Observing Variable Stars: SS Cyg

This image shows a comparison of the variable star SS Cyg to show the difference between its minimum and maximum brightness.

Image by Ivan Walton

Variable Star Observing 

At the start of the 19th century very few variable stars were known, certainly less than a few dozen. Nowadays, the  ‘Bible’ for all types of variable star (the General Catalogue of Variable Stars) lists over 40,000 variables!  

Even simple observation of these stars can yield information about their mass, radius, luminosity and evolution. And  what’s more it can be done with the simplest of equipment. We usually observe variable stars by measuring their  variations over time. These variations take the form of changes in brightness (magnitude) and are viewed as a  light-curve (magnitude against time).

Why observe variable stars? 

Variable stars can tell us much about stellar evolution and ultimately, the fate of our own Sun. Professional  astronomers need amateur observations as there are just too many variable stars for them to follow. Their  telescopes are often too large, and observing time on them is scarce. This is one of the few areas where an amateur  can make a real contribution to science. So at CADSAS we contribute data to the Variable Star Section (VSS) of the  British Astronomical Association (BAA) for a wide selection of variable stars. 

What are variable stars?  

Three main types of variable star are observed by the VSS: Pulsating, Eclipsing and Eruptive. 

Pulsating stars (Miras, semi-regulars [SRs] etc.) The variations are caused by the star physically pulsating, like a  balloon blowing up and down, but only the outer layers are involved. Periods range from hours to years, depending  on the type. The variation in brightness can be from about 0.5 magnitudes to over 10.  

Eclipsing binary stars (EBs) The variations are caused by two stars in our line of sight that physically revolve around  each other. Periods range from hours to many years. Some may be seen to undergo an entire eclipse in a single  night.

Eruptive stars (CVs, novae and supernovae) These stars show sudden increases in magnitude known as ‘outbursts’,  sometimes quite gentle and relatively frequent, and sometimes the gigantic explosions we call supernovae. Many  CVs (‘cataclysmic variables’) are actually binary stars in which the eruptions are due to an exchange of material from  one star to the other, often via an accretion disc. There are many subtypes of CVs and form one of the most  interesting areas for observing.