What are Variable Stars ?
Variable stars are stars that change in brightness over a period of time.
Some do it relatively quick, while others undergo the process more sedately over many weeks or months.
Since 1961, the Variable Star Section of the SPA has been helping beginners in this most fascinating branch of visual astronomy.
Charts for all the section stars can be found in the guides linked to the SPA VSS Programme Listing. Simply click on the name of any of the variable stars to access more information about the star and finder chart(s) for it.
The following notes are intended as a guide for the beginner.
They describe the naming of stars, different types, how often to observe, making a magnitude estimate and submitting observations to the section.
How are variable stars named?
Every variable star has a designated name and numerical number, by which it can be identified.
The very first variables were naked eye objects, and as such, they already had names. For example: Beta Persei, Beta Lyrae and Omicron Ceti, and it was decided that they would retain these designations.
It was also agreed Internationally that all new discoveries would be assigned a capital letter.
The letter R is used for the first discovered variable in any given constellation. This is followed by S, T and all the remaining letters until Z is reached.
The system then uses the double letters RR, RS….RZ, this is followed by SS…..SZ etc. After we reach ZZ, the system starts with AA, AB….AZ, until we reach QZ.
This letter system means, that 334 variable stars can be assigned to any given constellation, with the exception of letter combinations with J.
The next variable to be found was given the number 335, preceded by the letter V. So for example: V335 Cyg was the 335th variable star to be found in the constellation of Cygni. Due to the observations of large numbers of variables, it has become possible to classify them into groups based on the processes by which they vary in brightness.
Why do stars vary in brightness ?
Explaining that is the challenge for professional astronomers.
There are three main categories of variable stars: Eclipsing binaries, Pulsating and Eruptive.
These three main groupings are also split into many sub-divisions, which adds yet more interest to these remarkable stars.
Eclipsing variable stars are binary star systems, whose orbits, by chance, are edge on from our line of sight.
The brightness changes are not due to changes in the individual stars. Instead we see a change in brightness when one of the stars passes in front of the other.
Whether we see a total or a partial eclipse depends on the alignment of the orbital plane relative to our line of sight.
By making regular observations, it is possible to detect any discrepancy between observed and predicted time of mid-eclipse. These discrepancies can reveal the existence of mass transfer between the two stars or the gravitational effect of a third (unseen) star in the system.
The brightness changes in pulsating stars are due to changes in the stars themselves. The changes are related to the expansion and contraction of the star’s outer surface layers.
Some pulsating stars display regular rhythmic pulsation cycles over the course of a few days.
Others pulsate with no easily recognised period (or at least one that hasn’t been recognised yet), or in a semi regular fashion.
Pulsating stars tend to be giant stars. Many have (relatively) cool surface temperatures and therefore appear to be very red or orange in colour.
This grouping covers many different and varied types of stars. Their changing brightness is the result of violent processes that results in the ejection of material from the stars outer surface layers.
Types of Variable Star
Cepheid (Cep): Pulsating yellow super giant stars. That have periods from 1 to 70 days, and changes in brightness of up to 2 magnitudes. Most Cepheids pulsate with clockwork like regularity.
Semi Regular (SR): Giant pulsating stars (mostly red), that can change in visual brightness by 1 or 2 magnitudes. Periods can range from a few months to several years. Sometimes the period is ill defined when little change appears to take place, or when the star starts to show more than one period range.
RV Tauri (RV Tau): Pulsating super giant stars, that (usually) display alternating deep and shallow minima. They can vary by up to 3 or 4 magnitudes in brightness. Some show periods as short as 30 days ; others as long as 150 days.
Irregular (I): Pulsating stars that show little or no trace of periodic variation.
Mira (M): Giant pulsating stars that are sometimes referred to as long period variables. They can display large magnitude ranges over the period of a few hundred days. Their maximum brightness can often differ by as much as one magnitude or more from one cycle to the next. Most Mira type stars display a characteristic red or orange colour.
Eclipsing Binary (EA): These are eclipsing binary stars that stay at a constant brightness when outside of an eclipse. Periods can range from 0.2 days, up to 27 years, with a 1 to 2 magnitude change in brightness.
Eclipsing Binary (EB): Unlike the EA type, these change in brightness continually. The component stars have ellipsoidal shapes and are almost in contact with each other. Periods tend to be longer than 1 day with a change in brightness of 1 to 2 magnitudes. The EB stars always display a secondary minima that is observable.
Gamma Cassiopeiae (GCAS): Stars that change in brightness at irregular intervals, because shells of gas are thrown off the star due to its rapid rotation.
R Coronae Borealis (RCB): Very rare high luminosity stars, that is rich in carbon and helium. These stars undergo a large and rapid drop in brightness, due to the ejection of carbon from the star’s outer surface layers. The return back to normal brightness can often take several months, or occasionally much longer. These events are totally unpredictable, and regular monitoring of these stars is required.