Popular Astronomy

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Choosing stars to observe

 

Which type of variable star is suitable for you?

This depends on how often you can observe in a given month and how long you can observe for on each occasion.

The variable stars on the programme of the SPA VSS can be divided into five categories:

 

1. One-Nighters

These are stars for which it is possible to see significant variation in brightness in the course of several hours on a single night, although not necessarily on every night.

This category includes RR Lyrae and the Algol type eclipsing variables.

One such eclipsing variable is RZ Cassiopeiae, whose primary eclipse is shown in the accompanying light curve.

In the case of RZ Cas, the whole eclipse lasts for nearly 5 hours, but most of the brightness changes occur during the middle 3 hours.

Algol (beta Persei) is the most famous example, although its eclipses are about twice as long as those of RZ Cas.

 

2. One-Weekers

These are stars for which significant changes in brightness can be seen over the course of a week or two, in some cases from one night to the next.

These would suit people who are able to observe for a short time on every clear night.

This category includes the Cepheid variables, such as Delta Cephei,  and the beta Lyrae type eclipsing variables.

 

3. Medium-Rangers

These are stars that vary more slowly, but which show significant changes in brightness over the course of several months or a year.

The brightness changes don't repeat exactly from one cycle to the next, although there are clear signs of periodicity, all is illustrated by the accompanying light curve for AF Cygni.

Sometimes these stars spring surprises - by brightening beyond the published upper limits of their brightness ranges or fading below the published lower limits.

They would suit people who can observe several times per month, but not necessarily on every clear night.

This category includes R Scuti and the semi-regular variables.

 

4. Big-Rangers

These are stars that show large changes in brightness over the course of a year.

Their brightness changes are so large that for several months they become too faint to be seen using binoculars and are only visible telescopically.

The accompanying light curve for R Serpentis shows its brightness changes when visible in binoculars around its summer 2015 maximum.

They would also suit people who can observe several times per month, but not necessarily on every clear night.

This category includes the Mira type variables.

 

5. Rebels

These are variable stars that are totally unpredictable.

These variables would mostly appeal to people who can observe on every clear night, but other observers may also enjoy monitoring them when their circumstances permit.

The category includes R Coronae Borealis and Novae.