical range always the same range
Period of variation 12.941 days (and slowly increasing)
Frequency of observation Check on every clear night
Observe using Can be observed with the naked eye, but 40mm or 50mm binoculars may also help, especially during eclipses
Visibility Can be observed all year round, but is rather low in the evening sky from January to April

Here is a finder chart that will allow you to locate Beta Lyrae.

You can follow the brightness changes in Beta Lyrae by comparing its brightness with that of the lettered comparison stars.

At its brightest, Beta Lyrae is almost as bright as comparison A.

During primary eclipses, it becomes almost as faint as comparison G.