PAUL ABEL
PRO ASTRONOMER
Paul’s Visual Observing Guide continues with a look at how you can track the waxing and waning of variable stars.
At first glance, the stars in the night sky all appear alike – cold points of light lying far away. However, after a few minutes subtle differences become clear: some are different colours, some appear fainter than others, and so on. But there is one property of some stars that can take longer to pick up: not all stars stay at the same brightness all the time.
These wonderful objects are called variable stars. Many of them ‘vary’ – that is, complete a cycle where they dim and then brighten again– over the course of a night, but some take over a year. Studying these stars has provided some fascinating insights into modern astrophysics and the private lives of stars.
It took a surprisingly long time for variable stars to be identified. In the modern era the first one was discovered by Johannes Holwards in 1638 when he noticed that Omicron Ceti varied over the course of 11 months. Since then, many thousands of variable stars have been identified. While the astrophysics of variable stars is of great interest to professional astronomers, most observations of these stars come from amateurs.
The main focus of variable star observation is making estimates of magnitude. These estimates can be plotted against time to generate a light curve like the one above, which reveals the period of time in which the star varies, its brightest and faintest magnitudes, and any long-term fluctuations hidden
in its constant changes. There are many different types of variable star, and organisations such as the British Astronomical Association (BAA) and the American Association of Variable Star Observers (AAVSO) run a vast number of variable star campaigns, in which stars are monitored by many amateur astronomers,
and their observations are used to generate the much-needed light curves.
Getting started
Becoming part of the movement that monitors these stars is simple and there are many variables you can make a start with. Gary Poyner of the BAA Variable Star Section has provided some good targets for beginners on the opposite page; as a rule it is best to start with a couple of stars then build up to a manageable list.
Observing variable stars doesn’t require expensive kit: you can observe with a small or large telescope – there are plenty of stars to go around! It’s a good idea to have a selection of eyepieces as it’s easier to locate stars at low magnification, then switch to high magnification to home in on the ones close to the limiting magnitude of your telescope. A sturdy notebook to act as your log, a reliable pen and a red light are also helpful.
A correct star chart is also essential. It must show the location of the variable star in the sky, and indicate comparison stars to use for making magnitude estimates. This is important since we can’t use other variable stars for making comparisons. The BAA Variable Star Section and the AAVSO are the two most reliable sources for charts – just go to their websites, find the star you want a chart for and print it out. Remember to record which chart you are using in your logbook.
Make sure you’re fully dark adapted before you start observing. The hardest part is tracking down the star in question, and for this I always use a lower power eyepiece (100x or so). If your telescope has
setting circles, use them to get your scope aimed at the right location, otherwise use binoculars to track down the right area of sky. Don’t be disheartened if it takes a number of attempts to find your chosen star– you’ll get there in the end.
Contributing to science
The main task is to estimate the magnitude of the variable (see ‘Estimating star magnitudes’, above, to find out how). Record these in your logbook at the telescope along with the date, time (in UT) and telescope details. You will need to do this for each observation, and I find it best to keep a separate book for each variable star.
It’s essential that you report your observations so that they can be used by the whole community. Both the BAA ( w w w.britastro.org/vss ) and the AAVSO ( w w w.aavso.org ) will be grateful for them. These organisations have knowledgable astronomers who can help you with any problems you may experience. You can upload your observations using their websites, or alternatively you can post a paper version. Both sites also allow you to generate light curves for your data so you can compare them with other observers.
Variable star observing is very rewarding. There is something special about studying the life of a star that may be hundreds of lightyears away from us on planet Earth. Your chosen star will need observing indefinitely, so a variable star is a lifelong friend.
Light curves can reveal patterns: this one shows the fluctuations in the brightness of cataclysmic variable star SS Cygni over a period of 10 years
